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Atom
STRUCTURE OF THE ATOM
Atom is the smallest particle of an element .It is made up of
three fundamental particles: Electrons -Negatively charged
Protons -Positively charged
Neutrons -No charge (Neutral)
Now we will study how these particles were discovered.
Different scientists did different experiments and discovered
these particles.
Discovery Of Electrons
J.J. Thomson in 1897 discovered electrons in an atom. This
was done through a discharge tube experiment by passing
electricity at very high voltage through a gas at very low
pressure to break the atoms of gas into smaller particles.
The invisible rays, which, formed at cathode during the
cathode ray experiment and emitted greenish-yellow
light, were called cathode rays.
Properties of Cathode Rays
1.
2.
3.
4.
5.
Cathode rays travel in straight line. It is due to the fact
that cathode rays cast shadows of the objects placed in
their path.
Cathode rays can produce mechanical effects .It is due to
the fact that they can rotate a light paddle wheel if placed
in discharged tube.
Cathode rays are negatively charged. This is due to the
fact that cathode rays are deflected towards the positive
plate when an electric field is applied in the path of
cathode rays.
The nature of cathode rays does not depend on the
nature of gas taken in the discharge tube or the material
of cathode.
The mass of cathode ray particle is very small as
compared to mass of atom from which it is formed.
Electrons
Discovery Of Proton
E. Goldstein in 1886 discovered canal rays or positive rays and
protons in an atom. This was done through a discharge tube
experiment with some different equipments and methods.
Passing electricity at very high voltage through a gas at very
low pressure to break the atoms of gas into smaller particles
A stream of positively charged particle shot out from
anode of a discharge tube when a current is passed
through a gas at very low pressure are called anode
rays.
Properties Of Anode Rays
1.
2.
3.
4.
5.
Anode rays travel in straight lines as they cast shadows of
the objects placed in their path.
Anode rays can produce mechanical effect as they can
rotate a light paddle when placed in their path.
Anode rays are positively charged as they are deflected
towards negative plates when an electric field is applied.
The nature of anode rays depends on the gas taken in the
discharge tube.
The mass of anode ray particle is almost equal to the
mass of the atom from which it is formed.
Protons
The proton is positively charged particle found in the atoms of
all elements. The protons are located in the nucleus of an
atom.
Mass Of Protons
Relative mass = 1amu or 1840 times to that of an electron
Absolute mass = 1.6 × 10 −24 grams
Charge Of Proton
Relative Charge = +1
Absolute Charge = 1.6 × 10 −19 C
The electron is a negatively charged particle found in the atom
of all elements. Electrons are located outside the nucleus in an
atom.
Neutron
Mass of Electron
How neutrons were discovered?
1
Relative mass of electron =
of mass of hydrogen atom
1840
Absolute mass of electron = 9 × 10 −28 grams
Charge of Electron
Relative charge of electron = −1
Absolute charge = 1.6 × 10 −19 coulomb
Because electron has smallest negative charge of 1 unit, that
is why it is taken as unit of negative charge.
The neutron is a neutral particle found in the nucleus of an
atom.
After the discovery of protons and electrons the mass of the
atom was started measured on their basis. But the problem
was that with this method wrong atomic masses were
obtained. For example an oxygen atom contains 8 protons and
8 electrons. Now the mass of electron is negligible. So the
atomic mass of oxygen should be only 8, which is the mass of
protons. This is wrong because the actual mass of carbon is
16. Then a scientist named Chadwick discovered that there is
another fundamental particle in an atom, which is called
neutron. This particle is bearing rest of the mass of the atom.
Mass Of Neutron
Relative mass = 1 amu
Absolute mass = mass of proton
Charge Of Neutron
There is no charge on neutron.
1.
2.
Atomic Nucleus
To show the structure of an atom various models were given
by various scientists. These models show the presence and
location of the three particles present in it with the presence of
nucleus also. Now we will discuss some of the models of atom
one by one.
JJ Thomson’s Model Of An Atom
This model is also called Plum Pudding Model or Apple Pie
Model. JJ Thomson gave the first model of the atom. In this
model electrons were uniformly distributed in the entire atom.
The mass of the atom was supposed to be uniformly
distributed.
Thomson Proposed That
(i) An atom consists of a positively charged sphere and the
electrons are embedded in it.
(ii) The negative and positive charges are equal in
magnitude. So the atom as a whole is electrically neutral.
3.
Most of the alpha-particles pass straight without any
deflection indicating that most of the space inside the
atom is empty..
A few alpha-particles are deflected through small angles
and some are deflected through large angles indicating
that the positive charge of the atom occupies very small
space.
A very few alpha-particles turn back completely indicating
that all the positive charge and mass of the gold atom
were concentrated ina very small volume within the atom.
The Conclusions Made Were
1. There is a lot of empty space in an atom as most of the
alpha-particles passed straight.
2. There is a center of positive charge in atom, which repels
positively charged particles and deflects their path.
3. This center of positive charge in an atom is known as
nucleus.
4. The nucleus is very dense and hard which does not allow
alpha-particles to pass through them that are why a few
alpha-particles completely turned back.
Rutherford’s Structure Of An Atom
(i) There is a positively charged centre in an atom called the
nucleus. Nearly all the mass of an atom resides in the
nucleus.
(ii) The electrons revolve around the nucleus in well defined
orbits.
(iii) The size of the nucleus is very small as compared to the
size of the atom.
Drawback Of Rutherford’s Atomic Model Of An Atom
Rutherford Model Of An Atom
Rutherford did an experiment called Alpha Particle
Scattering or Gold Foil experiment. This experiment was
done by striking alpha particles (Helium ions with charge of +2
& mass of 4) on thin gold foil.
It was supposed that when electrons will revolve in a high
speed around the nucleus they would loose energy. When
energy of electrons will decrease their speed will also be
decreased. Due to this the electrons will be attracted towards
nucleus more strongly. By coming more and more close to the
nucleus following a spiral path ultimately they will fall into the
nucleus. Such an atom is not stable.
The Bohr’s Atomic Model
According to Bohr’s atomic model the electrons revolve around
the nucleus in particular orbits having fixed energy levels.
When electrons revolve in fixed orbits they do not loose
energy and in turn do not fall into the nucleus and that is why
they are stable.
The number of electron shells (energy levels) in which
electrons revolve is also fixed. The nucleus of an atom consists
of protons and neutrons and is positively charged. An atom
remains neutral due the presence of equal number of protons
(positive charge) and electrons (negative charge).
The Orbits, Shells, Energy Levels In An Atom
Observations were: -
There are six energy levels in an atom, which are denoted
by the numbers 1,2,3,4,5,6.
There are four shells which are denoted by K, L, M and N.
Example
4
2 He
12
6C
16
8O
Valence Electrons And Valency
Valence Electrons
The electrons, which are present in the outermost shell of an
atom, are called valence electrons. Only the valence electrons
take part in chemical reactions.
Valency
So it is concluded that: 1. No. of energy levels = No. of shells = No. of Orbits
2. The orbit nearest to the nucleus has the minimum energy
and the orbit farthest from the nucleus has the maximum
energy.
Rules To Be Followed For Writing The Number Of
Electrons In Different Energy Levels Or Shells.
(iv) The maximum number of electrons present in a shell is
given by the formula 2n2, where n is the orbit number.
Hence the maximum number of electrons in different
shells are as follows: -
Valency is number of electrons, which are loosed, gained or
shared by the atom of an element in order to become stable.
In other words, valency is the combining capacity of an
element with another element or compound. Valency is
determined by valence electrons.
The combining capacity of the atoms of other elements that is
their tendency to react and form molecules with the atoms of
same or other elements, is thus explained as an attempt to
attain a fully filled outermost shell.
To make an element stable the outermost shell of its atom
should be filled completely by the maximum number of
electrons, which can be accommodated in that shell. Three
things to be considered are: 1. If the maximum capacity is 8 and there are less than 4
electrons in the outermost shell, then atom loses these
electrons to become stable. Because less energy is
required in losing less than 4 electrons than to gain more
than 4 electrons to remove incomplete shell.
Example
For the first energy level
n=1
number of electrons = 2 × (1)2 = 2 × 1 = 2
Example
Sodium’s atomic number = 11
Sodium’s electronic configuration = 2, 8, 1
Now it has to lose this electron to become stable. Because
losing 1 electron is easier than to gain 7 more electrons
needed to become stable.
As it is losing its 1 electron in from outer most shell, its
valancy is 1.
For the second energy level
n=2
number of electrons = 2 × (2)2 = 2 × 4 = 8
With this, we can find the maximum number of electrons in an
electron shell as follows: Electron Shell
K Shell
L Shell
M Shell
N Shell
2.
Maximum Number Of Electrons
=
2
2 × (1)2 = 2 × 1
2 × (2)2 = 2 × 4
=
8
2 × (3)2 = 2 × 9
=
18
32
2 × (4)2 = 2 × 16 =
Example
Sulphur’s atomic number = 16
Sulphur’s electronic configuration = 2, 8, 6
Now it has to lose this electron to become stable. Because
gaining 2 electrons is easier than to lose 6 electrons to
become stable.
As it is gaining 2 electron in outer most shell, so its
valency is 2.
(ii) The maximum number of electrons that can be
accommodated in each shell or orbit is 8.
(iii) Electrons are not accommodated in a given shell, unless
the inner shells are filled. That is the shells are filled in a
step wise manner.
Atomic Number
The number of protons, which are equal to number of
electrons, is called atomic number. It is denoted by Z.
Example
Z = 1 for hydrogen
=> 1 electron & 1 proton
Z = 6 for carbon
=> 6 electrons & 6 protons
Mass Number
The total number of protons and neutrons present nucleus of
an atom is known as mass number.
An element is represented as
Mass Number
Symbol
Atomic Number
If the maximum capacity is 8 and there are more than 4
electrons in the outermost shell, then atom gains
electrons to become stable. Because less energy is
required in gaining less than 4 electrons than to lose more
than 4 electrons to fill the maximum capacity.
3.
If the maximum capacity is 8 and there are 4
electrons in the outermost shell, then atom can
gain or loose electrons to become stable.
As it can loose or gain 4 electrons from outer
most shell, its valancy is 4.
Metals And Non-metals
1.
Elements having 1, 2, or 3 valence electrons in
their atoms are metals, except hydrogen and
helium. Though, hydrogen has 1 and helium has
2 valence electrons in their outer most shells, but
both are non-metals.
2.
Elements having 4, 5, 6 or 7 valence electrons in
their atoms are non-metals, e.g. carbon,
nitrogen.
Isotopes
Isotopes are the atoms of the same element having same
atomic numbers but different mass numbers or atomic
masses. All the isotopes of an element have same chemical
properties because they have same number of valence
electrons. Atomic number of the isotopes is same because
number of protons and electrons are same in all atoms. Mass
number is different because the number of neutrons is
different in atoms of the same elements. The isotopes of the
elements are having same chemical properties but different
physical properties
Example
Carbon has two isotopes
C-12
C-14
12
6C
14
6C
In both atoms of carbon above, atomic number, number of
protons and number of electrons are same. But mass number
differs because number of neutrons is different.
Number of neutrons in C-12 = 12 – 6 = 6 neutrons Number
of neutrons in C-14 = 12 – 6 = 8 neutrons
Oxygen has three isotopes
16
8O
17
8O
18
8O
Atomic number = 8
Atomic masses = 16, 17, 18
Having 8, 9, 10 neutrons respectively
Why some elements are having fractional atomic
masses?
The mass of an atom of any naturally occurring element is
taken as the average mass of all the naturally occurring atoms
of that element. If an element has no isotopes, then the mass
of its atom would be the same as the sum of protons and
neutrons in it. But if an element occurs in isotopic forms, then
we have to know the percentage of each isotopic form and
then average mass is calculated. That is why the average
atomic mass of chlorine atom having two isotopes 35Cl 17 , 37Cl 17
is 35.5 u. This does not mean that one atom of chlorine has
fractional mass 35.5 u. It means that if you take a certain
amount of chlorine, it will contain both isotopes of chlorine and
the average mass is 35.5u.
Uses of Isotopes
1. Radioactive isotopes like Arsenic 74, Sodium 24 are used
as ‘tracers’ in medicine to defeat the presence of tumors
and blood clots in human body.
2. Radioactive isotopes like Cobalt -60 are also used for the
treatment of Cancer.
3. Radioactive isotopes like U-233. U-238 can undergo
fission process for the production of energy.
4. Radioactive isotopes are used in industry to detect the
leakage in underground pipeline, gas pipes and water
pipes.
5. Radioactive isotopes like C-14 are used to do carbondating to measure the age of dead plants and animals
(fossils).
Isobars
Atoms of different elements with different atomic numbers but
same atomic mass are called isobars.
Example
Let us consider two elements calcium having atomic number
20 and argon having atomic number 18 but both have the
same atomic mass as 40. It means that the total number of
nucleons is same in the atoms of this pair of elements and so
both are called as isobars.