Chemistry IX | Structure of Atom | Part 2

Structure of Atom | Part 2

I. Thomson’s Atomic Model

The first simple model of an atom was proposed by J.J. Thomson. According to this model an atom consists of a uniform sphere (radius approx 10 – 10 m) of positive charge with electron embedded in such a way so as to give most stable electronic arrangement.

II. Rutherford’s Atomic Model (1912)

The clip_image003-particles scattering experiments performed by Rutherford are concerned with the discovery of the nucleus.

The following conclusions can be drawn.

Most part of atom is empty space because most of clip_image003[1]-particles passed straight through the atom.

Atom has a very small rigid, positively charged body called the nucleus. clip_image003[2]-particles are repelled from the metal foil due to this positively charged part.

The whole mass of the atom is centralized in the nucleus.

Main Postulates of  Rutherford’s Atomic Model

An atom mainly or largely consists of free space. Each atom contains a heavy positively charged body at its center called the nucleus.

The Negatively charged electrons are revolving around the nucleus.

Rutherford atomic model failed because he could not explain the existence of revolving electrons.

III. Bohr’s Atomic Model (1913)

Niels Bohr proposed atomic model which is based upon the Planck’s quantum theory of radiations.

The electrons in an atom revolve around the nucleus only in certain circular paths called orbits or shells. Each orbit has a definite amount of energy.

Electron in the particular orbit, does not lose or gain energy.

The energy is emitted when an electron jumps from higher energy level to lower energy level and energy is absorbed when an electron jumps from lower energy level to higher energy level.

Bohr’s theory failed due to the dual nature of matter and uncertainty principle.

Electronic Configuration of Elements

The arrangement of electrons in different orbits of an atom is known as electronic configuration of the element. Following rules are used to write the electronic configuration.

Aufbau’s Principle :

According to this principle, “Sub-shells are filled with electrons in the increasing order of their energies.” i.e., sub-shell of lower energy will be filled first with electrons.

The order of increasing energies is summed as 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 6s, 4f, 5d,

6p, 7s, 5d, 6d, 7p.

Note that Cr24 and Cu29 not obey this law

Cr24 = 1s2 2s2 2p6 3s2 3p6 3d5 4s1

Cu29 = 1s2 2s2 2p6 3s2 3p6 2d10 4s1

Principal Quantum Number (Bohr)

It determines size and energy of electron.

It is denoted by n, where n = 1, 2, 3, ………clip_image008 (only positive integers).

The various shells are designated as K, L, M ……. respectively

Maximum numbers of electrons in any orbit is 2n2, where n is Principal Quantum number.


Exactly half filled sub-shells have lesser energy and thus assume more stability than any other arrangement. Thus p3 is more stable arrangement than p2, p4 or p5.

Iso Electronic Species

Species having same number of electrons are known as isoelectronic for e.g., NH3 or H3O+ are isoelectronic.


It was discovered by Prof. Henry Becquerel in 1896.

Some substances are found in the nature which destruct itself and emit invisible rays the phenomena is called Radioactivity and the substances are called Radio active elements.

Alpha Rays (clip_image010)

They are deflected towards negative plate in electric field.

It has two units positive charge and four unit mass that is doubly ionized Helium atom.

They produce maximum ionization in gases because of maximum kinetic energy.

Their penetration power is very small because of large mass.

Beta Rays (clip_image012)

They are deflected towards positive plate in electric field.

clip_image014-particles carries one unit negative charge and negligible mass that is electron.

They produce lesser ionization in gases than clip_image003[3]-particle.

Emission of one clip_image014[1]-particle from unstable nucleus results the increase in number of protons by one unit.

Gamma Rays (clip_image018)

clip_image020 rays are electromagnetic radiation of very high frequencies even more than X-rays. Thus, the clip_image020[1] rays have no charge and no mass.

They are not deflected from their path in electric or magnetic field.

They produce minimum ionization in gases.

They posses maximum penetration power.


Isotopes have same number of protons but different number of neutrons.

Isotones have same number of neutrons.

Radioactivity does not change with temperature, pressure etc.

Entranuclear part of atom contain electrons.

t1/2 = clip_image023 (clip_image025 = decay constant).

Average life = clip_image027 = 1.44 clip_image029 t1/2

Nuclear fusion is source of sun’s energy.

The principle of atom bomb is based on nuclear fission and that of Hydrogen bomb is based on nuclear fusion

clip_image014[2]particle is strongest reducing agent.

Cu and Cr are exceptions to Aufbau’s principle

Energy of an electron in

nth orbit (En) = clip_image033ev/atom

Radius of nth orbit (rn) = clip_image035 A0

Velocity of e – in nth orbit = clip_image037 cm/sec

Z = atomic number


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