Download AJAY PARMAR GROUP TUITION

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AJAY PARMAR
GROUP TUITION
Chapter – 5, Structure of atom
Some important definitions:
Orbit:
The atomic level at a definite distance from nucleus is called energy level of electron or
an orbit
Orbital:
The sub orbit of an orbit is called orbital.
Stationary orbit:
The orbits in which electrons revolve around nucleus without losing energy are called
stationery orbit
Atomic number:
The number of protons or electrons in the nucleus of that atom is called atomic number of
that atom.
Atomic mass:
The total number of protons and neutrons present in its nucleus is called atomic mass of
that atom.
Valence electrons: The electrons in the outermost orbit of an atom are called valence electrons of that atom
Valency:
The number of electrons in the outermost orbit is also called valency that an atom
Isotopes:
The atoms possessing same number of protons but different number of neutrons i.e. same
atomic number but different atomic mass are called isotopes.
Radioactivity:
The property by which some elements of high atomic mass radiates ,  and 
rays is called radioactivity.
1. What is indicated by hydrogen and sodium spectrum?
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The study of hydrogen and sodium spectrum suggests that hydrogen atom contains one electron while
sodium atom has more than one electron.
2. Write in brief about the production of cathode rays and its properties.
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+ High Voltage
G
A
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B
E
P
A thick walled electric discharge glass tube containing two metallic electrodes A and B is taken.
Vacuum is created in this discharge tube with the help of vacuum pump P.
Electrode A acts as cathode and electrode B acts as anode respectively.
When they are connected to high voltage, the cathode rays are produced from cathode A.
They move towards the anode with speed.
Scientist J. J. Thomson and Crookes proved that the cathode rays are really the flow of electrons having
negative charge.
Properties of Cathode rays:
They travel in straight line.
As they are the flow of electrons they are negatively charged.
They are deviated under the effect of electromagnetic field.
They cast shadow of the objects kept in their path.
When they strike the wall of the tube or anticathode, they produce X – rays.
3. What are cathode rays?
 The flow of electrons, produced from cathode of a vacuum discharge tube when its electrodes are connected to high
voltage, is called cathode rays.
4. What effect is experienced by cathode rays in electromagnetic field?
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The cathode rays are deviated in the electromagnetic field.
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GROUP TUITION
5. Mention the production of X-rays, its properties and uses.
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Production of X rays
[How X-rays can be produced from flow of electrons?]
Scientist Rontgen found that when cathode rays strike the wall of the tube or anticathode, new types of rays
were produced from them. He called these rays as X – rays or Roentgen rays.
Properties of X – rays [Write properties of X-rays.]
They are electromagnetic waves with extremely short wavelength.
They travel in a straight line like rays of light.
They do not deflect in electromagnetic field.
They can penetrate through opaque material like black paper.
They can also affect photographic plates placed in opaque material.
Uses of X– rays
[Write uses of X-rays.]
to detect a fracture in the bone,
to observe defect in lungs,
to diagnose cancer of oesophagus,
to diagonise physical disabilities,
to find in joints of metal industries, etc
6. What type of rays are obtained from uranium metal?
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 rays,  rays and  rays are obtained from uranium metal.
 rays possess positive charge and  rays possess negative charge.
Any radioactive substance, when kept in a cavity of lead block, emits radioactive rays i.e.  rays,  rays and
 rays in all the directions.
In the presence of electromagnetic field  rays are deflected towards S pole. Hence they must be positively
charged.
 rays are deflected towards N pole. Hence they must be negatively charged.
 rays
 rays
 rays
Magnetic
field
Radioactive
substance
Lead Vessel
7. How John Dalton’s conclusion that atoms are indivisible proved to be wrong?
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Some radioactive minerals emit ,  and  rays, which can pass through opaque material like paper.
These rays must be emitting from atoms.
Thus atom can be split and is not indivisible as thought earlier.
8. Give difference between cathode rays and  – rays?
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Cathode rays are negatively charged while  rays are positively charged.
Cathode rays are attracted towards anode in an electromagnetic field while  rays are attracted towards
cathode of the electromagnetic field.
X – rays are produced form cathode rays only and not from  rays.
9. Why Thomson's atomic model was not accepted?
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Thomson’s atomic model:
An atom is like a spherical ball and the positive electric charge is uniformly spread on its total volume.
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GROUP TUITION
Protons and the electrons are arranged at definite places.
If this would have been true then due to attraction of positive and negative electric charges they would have
mixed with each other and become charge less.
Also this arrangement could not explain the different chemical properties of different elements.
Thus, this model was not accepted.
Electron
Atom Nucleus
10. Explain Rutherford's experiment in brief and give the results.
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Rutherford’s experiment:
Experiments of Ernest Rutherford
Rutherford presented the experiment to show how electrons are arranged in the atom.
Rutherford produced alpha () rays from radioactive element polonium (Po).
These  rays were incident from one side on the foil (0.004 mm thick) of gold.
Observations of Rutherford’s experiment:
Most of the  rays passed straight through the foil.
Very few of them did not pass through, but reflected back by different paths. The proportion of alpha rays
returning after deviation and the alpha rays passing through in straight way was about 1:1200 i.e. out of
1200  rays incident on gold foil only one  ray was deviated.
When the thickness of gold foil was doubled (0.008 mm) the number of a  rays reflecting after striking the
smallest atom
foil was also doubled.
nucleus
Alpha rays
Returned
 rays
Atoms in thin
strip of gold
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Highly hollow
atom in which
electrons is there
Conclusions of Rutherford’s experiement:
As most of the rays passed through the gold foil and only very few rays returned in a different path after
striking the gold foil he concluded that the atom must be hollow.
In the middle of the atom there must be very small, heavy and positive electric charge possessing nucleus.
Around this nucleus electrons with negligible weight and possessing negative charge must have been
arranged.
He found out from calculations that atomic nucleus is 105 times smaller than the total area of the atom.
11. What information about atomic nucleus was given by Rutherford from his experiment?
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From his experiment, Rutherford determined that in the middle of the atom there must be very small, heavy
and positive electric charged centre which is called nucleus.
He found out that atomic nucleus is 105 times smaller than the total area of the atom.
12. On what basis Rutherford said that atom is hollow?
Rutherford observed that:
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Most of  rays passed through the foil but only one out of 1200  rays of them did not pass through and got
reflected in different directions.  He suggested that an atom is hollow.
Also he found out from calculations that atomic nucleus is 105 times smaller than the total area of the atom.
Hence, we can say that atom is relatively very hollow.
13. Write a short note about Niels Bohr's atomic model.
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Niels Bohr’s atomic model
According to Niels Bohr electrons move around the nucleus in atomic levels at a definite distance from the
nucleus. This atomic level is called energy level or orbit.
The energy of the orbit which is nearest to the nucleus is least.
The energy of the orbits away from the nucleus gradually increases.
According to atomic model of Bohr, the arrangement of the electrons around the atomic nucleus is as
follows:
(1) The first orbit, nearest to the nucleus, i.e. orbit K, can accommodate two electrons.
(2) The second orbit, i.e. orbit L, can accommodate 8 electrons.
(3) The third orbit, i.e. orbit M, can accommodate 18 electrons.
(4) The fourth orbit, i.e. orbit N, can accommodate 32 electrons.
(5) The second, third and fourth orbits have subsidiary orbits which are called orbitals.
(6) Generally the electrons are arranged in an order i.e. only after filling the first orbit completely the extra
electrons are arranged stepwise.
14. Mention the symbols of first, second and third orbit of an atom and how many electrons can be
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accommodated in each of them.
The symbols of first, second and third orbit of an atom are K, L and M.
Orbit K can accommodate maximum 2 electrons.
Orbit L can accommodate maximum 8 and orbit M can accommodate maximum 18 electrons.
15. Describe the arrangement of electrons in orbits of first 20 elements of the periodic table.
[Give arrangement of electrons of following elements in their orbits: 11Na, 13Al 19K, 16S, 8O.]
[Write electronic structure of following atoms on the basis of Bohr’s model: 10Ne, 12Mg, 15P, 17Cl,
20Ca.]
Atom
Symbol
Atomic
Number
Hydrogen
Helium
Lithium
Beryllium
Boron
Carbon
Nitrogen
Oxygen
Fluorine
Neon
Sodium
Magnesium
Aluminium
Silicon
Phosphorus
Sulphur
Chlorine
Argon
Potassium
Calcium
H
He
Li
Be
B
C
N
O
F
Ne
Na
Mg
Al
SI
P
S
Cl
Ar
K
Ca
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Number
of protons
Number
of
neutrons
Number
of
electrons
Arrangement of electrons in orbits
K
L
M
N
Valency
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
–
2
4
5
6
6
7
8
10
10
12
12
14
14
16
16
18
22
20
20
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
–
–
1
2
3
4
5
6
7
8
8
8
8
8
8
8
8
8
8
8
–
–
–
–
–
–
–
–
–
–
1
2
3
4
5
6
7
8
8
8
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1
2
1
0
1
2
3
4
3
2
1
0
1
2
3
4
3,5
2
1
0
1
2
16. What is meant by valency? What is the relation between valency and electronic structure? Explain it.
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Electrons of an atom are arranged in different orbits in the increasing order in energy around the nucleus.
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GROUP TUITION
Electrons in the outermost orbit are responsible for emission of the spectra and the chemical properties of
the elements. They are called valence electrons.
The number of electrons in the valence orbit is the valency of the atom.
Valency = Number of electrons in valence orbit.
e.g.
Sodium : Atomic number of sodium is 11.  Its electronic configuration is 2, 8, 1. It has 1 electron in the
outer most orbit. Thus its valency is 1.
Calcium: Atomic number of calium is 20.  Its electronic configuration is 2, 8, 8, 2. It has 2 electrons in
the outer most orbit. Thus its valency is 2.
17. What is called a valence electron?
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The electrons in the outermost orbit of an atom are called valence electrons of that atom
18. Mention the importance of valence electrons.
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The electrons in the outermost orbit i.e. the valence electrons are responsible for emission spectra and the
chemical properties of the elements.
19. What is called a stationary (stable) orbit?
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The orbits in which electrons revolve around nucleus without losing energy are called stationery orbit .
20. Give definitions: Atomic number, atomic mass, isotope.
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Atomic number: The number of protons or electrons in the nucleus of that atom is called atomic number of
that atom.
Atomic mass:
The total number of protons and neutrons present in its nucleus is called atomic mass of
that atom.
Isotopes:
The atoms possessing same number of protons but different number of neutrons i.e. same
atomic number but different atomic mass are called isotopes.
21. Write a short note on discovery of neutron.
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Rutherford suggested that, as hydrogen has 1 proton and the next element helium has two protons, the mass
of helium should be almost double of that of hydrogen.
But actually the mass of helium atom was found to be about four times the mass of hydrogen atom.
So an atom must be consisting of some particles having mass almost equal to that of proton.
These particles are mentioned as neutrons (n).
Chadwick discovered this fundamental particle of the atom called neutron. [Which scientist and by which discovery
invented neutrons?]
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He found that neutrons are neutral and its mass is almost equal to the mass of proton.
Hence the mass of an atom of any element is the sum of number of protons and number of neutrons in the
nucleus.
It is called atomic mass denoted as A.  A = p + n
22. Write a short note on isotopes and radioactivity.
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Isotopes
Atoms of an element having same number of protons and different number of neutrons i.e. the atoms having
same atomic number and different atomic masses are called isotopes of each other.
The word isotope means iso = same and topos = place. Thus the atoms of the elements whose position in
the periodic table is same are isotopes.
Suppose the number of protons in the nucleus of two or more atoms of an element is same but the number of
neutrons is different; then there will be change in atomic masses of the same element.
As the number of protons of isotopes is same their chemical properties are similar.
Some examples of isotopes are given below
Isotopes of Hydrogen
Isotopes of Oxygen
Isotopes of Uranium
16
17
18
232
235
238
Protium Deuterium Tritium
8O
8O
8O
92 U
92 U
92 U
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1
1
H
2
1
H or 21 D
1
1
1
GROUP TUITION
T or 31 H
1
1
2
3
1
Protons
1
8
8
8
92
92
92
Electrons
1
8
8
8
92
92
92
Neutrons
0
8
9
10
140
143
146
Radioactivity:
The property by which some elements of high atomic mass radiate ,  and  rays is called radioactivity.
E.g. Uranium U.
This is an old concept because element like lead (Pb) possesses high atomic mass even then it is not
radioactive. As the atomic mass of an element increases and if number of neutrons also increases, the atom
becomes unstable and so they emit radiation to make nucleus stable and hence these elements are called
radioactive elements. If the ratio of neutrons to protons exceeds 1.6, the property of radioactivity is
acquired.
Radioactive isotopes are used in determining age of old trees, age of fossils of man and animals, radiometric
dating, medical treatments, industries etc and also in the treatment of diseases like cancer.
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23. Give names of isotopes of hydrogen. Mention the number of protons and neutrons in them.
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The three isotopes of hydrogen are : protium ( 11 H ) , deuterium ( 21 H OR 21 D ) , tritium ( 31T ) or ( 31 H ).
Protium has 0 neutrons, deuterium has 1 neutron and tritium has 2 neutrons.
24. Which isotopes are called radioactive? Give examples.
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The isotopes of some elements having, high atomic masses possess the property of radioactivity and
hence they are called radioactive. e.g. Uranium, radium, polonium, etc
25. Give information about properties of isotopes.
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The radioactive isotopes are unstable and they emit very small particles like alpha () and beta () and
neutral gamma () rays.
26. Mention the importance of radioactive isotopes.
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Radioactive isotopes are used in (1) determining age of old trees, (2) age of fossils of man and animals, (3)
radiometric dating, (4) medical treatments like in treatment in cancer, (5) industries etc.
27. Distinguish between cathode rays and X – rays, protons and electron, protons and neutrons, neutrons
and electrons.
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2.
3.
4.
1.
2.
3.
4.
1.
2.
3.
4.
1.
2.
3.
Cathode rays
They are the streams of electrons
They are affected by electromagnetic fields
They cannot pass through opaque objects
They are used to produce X – rays.
Protons
They are found in the nucleus of an atom
They carry positive charge
Mass of a proton = 1.00723 u
It is 1836 times heavier than an electron
Proton
They are found in the nucleus of an atom
They carry positive charge
Mass of a proton = 1.00723 u
It is 1836 times heavier than an electron
Neutron
They are found in the nucleus of an atom
They carry no charge
Mass of a neutron = 1.00833u
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2.
3.
4.
1.
2.
3.
4.
1.
2.
3.
4.
4.
X rays
They are electromagnetic waves
They are not affected by electromagnetic fields
They can pass through opaque objects
They are used to find fracture in bones, etc.
Electron
They revolve around the nucleus of an atom
They are negatively charged particles
Mass of an electron = 0.00055 u
It is 1836 times lighter than the protons
Neutron
They are found in the nucleus of an atom
They carry no charge
Mass of a neutron = 1.00833u
It is1838 times heavier than electron
It is 1838 times heavier than electron
Electron
1. They revolve around the nucleus of an atom
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AJAY PARMAR
2. They are negatively charged particles
3. Mass of an electron = 0.00055 u.
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GROUP TUITION
4. It is 1838 times lighter than the neutrons
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