Study Portal 2 – Chapter 2

Structure of Atoms



The name atom is derived from the Greek word atoms which means “indivisible”, this initial concept of atom is believed to be given by the Greek philosopher Democritus (400BC) but with the advancement of science they are no more indivisible now, and can be easily broken into smaller particles.


Q1.   Describe the contribution of scientists towards the structure of atom.

Ans.    John Dalton

In 1808 john Dalton put his theory about atom and said “Matter is made up of very small indivisible particles”.

William Crook

In 1876 w.crook conducted a series of experiments in the discharge tube the electrical current was passed at low pressure through the gases. From his experiment he concluded that atom was divisible. He also found electron and protons.

J.J. Thomson

He performed many experiments with the discharge tube and determined (charge / mass) ratio of an electron.


By performing experiments on discharge tube he discovered proton and thus the divisibility of atom.

R.A. Milliken

In 1909 he measured the charge on electron.


He performed a series of experiments using Alpha particles. He proposed a planetary model for an atom.

Mosley and Bohar

They performed many experiments on structure of atom and revealed that atom had a complex structure.

Q2.   Write discovery of Electrons, Protons and Neutrons.

Ans.    Discovery of Electron

It was observed that when high voltage is passed through the cathode ray tube at a very low pressure then a beam of ray is produced. The direction of these rays is form cathode to anode, hence they originate from the cathode electrode, so are called cathode ray. When an object is placed in the path of these rays, it is observed that they cast sharp shadow. It means they travel in straight line. When a fly is placed in their path it is moved showing that these rays are actually particle in nature.

When these rays are subjected to electric or magnetic fields, they were deflected towards the positive plate showing their negative nature.

These experiments proved that cathode rays are actually tiny particles having negative charge and are emitted from cathode and travel towards his anode. These particles were named as “electron” by G.J Stoney in 1891, thus the electron was discovered.

Discovery of Proton

In 1886 gold stein observed that glow appeared in the discharge tube at opposite end of anode at back of a cathode. If a perforated cathode is used the rays that caused this must come from anode. Further investigation showed that these rays have positive charge have momentum. They were named proton.

Discovery of neutron

In 1920, Ernest Rutherford postulated that there were neutral massive particles in the nucleus of atoms this conclusion arose from the disparity between an elements atomic number and its atomic mass. James Chadwick proved the existence of these particles in 1930.

Q3.      Describe Thomson’s atomic model.

Ans.    According to Thomson’s an atom consists of a sphere of positive electricity in which electrons are embedded. The electrons are embedded like the plum in a pudding.

Q4.      Describe Rutherford’s atomic model.

Ans.    In 1911, Rutherford bombarded a thin sheet of gold of thickness 0.00004 with alpha particles. An alpha particle is a helium nucleus. A thin lead plate with a hole cut in it, was used to produce a beam of alpha particles. A circular screen coated with zinc sulphides was placed on the other side of the foil.

Rutherford observed that most of the alpha particles passed through the gold foil. These alpha particles caused illustration on the zinc sulphides screen very few alpha particles were deflected at some angles after passing through the gold foil.

A very few particles i.e. one in 10,000 were offered interference to the passage of their flow because of the presence of some heavy charge in the centre of the atom. It was found from this experiment that most of the space in an atom was empty. The scattering of alpha particles there is a heavy positive charge at the centre of the tom which causes repulsion.

Defects of Ruther ford’s Atomic Mode

According to Clark Maxwell a charged particle revolving round the neutron under the influence of an attraction force continuously loses energy. Since electron is also charged particle, it must emit radiations and thus lose energy continuously. As electron loses energy, it should come near the nucleus. A spiral path should be traced by the electron. As a result of this, the electron would ultimately fall into the nucleus. But the revolving electrons never fall into the nucleus. Hence Rutherford’s picture of an atom is faulty.

Rutherford’s Atomic Model and the Solar System

Chemistry believes that the structure of the atom is rather like solar system. At the centre of the atom is the nucleus which possesses a much larger mass than all the rest of the atom. Since most of the solar system is empty space with only a few plants here and there, in the same way, most of the atom is empty space. Hence we can say that the Rutherford’s picture of an atom is like the solar system.

Q5.      Describe in detail the bohar’s atomic model.

Ans.    In 1913 Neil Bohar modified Rutherford’s model of atom with the help of quantum theory proposed by man Planck in 1900.

Some basic postulates of the bohar’s atomic model are as under:

  • The electrons in an atom revolve around the nucleus in certain definite circular path called orbits.
  • These orbits are known as energy levels or energy shells. These energy levels are numbered as 1,2,3,4,5,—etc. they are also called K,L,M,N,—etc. shells respectively starting from the nucleus.
  • As long as an electron in any particular energy level or orbit, it neither loses nor gains the energy. But electron can lose energy when it jumps from an orbit of higher energy (E2) to one of the lower energy (E1).
  • The energy difference (E2-E1) is given out during the jumping of electron in the form of radiation ∴△E = E2-E1          = hv,

Where      h     = plank’s constant.

And value of h            = 6.625 x 10 -34 js

V = (spelled as nu) is the frequency of radiation.

hv  = energy possessed by one quantum.

The electron, however, gets promoted to a higher energy level by absorbing a required quantum of energy. The electron in the higher energy level is said to be in the excited state.

  • The electron is allowed to revolve in those energy levels in which the angular momentum of the electron is an integral whole number multiple of h/2p.
  • The angular momentum of an electron moving in a circular orbit is equal to mvr, where “m” and “v” are the mass and velocity of the electron moving in an orbit of radius “r”. thus, according to the bohar’s atomic model angular momentum is Mvr = hn/2p, where n = 1,2,3,4,… . According to this postulates we can says that angular momentum is quantized.
  • The centrifugal force acting on the moving electron around the nucleus of an atom is balanced by the attractive force between the nucleus and the electron.

Formula for accommodation of electron in shells

The formula for accommodating the maximum number of electron in shell is 2n2 where “n” is the number of shell.

A shell is further divided into sub shells.

If         n = 1 has only one sub shell i.e. 1S

n = 2 has two sub shells i.e. 2S, 2p

n = 3 has three sub shells i.e. 3S, 3P, 3d

n = 4 has four sub shells i.e. 4S, 4P, 4d, 4f

Q6.   What is meant by electronic configuration? Explain the electronic configuration of first 18 elements.

Ans.    The position of electrons in an atom is called its electronic configuration. In order to calculate electronic configuration it is kept in mind that electrons will fill that orbital first whose energy is least.

Electronic configuration of first 18 elements

Atomic Number Symbol Electronic Configuration
1 H 1S1
2 He 1S2
3 Li 1S2, 2S1
4 Be 1S2, 2S2
5 B 1S2, 2S2, 2P1
6 C 1S2, 2S2, 2P2
7 N 1S2, 2S2, 2P3
8 O 1S2, 2S2, 2P4
9 F 1S2, 2S2, 2P5
10 Ne 1S2, 2S2, 2P6
11 Na 1S2, 2S2, 2P6, 3S1
12 Mg 1S2, 2S2, 2P6, 3S2
13 Al 1S2,2S2, 2P6, 3S2, 3P1
14 Si 1S2,2S2, 2P6, 3S2, 3P2
15 P 1S2,2S2, 2P6, 3S2, 3P3
16 S 1S2,2S2, 2P6, 3S2, 3P4
17 Cl 1S2,2S2, 2P6, 3S2, 3P5
18 Ar 1S2,2S2, 2P6, 3S2, 3P6

Q.7.     Define isotopes?

Ans.    The atoms of the same elements which have same atomic masses are called isotopes. Almost all of the elements form isotopes. Table shows isotopes of some elements and their percentage ion nature.

Element Protons Mass No. Symbol Percentage
H 1 1 1H1 9.985
1 2 1D2 0.015
1 3 1T3 Trace
C 6 12 6C12 98.90
6 13 6C13 1.10
6 14 6C14 Trace
Cl 17 35 17Cl35 75.77
17 36 17Cl36 Trace
17 37 17Cl37 24.23
U 92 233 92U233 Trace
92 234 92U234 0.0055
92 235 92U235 0.7200
92 238 92U238 99.2745

Uses of Isotopes

The isotopes are used in many fields of life e.g.

  • They are used in isotopic labeling, normally, atoms of a given elements are undistinguishable from each other. However, by using isotopes of different masses, they can be distinguished by mass spectrometry.
  • As tracers in chemical reactions, this technique is called radio isotopic labeling. If radioactive isotopes are used in chemical reactions, they can be detected by the radiation they emit and thus mechanisms of different reactions can be understood quite easily.
  • The isotopes are also used to understand the mechanisms of various natural processes like photosynthesis.
  • A technique similar to radio isotopic labeling is radiometric dating using the known half-life of an unstable element we can calculate the amount of time that has elapsed since a known level of isotope existed the most widely known example is radiocarbon dating used to determine the age of carbonaceous matials.
  • Several forms of spectroscopy rely on the unique nuclear properties of specific isotopes.
  • The isotopes also used for the treatment of different types of cancers in the body, the cancerous cells are killed by using radio active rays of radio isotopes. This process is called radio therapy.
  • Isotopes are also used for the investigation of different diseases.
  • The isotopes are also used to generate electricity. In Pakistan kanupp (Karachi) nilore (Islamabad) and chashma nuclear energy centers produce electricity based on this method.
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