Download CHAPTER #2 STRUTURE OF ATOM CLASS:9th The word atom

CHAPTER #2
STRUTURE OF ATOM
CLASS:9th
The word atom was first introduced by Greek philosopher Democritus. He gave the concept that matter is
made up of very small indivisible particles called atoms. The origin of word atom was Greek word atomos
means indivisible, uncuttable (a- not, tomos-a cutting)
Q: what is Rutherford’s atomic model? , what were main defects in the Rutherford’s atomic model?
Ans: RUTHERFORD’S ATOMIC MODEL:
Rutherford's atomic model shows the existence of nucleus in the atom, nature of charge on the nucleus
and the magnitude of charge on the nucleus.
APPARATUS FOR EXPERIMENT
 Alpha particles.
 Gold foil. (0.0004 cm thick)
 Zinc sulphide screen.(ZnS)
EXPERIMENT
In his experiments, Rutherford bombarded alpha particles on very thin metallic foils such as gold foil of
0.0004cm thickness. He obtained alpha particles (He2+) from a radioactive element radium or polonium. A
In order to record experimental observations, he made use of circular screen coated with zinc sulphide.
OBSERVATIONS
1. He observed that most of the alpha particles were pass through the foil undeflected.
2. Very few particles were deflected when passed through the foil.
3. One particle out of 1 million particles was deflected greater than 90o.
4. Few particles were deflected at different angles.
5. Massive alpha particles were not deflected by electrons.
CONCLUSIONS DRAWN FROM THESE EXPERIMENTS
From these experiments Rutherford proposed a planetary model for an atom. He drew the following
conclusions.
1.
Since most of the a-particles pass through the foil undeflected, therefore, most of the
space occupied by the atom is empty.
2.
The deflection of a few particles proved that there is a centre of heavy positive charge in an
atom which causes repulsion as shown in the figure. The heavy part of the atom is called its
nucleus.
All the space outside the nucleus is occupied by the fast revolving electron. The massive a3.
particles are not deflected by these electrons.
4.
The centrifugal force which is produced due to rotation of electrons balances the
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electrostatic force of attraction between the nucleus and electron. This prevents the electrons
from falling into nucleus.
RUTHERFORD’S ATOMIC MODEL
1. Major portion of the atom is empty.
2. The whole mass of the atom is concentrated in the center of atom called nucleus.
3. The positively charged particles are present in the nucleus of atom.
4. The electrons revolve around the nucleus in different circular orbits.
5. Size of nucleus is very small as compare to the size of atom.
DEFECT IN RUTHERFORD’S THEORY
1.
According to classical physics, electrons being charged particle should emit energy continuously. The
orbit of electron should become smaller and smaller until it falls into the nucleus. But it never falls
because if it falls at all, the atomic structure would collapse.
2. If the electron emits energy continuously, atom should form continuous spectrum. Actually atoms
give line spectrum.
NEIL BOHR’s ATOMIC THEORY:
In 1913 Neil Bohr presented his atomic model, he introduced circular orbit concept on basis of Plank’s
Quantum theory.
Main postulates of Bohr’s atomic theory are:
1. Electrons revolve around the nucleus in circular path, which are known as "ORBITS" or "ENERGY
LEVEL".
2. Energy of an electron in one of its allowed orbits is fixed.As long as an electron remains in one of its
allowed orbit, it can not absorb or radiate energy.
3. If an electron jumps from lower energy level to a higher energy level, it absorbs a definite amount of
energy.
...
4. If an electron jumps form higher energy level to a lower energy level, it radiates a definite amount of
energy.
..
5. Energy released or absorbed by an electron is equal to the difference of energy of two energy levels.
Let an electron jumps from a higher energy level E2 to a lower energy level E1.The energy is emitted in the
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form of light .Amount of energy released is given by:
E = E2 - E1
E2 - E1= h
Where
h = Planck's constant ( 6.6256 x 10-34 J.s)
= Frequency of radiant light
6. Spectrum of light emitted from an electron is a "LINE SPECTRUM".
7. Angular momentum of an electron is given by:
mvr=
Where n =1, 2, 3, ………
.m = mass of electron
V = velocity of electron
r = radius of orbit
ISOTOPES
Nuclei of the same element having same atomic number but different mass number are known as
isotopes.
OR
Nuclei of same element having same number of proton but different number of neutron are knows as
isotopes.
Example



35
37
17Cl , 17Cl
12
13
14
6C , 6C , 6C
234
, 92U235,92U238
92U
ISOTOPES OF HYDROGEN
There are three isotopes of hydrogen.
o
o
o
Protium
Deuterium
Tritium
Protium
Ordinary hydrogen is knows as protium .
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



It has one electron ,one proton but it has no neutron.
Mass number : 1
Charge number : 1
Symbol : 1H1
Percentage in natural hydrogen: 99.98%
Deuterium




It has one electron one proton and one neutron.
Mass number: 2
Charge number: 1
Symbol: 1H2 or D
Percentage In Natural Hydrogen: 0.0156%
Structure
Heavy water(D2O) consists of deuterium isotope of hydrogen.
TRITIUM




It has one electron, one proton and two neutrons.
Mass number : 3
Charge number : 1
Symbol : 1H3
1. It is a radioactive isotope. Not occurring naturally
2. It emits radioactive rays.
3. It is present in traces.
ISOTOPES OF CARBON
Carbon has three isotopes: C-12, C-13, C-14. Almost all carbon is carbon-12, its symbol is 6C12. Its %age
in nature is 98.9% , C-13 and C-14 are 1.1% in nature. C-14 is radioactive.
ISOTOPES OF CHORINE
There are two isotopes of chlorine, 17Cl35, 17Cl37 . The abundance in nature is 75% and 25 % respectively.
Chlorine is grayish yellow gas with sharp pungent irritating smell. It is fairly soluble in water.
ISOTOPES OF URANIUM
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

There are three isotopes of uranium, 92U234, 92U235,92U238
Natural abundance is 0.006%, 0.72%, 99.27% respectively
234
92U
235
92U
238
92U
Protons:
92
92
92
Electrons
92
92
92
Neutrons
142
143
146
Q : what are radioisotopes?
Ans: Isotopes which are produced artificially are called radioisotopes. Radioisotopsare unstable and emit
radioactive rays like. Alpha, beta and gamma rays.
Or
radioactive isotope, also called radioisotope, any of several species of the same chemical element with different masses
whose nuclei are unstable and dissipate excess energy by spontaneously emitting radiation in the form of alpha, beta,
and gamma rays.
Q: what are applications( uses) of isotopes?
Ans: Americum-241: Used in many smoke detectors for homes and businesses…to measure levels of toxic
lead in dried paint samples…to ensure uniform thickness in rolling processes like steel and paper
production.
Cadmium-109: Used to analyze metal alloys for checking stock, scrap sorting
Calcium -47: Important aid to biomedical researchers studying the cellular functions and bone formation in
mammals.
Carbon-14: Major research tool. Helps in research to ensure that potential new drugs are metabolized
without forming harmful byproducts. Used in biological research, agriculture, pollution control, and
archeology.
Iodine-123: Widely used to diagnose thyroid disorders and other metabolic disorders including brain
function.
Iodine-125: Major diagnostic tool used in clinical tests and to diagnose thyroid disorders. Also used in
biomedical research.
Iodine-131: Used to treat thyroid disorders.
Phosphorus-32: Used in molecular biology and genetics research.
Uranium-235: Fuel for nuclear power plants and naval nuclear propulsion systems…and used to produce
fluorescent glassware
Q: what I electronic configuration, which principle observe filling of electrons?
Ans:
Electronic configuration: The distribution of electrons in various subshells is called electronic configuration.
Shells: According to the Bohr theory electrons in an atom revolve around nucleus in one of the circular
paths called shells or orbits or energy levels.
Each shell is described by an n value and n can have values 1, 2,3……………………….
When
n=1 it is K shell
n=2 it is L shell
n=3 it is M shell
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n=4 it is N shell
Subshells: A shell or energy level is subdivided into subshells aor sub energy levels.n value of shell is
placed before the symbol of subshell.
For instance n=1 for K Shell and will be represented as 1s
n=2 for L Shell and will be represented as 2s, 2p
n=3 for M Shell and will be represented as 3s,3p,3d
n=4 for N Shell and will be represented as 4s,4p,4d,4f
Principle:”In filling of the subshells , electrons always enter in lower energy orbit firs”. This principle s
known by Auf Bau Principle.
Order of filling of subshells
1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p<8s………..
By: M. Zia Dogar
[email protected]
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cell:+92-321-4022401