CHAPTER - 4
STRUCTURE OF THE ATOM-1
(Development of Atomic Model)
Particles within atom
Atoms have three types of sub atomic particles which are electrons, protons and neutrons.
Electrons with negative charge (e-), Protons with positive charge (p+) Neutrons have no charge (n).
The mass of an electron is 1/1826 of the mass of a proton.
The mass of a proton is nearly equal to the mass of a hydrogen atom and is taken as 1 unit.
The mass of a neutron is slightly more than mass of proton.
Electron was first sub atomic particle discovered , followed by proton &
neutron.
Charge on an electron is equal to charge on a proton although opposite in nature.
Hydrogen is only element that do not have neutron.
Discovery of sub atomic particles (ELECTRON)
In 1900, J.J.Thomson discovered the presence of the negatively charged particles called electrons in the atom.
This experiment is called Cathode Ray Tube (CRT) Experiment.
CRT is a glass tube which is partially evacuated i.e. much of the air was pumped out of the tube.
CRT were initially designed by Hittorf. He observed that on applying
voltage a stream of particles (ray) was emitted from the negatively charged electrode (cathode) which move towards positively charged electrode
(anode).
The CRT used by Thomson was slightly different from others.
These rays were named as Cathode Ray by Goldstein as they are
emitted from cathode and the whole construction is called cathode ray
tube.
Discovery of sub atomic particles (ELECTRON)
Discovery of sub atomic particles (ELECTRON)
When experimentally studied cathode rays showed following properties:
1. They travel in straight lines.
2. They are deflected by electric field as they carry charge. Cathode rays are deflected by positively charged plate thus consists of negatively charged particles.
3. Cathode rays are also deflected by magnetic field.
3. Cathode rays show particle nature.
4. They are independent of the material composition of the cathode i.e. similar
properties for cathode rays were observed irrespective of material of cathode and gas filled in tube.
Discovery of sub atomic particles (PROTONS)
In 1886, E. Goldstein performed the experiment using perforated cathode and discovered new radiations in gas discharge behind perforated cathode.
These radiations were named anode rays as they were emitted by anode.
Goldstein called these positive rays as canal rays as they appear to be produced by the holes or channels in the cathode.
These rays later led to the discovery of the positively charged protons in the atom.
Various properties of canal rays are:
1. They are deflected by electric and magnetic fields. Their deflection is opposite to that of cathode rays.
2. They travel in straight lines.
3. The velocity of canal rays is much smaller than the velocity of cathode rays.
4. Mass of canal rays vary with gas used in experiment.
In 1932 Chadwick discovered the presence of particles having no charge and
called them neutrons.
Discovery of sub atomic particles (PROTONS)
Structure of an atom (THOMSON’S MODEL)
According to Thomson an atom is similar to a Christmas pudding.
The pudding had positive charge and the electrons having negative charge were like plums on the pudding.
He 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.
The model failed as it was unable to explain the observations made by Rutherford later in gold foil experiment.
Structure of an atom (RUTHERFORD’S MODEL)
Rutherford’s proposed the model on basis of Alpha Particle Scattering Experiment or Gold Foil Experiment.
Rutherford bombarded a thin gold foil with a beam of fast moving alpha particles ( α – particles).
The foil was surrounded with a photographic sheet to
observe the path of alpha particles.
Structure of an atom (RUTHERFORD’S MODEL)
Rutherford made following observations in the experiment.
i) More than 99% of the α – particles passed straight through the gold foil.
ii) Less than 1%of the α – particles were slightly deflected by small angles.
iii) Very few 1/2000) of all α – particles appeared to rebound on their original path
Designing Rutherford’s Model
i) As >99% of alpha particles pass through most of the space inside an atom (foil) is empty.
ii) As <1% particles were deflected by small angles the atom must have a small nucleus having positive charge which produces repulsive forces with alpha particles.
iii) Small number of particles rebound due to strong repulsive forces so the size of the nucleus must be very small.
Features of Rutherford’s model of an atom
i) An atom has a positively charged nucleus at its centre and most of the mass of the atom is in the nucleus.
ii) The electrons revolve around the nucleus in different orbits.
iii) The size of the nucleus is very small as compared to the size of the atom.
Demerits of Rutherford’s model of the atom :-
1. Maxwells rule: “Any particle in a circular orbit would undergo acceleration and would radiate energy.” So the revolving electrons would lose energy and fall into the nucleus and thus atom would be unstable.
2. No information about the distribution of electrons in orbits was given.
Positively charged nucleus
CHAPTER - 4
STRUCTURE OF THE ATOM-2
(Bohr’s Atomic Model)
Bohr’s model of an atom
1. An atom has a positively charged nucleus at its centre and contains most of its mass.
2. The electrons revolve around the nucleus in discrete energy levels or orbits or shells.
3. While revolving in the discrete orbits the electrons do not radiate energy.
4. As we move away from nucleus the energy level of energy shell
increases.
Bohr Bury Scheme
It explains the distribution of electrons in energy levels:
1. The maximum number of electrons in a shell is given by the formula 2n
2where n is the number of the shell 1, 2, 3 etc.
First shell or K shell can have = 2n2 = 2 x 12 = 2x1x1 = 2 electrons Second shell or L shell can have = 2n2 = 2 x 22 = 2x2x2 = 8 electrons Third shell or M shell can have = 2n2 = 2 x 32 = 2x3x3 = 18 electrons
Fourth shell or N shell can have = 2n2 = 2 x 42 = 2x4x4 = 32 electrons
2. The maximum number of electrons that can be filled in the outermost shell is 8.
3. Electrons cannot be filled in a shell unless the inner shells
are filled.
Composition of the atoms of the first eighteen elements
Name of element
Symbol Atomic Number
Number of Protons
Number of Neutrons
Number of Electrons
Distribution Of Electrons K L M N
Valency
Hydrogen H 1 1 - 1 1 - - - 1
Helium He 2 2 2 2 2 - - - 0
Lithium Li 3 3 4 3 2 1 - - 1
Beryllium Be 4 4 5 4 2 2 - - 2
Boron B 5 5 6 5 2 3 - - 3
Carbon C 6 6 6 6 2 4 - - 4
Nitrogen N 7 7 7 7 2 5 - - 3
Oxygen O 8 8 8 8 2 6 - - 2
Fluorine F 9 9 10 9 2 7 - - 1
Neon Ne 10 10 10 10 2 8 - - 0
Sodium Na 11 11 12 11 2 8 1 - 1
Magnesium Mg 12 12 12 12 2 8 2 - 2
Aluminium Al 13 13 14 13 2 8 3 - 3
Silicon Si 14 14 14 14 2 8 4 - 4
Phosphorus P 15 15 16 15 2 8 5 - 3,5
Sulphur S 16 16 16 16 2 8 6 - 2
Chlorine Cl 17 17 18 17 2 8 7 - 1
Argon Ar 18 18 22 18 2 8 8 - 0
Atomic structure of the first eighteen elements
H He
Li Be B C N O F Ne
Na Mg Al Si P S Cl Ar
Valency
“Valency is the combining capacity of an atom of an element. “
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In simpler terms it is the number of bonds formed by an atom with other atoms present in molecule
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The electrons present in the outermost shell of an atom are called valence electrons as only they participate in bond formation.
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Every atom wants to complete eight electrons in its valance shell.
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Completely filled outermost shell is called octet. Helium atom can only accommodate 2 electrons i.e. duplet.
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Octet and duplet are highly stable and atoms with these don't react with other atoms. So they only exist as monoatomic gases or inert gases.
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Helium is only inert gas with two electrons in its outer
most shell.
Valency
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If any atom has completely filled valance shell it does not participate in bond formation and are called inert elements.
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If an atom has incompletely filled outermost shell it try to attain stability by losing, gaining or sharing electrons with other atoms to attain octet.
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If an atom has 3 or less electrons in its valence shell it exposes inner completely filled shell by loosing electrons.
For these elements:
Valency = number of valence electrons
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If an atom has 5 or more electrons in valence shell it attains inert shell configuration by gaining remaining electrons.
Valency = 8 - number of valence electrons
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If an atom has 4 electrons in valence shell it shares these electrons with other atoms.
Valency = 4
Valency
Composition of the atoms of the first eighteen elements
Name of element
Symbol Atomic Number
Number of Protons
Number of Neutrons
Number of Electrons
Distribution Of Electrons K L M N
Valency
Hydrogen H 1 1 - 1 1 - - - 1
Helium He 2 2 2 2 2 - - - 0
Lithium Li 3 3 4 3 2 1 - - 1
Beryllium Be 4 4 5 4 2 2 - - 2
Boron B 5 5 6 5 2 3 - - 3
Carbon C 6 6 6 6 2 4 - - 4
Nitrogen N 7 7 7 7 2 5 - - 3
Oxygen O 8 8 8 8 2 6 - - 2
Fluorine F 9 9 10 9 2 7 - - 1
Neon Ne 10 10 10 10 2 8 - - 0
Sodium Na 11 11 12 11 2 8 1 - 1
Magnesium Mg 12 12 12 12 2 8 2 - 2
Aluminium Al 13 13 14 13 2 8 3 - 3
Silicon Si 14 14 14 14 2 8 4 - 4
Phosphorus P 15 15 16 15 2 8 5 - 3,5
Sulphur S 16 16 16 16 2 8 6 - 2
Chlorine Cl 17 17 18 17 2 8 7 - 1
Argon Ar 18 18 22 18 2 8 8 - 0
Atomic structure of the first eighteen elements
H He
Li Be B C N O F Ne
Na Mg Al Si P S Cl Ar
Atomic number and Mass number
a) Atomic number (Z): The atomic number of an element is the number of protons present in the nucleus of the atom of that element.
All the atoms of an element have the same atomic number.
Eg :- Hydrogen (Z) = 1 (1 proton) Helium (Z) = 2 (2 protons) Lithium (Z) = 3 (3 protons)
b) Mass number (A): The mass number of an element is the sum of the number of nucleons (protons and neutrons) present in the nucleus of an atom.
Mass of an atom is almost equal to the mass of the protons and neutrons in the nucleus of the atom as mass of electrons is negligible as compared to them.
Eg :- Carbon – Mass number = 12 (6 protons + 6 neutrons) Mass = 12u Aluminium – Mass number = 27 (13 protons + 14 neutrons) Mass = 27u
Sulphur – Mass number = 32 (16 protons + 16 neutrons) Mass = 32u
Atomic number and Mass number
Notation of an atom:
How many protons, neutrons and electrons?
Isotopes
Isotopes are atoms of the same element having the same atomic number but different mass number.
Eg :- Hydrogen has three isotopes. They are Protium, Deuterium (D) and Tritium (T).
H H H
Protium Deuterium Tritium Carbon has two isotopes. They are :-
C C
Chlorine has two isotopes They are :-
Cl Cl
Isotopes of oxygen are O-16 & O-17.
Isotopes of nitrogen are N-14 & N-15.
1
1 1 1
2 3
12
6 6
14
35 37
17 17
Isotopes (Applications)
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Uranium isotope U-235 is used in producing nuclear energy.
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Iodine isotope I-125 & I-123 are used in medicine for treatment of goiter, cancer etc.
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Cobalt isotope cobalt-60 is extensively employed as a radiation source to control the development of cancer.
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Radioisotopes of different elements are used to determine the mechanism of a chemical reaction.
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