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Structure of Atom
Learning Objectives
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Ancientviewonatomicstructure
Sub-atomicparticlesandtheirdiscovery
Modernconceptofanatom
Atomicnumber
Massnumber
Relativeatomicmassormassnumber
Isotopes
Arrangementofelectrons
Valency
Radioactivity
Everything around us is made up of matter. Matter is anything that occupies space and has mass.
Atoms are the building blocks of matter and cannot be chemically subdivided by ordinary means. The
word atom is derived from the Greek word atomos which means indivisible.
Around 20th century, through various experiments with electromagnetism and radioactivity,
physicists discovered that the so-called indivisible atom was actually a cluster of various subatomic
particles (mainly electrons, protons and neutrons) which can exist separately from each other.
In this chapter, let’s know how ancient scientists and philosophers discovered the structure of an
atom.
ANCIENT VIEW ON ATOMIC STRUCTURE
Maharishi Kanada, a great Indian philosopher of 600 BC, proposed that matter consists of indestructible
particles called paramanu (param means ultimate and anu means particle). A paramanu does not
exist in free state, rather it combines with other paramanus to form a bigger particle called the anu.
In other words, he believed that an anu may be made up of two or more subatomic particles called
paramanu.
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The whole universe is made up of five basic elements—earth, water, fire, air and space. Our body is also made up of these
five basic elements of nature, also known as pancha mahabhutas. The pancha mahabhutas are related to our five senses
of smell, taste, hearing, touch and sight.
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Dalton’s Atomic Theory
John Dalton proposed an atomic theory on the basis of his research. The
postulates of this theory are as follows:
1. All matter consists of tiny particles called atoms.
2. Atoms are indivisible or indestructible. In chemical reactions, atoms
rearrange, combine or separate but they do not themselves break
apart.
3. All atoms of a particular element are identical in mass and other
properties.
John Dalton
4. The atoms of one element are different from the atoms of all other elements, i.e., atoms of
different elements have different mass, different size and different chemical properties.
5. Atoms of an element combine with atoms of another element (or elements) in whole numbers
to form compounds.
SUB-ATOMIC PARTICLES AND THEIR DISCOVERY
John Dalton proposed that an atom is solid,
indestructible and indivisible sphere. But studies and
discoveries in the 19th and 20th centuries modified
his theory.
In 1897, the English scientist Sir J.J. Thomson proved
that an atom can be split into even smaller parts. His
discovery of the electron was the first step towards a
detailed model of the atom.
Discovery of Electron
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Joseph John Thomson was
born on December 18, 1856
near Manchester, England.
He won the scholarship at
Trinity College, one of the
most prestigious colleges
at Cambridge University. In
1906, he was awarded the
Nobel Prize in Physics for his researches on the
discharge of electricity in gases.
We know that under ordinary conditions, gases are poor conductor of electricity. But gases become
good conductor of electricity when
(i) the gas is stored at very low pressure (0.01–0.001 mm of mercury) and
(ii) a very high voltage is applied through the gas (more than 10,000 volts).
J.J. Thomson applied these conditions in electric discharge tubes to conduct his experiment on a gas.
A discharge tube is a cylindrical glass tube with sealed electrodes at each end. Electrodes are the
metal plates connected to a high voltage source. The metal plate connected to the negative terminal
of the voltage source is called the cathode. The metal plate connected to the positive terminal of the
voltage source is called the anode. A small tube at the side of the cylindrical glass tube is connected
to a vacuum pump to reduce the pressure of the gas, as desired.
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high voltage generator
–
+
discharge tube
air at very low
pressure
green glow
+
–
cathode rays
anode
cathode
to vacuum pump
Fig. 1.1: Cathode rays in discharge tube
When a high voltage electric current is passed through the discharge tube containing small quantity
of a gas, at very low pressure, a green fluorescence is seen coming from the glass opposite to the
cathode. This fluorescence was observed due to a stream of rays originated from the cathode, hence
called the cathode rays.
electric
field
–
fluorescent screen
+
+
–
high voltage
Fig. 1.2: Line representation of J.J. Thomson’s experiment
Characteristics of cathode rays
• Cathode rays are produced from the
cathode in a discharge tube when an
electric field is applied to a gas at reduced
pressure.
• Cathode rays consist of negatively-charged
particles because when an electric field
was applied in the path of cathode rays,
the rays got attracted (or bent) towards
the positively-charged plate.
+
–
–––
+++
cathode
anode
Fig. 1.3: Negatively-charged cathode rays bending towards
positively-charged discharge plate
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• Cathode rays are a stream of fast moving charged particles. It was observed that when a lightweight wheel was placed in the path of cathode rays, the wheel began to rotate.
• Cathode rays travel in a straight line. A shadow was formed on the wall opposite to cathode
when a solid object was placed in the path of cathode rays. A shadow will be formed only if
+
cathode rays travel in a straight line.
–
cathode
+
–
opaque
object
cathode
cathode rays
wheel
shadow of
opaque
object
anode
Fig. 1.4: A stream of fast moving charged
particles rotate plate
Fig. 1.5: Cathode rays travel in straight
lines the wheel
J.J. Thomson found that the cathode rays consisted of negativelyThirst for Knowledge
charged particles. These particles were named as electrons.
An electron is represented by the
In order to get an estimate of the size of these particles, he
symbol –1e0. The superscript 0 rep­
resents its mass and the subscript –1
calculated their charge to mass ratio (e/m). This ratio was found
represents its electrical charge.
to be equal to 1.78 × 108 C/g (coulomb per gram). Thomson also
noticed that the value of charge to mass ratio was the same for all electrons irrespective of the nature
of the gas and the material of the electrodes. This proved that electrons are contained in all matter
and hence in all atoms. Therefore, the atom is not indivisible and is composed of smaller particles.
Characteristics of electrons
• An electron is an integral part of an atom and all matter.
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• The charge to mass ratio (e/m) of electron was found to
The charge of an electron was found to
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be 1.602 × 10–19 coulombs, in 1908, by
be 1.78 × 10 C/g (coulomb per gram).
R.A. Millikan.
• The value of e/m is the same irrespective of the nature of
the gas and the material of the electrodes used in the discharge tube.
• The mass of an electron was found to be 9.108/10–28 g, which was about 1/1837 of a hydrogen
atom.
• The charge on an electron is one unit negative charge, i.e., 1.602 × 10–19 coulombs.
Discovery of Proton
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An atom is electrically neutral. This suggests that if negatively-charged electrons are present in an
atom, so there must be some positively-charged particles present in the atom to neutralize the
negative charge of the electrons.
E. Goldstein, a German scientist, performed an
experiment in the discharge tube, with a perforated
cathode (a cathode with small holes to allow
passage of positive rays). During the experiment,
he observed another set of rays moving in direction
opposite to that of the cathode rays. Since these
rays originated from the anode, they were named
as anode rays. They were also called canal rays.
The anode rays consist of the positively-charged
particles called protons.
anode
cathode rays
perforated cathode
anode rays
Fig. 1.6: Emission of anode rays
Characteristics of anode rays
• Anode rays are produced from the anode in a discharge tube when an electric field is applied
to a gas at reduced pressure.
• Anode rays consist of positively-charged particles
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• Since an atom is electrically neutral,
because when an electric field was applied in the path of
the number of electrons in an atom
anode rays, the rays got attracted (or bent) towards the
is equal to the number of protons in
negatively-charged plate.
that atom.
• A proton is about 1840 times
• Anode rays are a stream of fast moving charged particles.
heavier than an electron.
• Anode rays travel in a straight line.
Characteristics of proton
• A proton is an integral part of an atom.
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• The charge to mass ratio (e/m) of proton is not fixed, i.e.,
A proton is represented by the symbol
p1. The superscript 1 represents
+1
it varies.
1 amu mass and the subscript +1
• The e/m value depends on the gas being used in the
represents one unit positive charge.
discharge tube.
• The mass of a proton is almost equal to the mass of an atom of hydrogen, i.e., 1.672 × 10–24 g.
• The charge on a proton is equal in magnitude to the charge on an electron, i.e., 1.602 × 10–19
C but opposite in nature and sign.
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In 1906, J. J. Thomson proposed the plum pudding model of the atom. In this
model, each atom was a sphere of positively­charged fluid called the “pudding” in
which electrons known as “plums” were embedded. This model failed to explain
many experimental observations about atoms. This model was displaced by
Rutherford’s model.
positively
charged
matter
negatively
charged
electrons
–
–
–
–
–
–
–
–
–
–
–
Discovery of Nucleus
In 1911, Ernest Rutherford carried out the famous gold foil
experiment. In his experiment, Rutherford bombarded a thin
sheet of gold of thickness 0.00004 cm with alpha (a) particles,
in an evacuated chamber.
Alpha Particles
Positively charged particles emitted by
radioactive substances like uranium,
radium, etc.
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Rutherford observed that most of the α-particles passed straight through the foil without any
deflection from their path. However, to his surprise many particles were deflected at very large
angles. As the mass of α-particles is about 8000 times that of an electron, it was evident that the force
which causes such large deflections was also strong. Rutherford then realised that the deflections of
the α-particles could only be caused by a centre of concentrated positive charge that accounts for
most of the atom’s mass. This central body of concentrated positive charge was called nucleus by
Rutherford.
radioactive sample emits beam
of alpha particles
lead block shield
gold foil
zinc sulphide screen
most alpha particles
hit here
some alpha particles are deflected
Fig. 1.7: Gold foil experiment
Rutherford’s model of an atom
Based on his experimental findings, Rutherford in 1911 published
his views on atomic theory. He suggested the following model of
the atom:
1. Most of the mass of an atom was concentrated in the
centre of the atom called nucleus.
2. The nucleus is the densest part of an atom and contains
the positively-charged particles, the protons.
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A large number of positively­charged
protons confined in a small nucleus
should lead to repulsion, thus, making
the nucleus unstable. But a nucleus
is stable. Rutherford explained that
protons in the nucleus are acted upon
by an attractive nuclear force that
balances the repulsion in protons. Thus,
a nucleus is stable.
3. The size of the nucleus is extremely small as compared
to the size of atom as a whole, as there is a lot of empty
space around the nucleus.
4. An atom is electrically neutral, i.e., an equal number of electrons surround the nucleus to
counter balance the positive charge in an atom.
5. Electrons revolve in circular orbits (shells) in the space available around the nucleus, just as in
the solar system the sun is at the centre and the planets revolve around it.
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The Rutherford model of an atom did not explain the stability of an atom. Positively-charged nucleus
and negatively-charged electrons would attract each other. An electrically-charged particle in motion
releases energy and therefore, the electrons should lose energy and get attracted towards the
nucleus, leading to the total collapse of the atom. But this does not happen. An explanation to
this problem was provided by Neils Bohr, a Danish physicist. He suggested that electrons possess a
specific amount of energy and are arranged in a succession of energy levels or orbits that allow them
to revolve around the nucleus.
Discovery of Neutron
By the time Rutherford gave his model of the atom, it was concluded that the mass of an atom was
entirely concentrated within the nucleus in the form of protons, since electrons had negligible mass.
However, during subsequent experiments, it was accounted that the mass of an atom is far more
than the total mass of proton present in the atom. It was realized that there must be a third type of
sub-atomic particle, which was present in the nucleus that had mass but no electric charge. Neutron
was discovered in 1932 by James Chadwick.
Characteristics of neutrons
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• Neutron is an integral part of an atom and is present
inside the nucleus.
• It is of the same size as that of a proton.
• It is electrically neutral, i.e., it has no charge.
• The mass of a neutron is slightly more than that of a
proton. The mass of a neutron is 1.676 × 10–24 g and that
of a proton is 1.672 × 10–24 g.
The protons and neutrons present in
the nucleus of an atom are collectively
called nucleons.
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A neutron is represented by 0n1. The
superscript 1 represents 1 amu which
is its mass and the subscript 0 represent
zero charge.
Table 1.1 shows a summary of sub-atomic particles.
Table 1.1: Sub-atomic particles
Particles
Proton
Neutron
Electron
Symbol
p1
+1
n1
0
e0
–
Relative charge
(1.602 × 10–19 C)
Atomic mass
(in grams)
Relative mass
(amu)
+1
1.672 × 10–24 g
1
9.108 × 10–28 g
1/1837
0
–1
1.676 × 10–24 g
1
Position
In the nucleus
In the nucleus
Outside the nucleus
Take a Break!
1. All atoms of a particular element are identical in _____________ and _____________.
2. Cathode rays are produced from the _____________ in a discharge tube.
3. The anode rays consist of the positively-charged particles called _____________.
4. The charge to mass ratio (e/m) of proton is fixed. (True/False)
5. _____________ revolve in fixed orbits around the nucleus.
6. Neutrons and protons are present outside the nucleus of an atom. (True/False)
MODERN CONCEPT OF AN ATOM
According to the modern concept of the atom:
1. An atom consists of three sub-atomic particles—electrons, protons and neutrons.
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2. There are two structural parts of an atom:
(i) the nucleus and
(ii) the extra-nuclear region or the empty space
that surrounds the nucleus.
3. The nucleus is positively charged central part of
the atom. It contains the protons and neutrons.
The protons and neutrons are held tightly in the
nucleus by strong nuclear forces. The positive
charge of nucleus is due to the protons present in
it. The neutrons have no charge. The entire mass
of the atom is concentrated in the nucleus (since
electrons have negligible mass).
orbits or
shells
P
N
electrons
nucleus
(protons + neutrons)
Fig 1.8: Modern concept of the atom
4. The extra-nuclear region of an atom consists of the electrons which revolve around the
nucleus in definite orbits or shells. Each orbit has a fixed energy level. The energy increases as
you go away from the nucleus. The electrons experience an outward pull (or centrifugal force)
due to their circular motion. This outward pull is counter balanced by the inward attraction
between these electrons and the protons present inside the nucleus.
5. An atom is electrically neutral as it contains equal number of electrons and protons.
ATOMIC NUMBER
The number of protons present inside the nucleus of an atom of an element is called its atomic
number. It is represented by the letter Z.
Since an atom has equal number of protons and electrons, the atomic number of the atom of an
element is also equal to the number of electrons present in it.
Atomic number (Z) = Number of protons
= Number of electrons
MASS NUMBER
The sum total of the number of protons and the number of neutrons present in the nucleus of an
atom of an element is called the mass number of that element. It is represent by the letter A.
Mass number (A) = Number of protons + Number of neutrons
The atomic number and the mass number are respectively written as subscript and superscript
A
alongside the symbol of an element. An element X is represented as Z X . This is called atomic notation
of an element.
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If the atomic number and the mass number of an element are known, then the number of neutrons
can be easily calculated.
Mass number (A) = Number of protons + Number of neutrons
Thinking Fountain
Number of neutrons = Mass number (A) – Number of protons
Calculate the number of electrons,
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Since Atomic number (Z) = Number of protons
protons and neutrons in krypton (36Kr ).
\ Number of neutrons = Mass number (A) – Atomic number (Z)
Number of neutrons = A – Z
The atomic notation and number of sub-atomic particles of first 15 elements are mentioned in
Table 1.2.
Table 1.2: Atomic notation and number of sub-atomic particles of elements
Element
Hydrogen
Symbol
H
Helium
He
Beryllium
Be
Carbon
C
Lithium
Boron
Li
B
Nitrogen
N
Fluorine
F
Oxygen
O
Neon
Ne
Magnesium
Mg
Silicon
Si
Sodium
Aluminium
Phosphorus
Na
Al
P
Atomic
notation
1
H
1
4
He
2
7
Li
3
9
Be
4
11
B
5
12
C
6
14
N
7
16
O
8
19
F
9
20
Ne
10
23
Na
11
24
Mg
12
27
Al
13
28
Si
14
31
P
15
Mass
number
(A)
Atomic
number
(Z)
Number of
protons
(p)
Number of
electrons
(e)
Number of
neutrons
(n)
1
1
1
1
0
7
3
3
3
4
4
9
11
12
14
16
19
2
4
5
6
7
8
9
2
4
5
6
7
8
9
2
4
5
6
7
8
24
12
12
12
28
31
13
14
15
RELATIVE ATOMIC MASS OR ATOMIC MASS
13
14
15
7
8
12
10
27
6
11
10
11
6
10
10
11
5
9
20
23
2
13
14
15
10
12
14
14
16
Atoms are too small to be weighed. So scientists focused on finding out a standard atom and calculating
the relative weight of atoms of other elements by comparing it with the mass of the standard atom.
Earlier hydrogen atom was chosen as the standard atom for this purpose. As time passed, the scale
of atomic weight based on hydrogen was abandoned and the
Thirst for Knowledge
term atomic weight was also replaced by another term called
The average mass of one atom of an
the relative atomic mass of an element.
The relative atomic mass of an element is the number of times
by which the average mass of one atom of an element is heavier
than 1/12th of the mass of a carbon atom.
element is used because elements may
have atoms of different masses. This is
because the number of neutrons in the
nucleus may differ from atom to atom.
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Relative atomic mass =
Average mass of one atom of an element
One-twelfth of the mass of one atom of carbon-12
It is expressed in atomic mass unit (amu).
ISOTOPES
Atoms of the same element having same atomic number but different mass number are known
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as isotopes. For example, 7 N and 7 N are the two isotopes of nitrogen. There are 7 protons and 7
electrons in both the isotopes. But the first atom has 7 neutrons and the second atom has 8 neutrons.
Properties of Isotopes
• The isotopes of an element have same atomic number. Thus the number of electrons are also
same.
• The isotopes of elements show same chemical properties because they have same atomic
number.
• Mass dependent properties such as density, melting point and boiling point will differ in
different isotopes. This is because the mass of the isotopes differs slightly.
ARRANGEMENT OF ELECTRONS
The arrangement of electrons in different orbits or shells around the nucleus of an atom is called
electronic configuration. We can write down the electronic configuration of an element if we know:
1. The number of electrons present in an atom of the element and
2. The maximum number of electrons which can be accommodated in different energy shells of
the atom.
The distribution of electrons in various energy shells of an atom is governed by a scheme called BohrBury scheme.
The rules of Bohr-Bury scheme of electronic configuration are as follows:
• The maximum possible number of electrons that can be present in any shell of an atom is
given by the formula 2n2, where n is the number of shells. The shells are labelled as K (n = 1),
L (n = 2), M (n = 3), N (n = 4) and so on.
Table 1.3 gives the number of electrons a shell can accommodate.
Table 1.3: Number of electrons that can be accommodated in a shell
Shell
Number of shell (n)
Maximum number of
electrons in the shell (2n2)
K
1
2(1)2 = 2
N
4
L
M
10
2
3
2(2)2 = 8
2(3)2 = 18
2(4)2 = 32
• The outermost shell also called the valence shell cannot have more than 8 electrons (octet)
except in case of the first shell which cannot have more than 2 electrons (duplet). The reason
behind this is that the presence of eight electrons in the outermost shell makes the atom very
stable. The shell preceding the valence shell cannot have more than 18 electrons.
The electronic configuration of the first 20 elements is given in Table. 1.4.
Table 1.4: Electronic configuration of elements
Element
Hydrogen
Symbol
Atomic Number
H
1
Helium
He
2
Beryllium
Be
4
Carbon
C
Lithium
Li
Boron
B
Nitrogen
N
Fluorine
F
Oxygen
Neon
O
Electronic configuration
K
L
M
1
2
3
2
1
5
2
3
2
6
2
2
7
4
2
8
5
2
9
6
2
7
Ne
10
2
8
Magnesium
Mg
12
2
8
2
Silicon
Si
14
2
8
4
Sodium
Aluminium
Na
Al
Phosphorus
P
Sulphur
S
Chlorine
2
13
8
2
15
8
2
16
8
2
8
1
3
5
6
17
2
8
7
K
19
2
8
8
Ar
Calcium
Ca
Potassium
11
Cl
Argon
N
18
2
20
8
2
8
8
1
8
2
6n
5p
4n
3p
5n
4p
Helium (2)
Lithium (3)
Beryllium (4)
6n
6p
7n
7p
8n
8p
10n
9p
10n
10p
Carbon (6)
Nitrogen (7)
Oxygen (8)
Fluorine (9)
Neon (10)
1p
Hydrogen (1)
2n
2p
Boron (5)
11
12n
11p
12n
12p
14n
13p
14n
14p
16n
15p
Sodium (11)
Magnesium (12)
Aluminium (13)
Silicon (14)
Phosphorus (15)
16n
16p
18n
17p
22n
18p
20n
19p
20n
20p
Sulphur (16)
Chlorine (17)
Argon (18)
Potassium (19)
Calcium (20)
Fig. 1.9: Arrangement of electrons in the first 20 elements
VALENCY
The outermost shell of an atom is known as valence shell or valence orbit. The electrons present in
the valence shell of an atom are called valence electrons.
Valence electrons are involved in compound formation. They readily participate in chemical reactions
and therefore, determine the chemical properties of an element.
Since every atom of an element prefers to be stable, its valence shell should have eight electrons
(except the first shell, which can only accommodate two electrons). This stability is achieved by
accepting, donating or sharing some or all valence electrons.
The number of electrons that an atom needs to accept, donate or share so as to form a complete
outer octet (8 electrons) (except the first shell that forms a duplet) is called valency. In other words,
valency is the combining capacity of an atom of an element.
Why do atoms of elements combine or react?
Atoms of elements combine or react to attain the electronic
configuration of the nearest noble or inert gas and thus, become
stable. This theory was proposed by a German chemist, Kossel.
Elements tend to combine in such a way that each element has
eight electrons in its valence shell (except hydrogen, lithium
and beryllium). This theory is commonly called the octet rule.
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Thirst for Knowledge
Kossel was born on 4 January, 1888
in Berlin, Germany. In 1916, he put
forth his theory of the ionic chemical
bond (octet rule). Octet rule was also
independently put forth by Gilbert N
Lewis in the same year.
In general, the valency of:
(i) metals is the same as the number of electrons in the valence shell. For example, the valency
of sodium is 1, with 1 valence electron and that of magnesium is 2, with 2 valence electrons.
(ii) non-metals is determined as (8 – the valence electron(s)). For example, the valency of nitrogen
is (8 – 5) = 3, oxygen is (8 – 6) = 2 and that of chlorine is (8 – 7) = 1.
Elements with valency one are called monovalent elements, for example, sodium and chlorine.
Elements with valency two are called divalent elements, for example, magnesium and calcium.
Elements with valency three are called trivalent elements, for example, lithium and phosphorus.
When an atom donates electrons to acquire a stable electronic configuration, it becomes a positivelycharged ion and is said to have positive (electropositive) valency. For example,
• Sodium (Na): It donates 1 electron and becomes sodium ion (Na+).
• Magnesium (Mg): It donates 2 electrons and becomes
magnesium ion (Mg2+).
• Aluminium (Al): It donates 3 electrons and becomes
aluminium ion (Al3+).
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All the metals and hydrogen have
positive valency, since their valence
shells have 1 to 3 electrons.
When an atom accepts electrons to acquire a stable electronic configuration, it becomes a negativelycharged ion and is said to have negative (electronegative) valency. For example,
(i) Chlorine (Cl): It accepts 1 electron and becomes chlorine ion (Cl–).
(ii) Oxygen (O): It accepts 2 electrons and becomes oxygen ion (O2–).
(iii) Nitrogen (N) : It accepts 3 electrons and becomes nitrogen ion (N3–).
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The bond formed between two atoms when they combine is called chemical bond. Atoms can combine either by transferring
electrons or by sharing electrons. The chemical bond formed by the complete transfer of electrons from one atom to the
other is called electrovalent or ionic bond and the compounds so formed are called electrovalent or ionic compounds.
The chemical bond formed by the sharing of electrons is covalent bond and the compounds so formed are called covalent
compounds.
Let’s see an example of formation of an electrovalent and a covalent compound.
Electrovalent compound
Sodium chloride: During the formation of sodium chloride, the sodium atom (2,8,1) loses an electron and becomes sodium
ion (Na+) and the chloride atom (2,8,7) accepts the electron given by sodium and becomes chloride ion (Cl–). Since sodium
ion and chloride ion are oppositely charged ions, they attract each other and form sodium chloride.
11p
12n
17p
18n
11p
12n
Na+(2,8) +
e–
sodium ion
electron
Na (2,8,1)
sodium atom
Na+ + Cl– Covalent compound
17p
18n
Cl (2,8,7)
chloride atom
+
NaCl
e–
electron
Cl– (2,8,8)
chloride ion
Oxygen molecule: Oxygen atom (2, 6) combines with another oxygen atom by sharing two electrons to form oxygen
molecule.
O
O
oxygen atom (2,6)
+
oxygen atom (2, 6)
O
oxygen molecule
O
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Variable Valency
Some elements show more than one valency. Such elements are said to have variable valency. For
example, copper, iron, silver, lead, tin and mercury have variable valencies. Table 1.5 shows some
metals which have variable valencies.
Table 1.5: Metals with variable valency
Element
Copper
Iron
Silver
Symbol
Cu
Fe
Ag
Lower valency
Cu+ or Cu(I), Cuprous
Cu2+ or Cu(II), Cupric
Ag+ or Ag(I), Argentous
Ag2+ or Ag(II), Argentic
Fe2+ or Fe(II), Ferrous
Lead
Pb
Pb2+ or Pb(II), Plumbous
Mercury
Hg
Hg+ or Hg (I), Mercurous
Tin
Radicals
Sn
Higher valency
Sn2+ or Sn(II), Stannous
Fe3+ or Fe(III), Ferric
Pb4+ or Pb(IV), Plumbic
Sn4+ or Sn (IV), Stannic
Hg2+ or Hg(II), Mercuric
Two or more non-metals that collectively accept or donate one or more electrons and become
+
negatively or positively-charged in the process are called radicals. For example, ammonium (NH 4 )
2–
3–
(valency 1), carbonate (CO 3 ) (valency 2) and phosphate (PO 4 ) (valency 3). Radicals occur in compound
form and behave as a single unit.
Take a Break!
1. Name the sub-atomic particles of an atom.
2. The extra-nuclear region of an atom consists of electrons. (True/False)
3. Atomic number is the number of neutrons present inside the nucleus of an atom. (True/
False)
4. The sum total of ___________ and ___________ is known as mass number of an element.
5. If the atomic number of nitrogen is 7, then the number of protons present in it will also be
7. (True/False)
6. If the mass number of aluminium is 27 and its atomic number is 13, then the number of
neutrons present in it would be _____________.
7. The _____________ of an element have same atomic number.
8. The outermost shell of an atom can accommodate _____________ electrons.
9. Oxygen donates two electrons and becomes oxygen ion. (True/False)
10. Ammonium is an example of a radical. (True/False)
RADIOACTIVITY
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In 1896, French scientist Henri Becquerel happened to leave a photographic plate covered with black
paper along with uranium sample in a drawer. He was highly surprised when he observed that the
plate got darkened even after wrapping it carefully. Obviously, it was due to some light rays that
must have penetrated through the black paper and reached the
photographic plate. Later he found out that uranium sample
was the source of invisible rays which have properties similar
to X-rays. These new rays were said to be radiation active or
radioactive rays.
Thirst for Knowledge
X­rays were discovered by Wilhelm
Conrad Röntgen in 1895. X­rays have
the power of passing through matter
that is opaque to ordinary light and to
affect photographic plates.
The phenomenon during which the nucleus of certain elements
emits radioactive rays is known as radioactivity. In this process, tremendous amount of energy is
released.
The substances that give out radioactive rays are called radioactive substances.
Rutherford studied the nature of radioactive rays further. He found that there are three kinds of
radiation—alpha (a) rays, beta (b) rays and gamma (g) rays.
Causes of Radioactivity
The cause of radioactivity is the disintegration of the nucleus
of an atom. We know that nucleus of an atom contains protons
and neutrons, collectively known as nucleons. These nucleons
are acted upon by two types of forces:
(i) the force of electrostatic repulsion between positivelycharged protons which tends to move them apart.
Thirst for Knowledge
• There is no known method of
stopping, starting, accelerating or
decelerating radioactivity.
• The nuclei of the atoms of all
elements with atomic number
greater than 83 are radioactive.
(ii) the nuclear force of attraction between protons and neutrons which tends to bind them
together.
These two forces balance each other, thereby making the nucleus of an atom stable. When under
certain conditions, this balance gets disturbed, the nucleus becomes unstable and disintegrates into
smaller fragments along with emission of radioactive radiation. An enormously large amount of
energy is released during this process.
Nuclear Energy
Nucleus is the storehouse of huge amount of energy. This energy is known as nuclear energy. This
stored energy can be released by two processes—nuclear fission and nuclear fusion.
Nuclear fission
Certain large atoms have unstable nucleus. Their nucleus can split into two smaller nuclei either
spontaneously or when bombarded with particles like neutrons. Enormous amount of energy is
released during this process. This process is called nuclear fission. In other words, nuclear fission
is the process of splitting of an atomic nucleus into two or more smaller nuclei with release of large
amount of energy.
For example, let’s consider the fission of nucleus of Uranium-235. When a neutron hits the nucleus of
Uranium-235, it splits into barium (Ba) atom and krypton (Kr) atom. It is accompanied by simultaneous
release of huge amount of energy and three neutrons.
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The three neutrons released in this process bombard three more
nuclei, each releasing three more neutrons along with energy.
This continuously growing reaction is called chain reaction. If
this reaction is not controlled it can lead to great explosion.
Thirst for Knowledge
An atom bomb is based on the principle
of uncontrolled nuclear fission.
Thirst for Knowledge
• In August 1945, at the end of World War II, atom bombs were dropped
on Hiroshima and Nagasaki in Japan. The atom bombs were based
on nuclear fission of uranium and plutonium and had far­reaching
devastating implications. It resulted in a loss of more than 1,40,000
lives. Birth defects in new born children continued to take place even
decades after the bombing.
• In 1986, the Chernobyl Nuclear Power Plant in Ukraine exploded. The
accident occurred due to a flaw in the nuclear reactor design and due
to human error. The leakage from Chernobyl reached to several parts
in western Soviet Union, eastern Europe and USA. Several casualties
took place.
Hiroshima and Nagasaki
Nuclear fusion
Fusion means to fuse or to join together. The process in
which two lighter nuclei combine (or fuse) together at
extremely high temperature to form a heavier nucleus
accompanied with the release of huge amount of energy
is called nuclear fusion.
For example, at extremely high temperature two nuclei
of deuterium (21H) (an isotope of hydrogen) fuse together
to form a nucleus of helium. In this process, lot of energy
is evolved.
2
H
1
deuterium
+
2
H
1
deuterium
4
He
2
D
energy
fusion
D
+ Energy
helium
The principal source of energy of the sun is the nuclear fusion
reaction. Several fusion reactions keep on taking place all the
time inside the sun.
He
Fig. 1.10: Nuclear fusion
Thirst for Knowledge
The hydrogen bomb is based on the
principle of nuclear fusion as well as
nuclear fission reaction.
Nuclear fusion has not been used to generate power because it takes place only at extremely high
temperature and pressure.
Uses of Radioactivity
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• Gamma radiation from Cobalt-60 (a radioactive material) is used to check leakage in oil, gas
and water pipelines, whether on land or under water.
• Cobalt-60 is used in the treatment of cancer (the treatment is termed as radiotherapy).
Iodine-131 is used to study and treat disorders of thyroid gland. Beta rays from Sodium-24
are used to examine blood circulation. Beta radiation from Phosphorus-30 is used to treat
leukemia.
• Exposure of gamma rays to all kinds of food stuff helps to sterilize and preserve the food stuff
from bacteria and insects. There is no danger to humans from this irradiated food.
• Controlled radiation is used to determine the age of rocks, pre-historic sites, fossils, dead
plants and animals and mineral deposits. This technique is called radiocarbon dating or
radioisotopic dating.
• Controlled radiation is also used to destroy bacterial diseases in fruits and vegetables.
• Controlled nuclear fission is used in nuclear power plants to generate electricity.
Harmful Effects of Radiation
• Nuclear radiation can easily penetrate through all kinds of living and non-living things.
• Radiation destroys living tissues, cells and blood corpuscles.
• Radiation causes harmful gene mutation, i.e., altering the genetic structures, leading to
different kinds of cancer and genetic disorders.
• Radiation interferes with cell division, leads to the birth of deformed babies and even kills
healthy cells.
• Exposure to harmful radiation leads to infertility and loss of hair.
Thirst for Knowledge
Artificial or Induced Radioactivity
Only a few elements are radioactive by nature. However, a few elements which are stable by nature, become radioactive
when they are bombarded with high speed neutrons.
The phenomenon during which a non­radioactive element changes to a radioactive one, when it is bombarded with a high
speed neutron is called artificial or induced radioactivity. The radioactive elements so obtained are called radioisotopes.
C­14, Na­24, P­32, Co­60 and I­131 are radioisotopes.
Nuclear Power Plants
Nuclear power plants are set up to generate electricity, where nuclear fission reaction is carried out
in a controlled manner. The heat energy produced during the nuclear fission reaction is converted
into electrical energy, so as to generate electricity.
steam generator
steam
steam
turbine
generator
liquid water
control rod
cooling tower
nuclear reactor
air
reactor core
air
water
condenser
pump
Fig. 1.11: Diagrammatic representation of nuclear reactor
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A nuclear power plant consists of:
(i) a nuclear reactor where nuclear fission is carried out and
(ii) a turbine and a generator for converting nuclear energy
into mechanical energy and then electrical energy
respectively.
Thirst for Knowledge
Some of the nuclear power plants
functioning in India are Tarapur, Kota,
Narora, etc.
A nuclear reactor needs the following things to carry out nuclear reaction in a controlled manner:
(i) Fuel: Uranium-235 or Plutonium-239 is used as fuel.
(ii) Moderator: A moderator is used to slow down the fast moving neutrons, since slow moving
neutrons are needed for the nuclear fission reaction in a nuclear power plant. Fast moving
neutrons slow down on striking the moderator. Heavy water (deuterium oxide) or rods of
graphite (12C ) are used as moderators.
(iii) Control rods: Control rods help to absorb neutrons so that only one neutron per fission is
available for carrying out the chain reaction. Boron steel or cadmium rods are used as control
rods.
(iv) Coolant: A coolant is required as a huge amount of heat energy is released during the nuclear
reaction. Heavy water and liquid sodium are used as coolants.
Safety requirements at nuclear power plant
• Nuclear power plants should be situated away from densely populated areas.
• The nuclear reactor must be enclosed in thick shield of concrete walls or steel or lead chamber
to ensure that the radioactive fuel or radioactive rays do not leak.
• Nuclear materials should be kept away from each other, in different containers so that there
is no explosion.
• All workers in nuclear power plants should wear lead-lined aprons and gloves and special lead
glasses to protect themselves from radiation.
• All workers must undergo periodic medical check-ups to ensure that they have not been
affected by radiation.
• Nuclear power plants should be made resistant to earthquakes.
Take a Break!
1. Name the scientist who discovered the phenomenon of radioactivity.
2. There are three kinds of radiation. (True/False)
3. The principle source of energy of the sun is the nuclear _____________ reaction.
4. Gamma radiation from Phosphorus-30 is used to check leakage in oil, gas and water
pipelines, whether on land or under water. (True/False)
5. Exposure to harmful radiation leads to ______________ and loss of hair.
6. In a nuclear reactor, ______________ and ______________ are used as coolants.
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VOCABULARY
• Discharge tube: A cylindrical glass tube with sealed electrodes at each end
• Cathode: A metal plate connected to the negative terminal of the voltage source
• Anode: A metal plate connected to the positive terminal of the voltage source
• Cathode rays: A stream of rays originated from the cathode
• Electrons: Negatively-charged particles that revolve around the nucleus in fixed energy shells
• Anode rays: A stream of rays originated from the anode
• Protons: Positively-charged particles that are found inside the nucleus
• Neutrons: Electrically-neutral particles that are found inside the nucleus
• Nucleus: The positively-charged central part of an atom containing the protons and neutrons
• Atomic number: The number of protons present inside the nucleus of an atom
• Mass number: The sum total of the number of protons and the number of neutrons present in
the nucleus of an atom of an element
• Relative atomic mass: The number of times by which the average mass of one atom of an
element is heavier than 1/12th of the mass of a carbon atom
• Isotopes: Atoms of the same element having same atomic number but different mass number
• Electronic configuration: The arrangement of electrons in different orbits or shells around the
nucleus of an atom
• Valence shell: The outermost shell of an atom
• Valence electrons: The electrons present in the valence shell of an atom
• Valency: The number of electrons that an atom needs to accept, donate or share so as to form
a complete outer octet
• Monovalent elements: Elements with valency one
• Divalent elements: Elements with valency two
• Trivalent elements: Elements with valency three
• Radicals: Two or more non-metals that collectively accept or donate one or more electrons and
become negatively or positively-charged
• Radioactivity: The phenomenon during which the nucleus of certain elements emits radioactive
rays
• Radioactive substances: The substances that give out radioactive rays
• Nuclear fission: The process of splitting of an atomic nucleus into two or more smaller nuclei
with release of large amount of energy
• Nuclear fusion: The process in which two lighter nuclei combine (or fuse) together at extremely
high temperature to form a heavier nucleus accompanied with a release of huge amount of
energy
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• Radiocarbon dating or radioisotopic dating: The technique of using controlled radiation to
determine the age of rocks, pre-historic sites, fossils, dead plants and animals and mineral
deposits
SUMMARY
• Around the turn of the 20th century, it was discovered that an atom was actually a cluster of
various sub-atomic particles (mainly electrons, protons and neutrons) which can exist separately
from each other.
• John Dalton proposed that an atom is solid, indestructible and indivisible sphere.
• In 1897, the English scientist Sir J.J. Thomson proved that an atom can be split into even
smaller parts. His discovery of the electron was the first step towards a detailed model of the
atom.
• J.J. Thomson found that the cathode rays consisted of negatively-charged particles. These
particles were named as electrons.
• E. Goldstein performed an experiment in the discharge tube with a perforated cathode and
found that the anode rays consisted of positively-charged particles called protons.
• In 1911, Ernest Rutherford carried out gold-foil experiment and found that most of the mass of
an atom was located in the centre of the atom called nucleus.
• In 1932, James Chadwick discovered that atoms have neutrons as parts of their structure.
• According to the modern concept of the atom, there are three sub-atomic particles—electrons,
protons and neutrons. Protons and neutrons together form the nucleus, while electrons revolve
around the nucleus in definite orbits or shells.
• Electrons are distributed in different energy levels or shells according to Bohr-Bury scheme of
electronic configuration.
• Atoms of element combine or react to attain the electronic configuration of the nearest noble
or inert gas and thus become stable.
• Radioactive radiation are of three kinds—alpha (α) rays, beta (β) rays and gamma (γ) rays.
• When Uranium-235 is bombarded with a neutron, it splits up into two stable nuclei—barium
and krypton and three free neutrons with the release of a huge amount of nuclear energy.
• At extremely high temperature, two nuclei of deuterium (21H) fuse together to form a nucleus of
helium.
• Nuclear power plants are set up to generate electricity, where nuclear fission reaction is carried
out in a controlled manner.
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EXERCISES
A. Answer the following questions.
1. What was the ancient view on atomic structure?
2. Give the postulates of Dalton’s atomic theory.
3. How was electron discovered?
4. List the characteristics of cathode rays.
5. Give the characteristics of electrons.
6. Write a note on the discovery of proton.
7. List the characteristics of anode rays.
8. Give the characteristics of protons.
9. Discuss the Rutherford’s model of an atom.
10. Give the characteristics of neutrons.
11. Describe the modern concept of an atom.
12. Explain atomic number and mass number.
Ar.
13. Calculate the number of electrons, protons and neutrons in 40
13
14. What do you mean by relative atomic mass?
15. What are isotopes? Give examples and list properties of isotopes.
16. What is electronic configuration?
17. Discuss Bohr-Bury scheme of electronic configuration.
18. Explain what are valence shell, valence electrons and valency.
19. Why do atoms of elements combine or react?
20. What is the valency of the following elements?
(a) Boron
(b) Fluorine
(c) Calcium
(d) Potassium
(e) Sulphur
21. What is radioactivity? Discuss the causes of radioactivity.
22. Discuss a few uses of radioactivity.
23. Explain nuclear fission. Give an example.
24. What is nuclear fusion? Support your answer with an example.
25. With the help of a diagram, describe the structure of a nuclear reactor.
26. Discuss the safety requirements at a nuclear power plant.
B.
Fill in the blanks.
1. Maharishi Kanada believed that an ____________ may be made up of two or more subatomic
particles called ____________.
2. ____________ rays consisted of negatively-charged particles called electrons.
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3. ____________ performed an experiment in the discharge tube, with a perforated cathode.
4. Neutron was discovered by ____________.
5. Nucleons consist of ____________ and ____________.
6. ____________ have same atomic number but different mass number.
7. Radioactivity was discovered by ____________.
8. The principle source of energy of the sun is the nuclear ____________ reaction.
C.
State whether the following statements are True or False. Correct the false statements.
1. J.J. Thomson proved that atom can be split into even smaller parts.
2. From the gold foil experiment, Rutherford concluded that most of the space in an atom is
occupied by nucleus.
3. A neutron is represented by 0n1.
4. An atom is electrically neutral as it contains equal number of electrons and protons.
5. Uranium and plutonium are used as coolants.
D.
Choose the correct answer for each of the following.
1. An atom consists of
(a) electrons.
(b) protons.
(c) neutrons.
(d) all of them.
2. Cathode rays
(a) travel in straight line.
(b) are produced from the anode in a discharge tube.
(c) consists of positively-charged particles.
(d) all of them.
3. Which of the following scientists suggested that electrons possess a specific amount of energy
and are arranged in a succession of energy levels?
(a) J.J. Thomson
(b) James Chadwick
(c) Niels Bohr
(d) E. Rutherford
4. Henri Becquerel discovered
(a) electrons.
(b) protons.
(c) neutrons.
(d) radioactivity.
22
5. Which of the following does not exhibit variable valency?
(a) Copper
(b) Iron
(c) Phosphorus
(d) Lead
6. Which of the following elements is used in the treatment of cancer?
(a) Phosphorus-30
(b) Cobalt-60
(c) Uranium-235
(d) Iodine-131
E.
Match the columns.
Column A
F.
Column B
Column C
1. J.J. Thomson
(a) Gold foil experiment
(i) Discovery of protons
2. Nuclear fission
(b) Generate electricity
(ii) Atom bomb
3. E. Goldstein
(c) Cathode rays
(iii) Principle source of energy
of the sun
4. Nuclear fusion
(d) Breaking up of heavy nucleus
when bombarded with slow
moving neutrons
(iv) Discovery of nucleus
5. Ernest Rutherford
(e) Anode rays
(v) Controlled nuclear fission
reaction
6. Nuclear power plant
(f) Joining of two lighter nuclei
at extremely high temperature
and pressure
(vi) Discovery of electrons
Fun Activity
Unscramble the following words.
1. ECRNETOLS
______________________________
2. NCNOESLU
______________________________
3. SDITECEARHBUG
______________________________
4. ODRRMOTEA
______________________________
5. CIARALDS
______________________________
6. TAMCIO BRUNEM
______________________________
7. NERACUL UOSIFN
______________________________
8. CAIDORBOANR NIGADT
______________________________
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