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


Chemistry IX | Structure of Atom | Part 1

Structure of Atom | Part 1


Atom is the smallest particle of an element, does not found in free state but takes part in chemical reaction.

Main Postulates of Dalton’s Atomic Theory

All matter is made up of very small particles known as atoms.

Atom can neither be created nor destroyed, it is the smallest particle that take part in a chemical reaction.

All atoms of an element are alike and of different element are different atoms.

Atoms combine together in the ratio of small whole numbers.

Drawbacks of Dalton Atomic Theory

It does not explain why atoms of different elements differs in their mass, valencies etc.

It does not explain the existence of isotopes and isobars.

It does not explain the reason for difference in atoms of different elements.

Discovery of electron, proton and neutron discarded the indivisible nature of atom proposed by Dalton.


Electron was discovered by J.J. Thomson in 1897.

Magnitude of charge on electron = 1.6 clip_image00310 – 19 coulomb

Mass of electron = 9.4 clip_image003[1] 10 – 28 gram.

When an electric discharge at high voltage is passed through a gas at low pressure, a stream of rays is emitted from the cathode surface. These are called as cathode rays which are made up of electrons.

Properties of Cathode Rays

Cathode rays always travel in straight line. Whenever an object is placed in the path of cathode rays, it forms a shadow on the walls opposite to the cathode.

Cathode rays consists of material particles and produce mechanical effects.

The beam of cathode rays are deflected towards the positive charged plate, hence the particles in the cathode rays carry negative charge.

Cathode rays are also deflected from their path in magnetic field. 

Cathode rays possess heating effect. When these strike a thin metal foil, it gets heated up.

It affects the photographic plates.

The cathode rays produce X-rays when they strike metals with high melting point like tungsten, etc.

The value of e/m was found to be 1.76 clip_image003[2] 108 coulomb per gram for electrons.

The charge of the electron was measured by R.A. Millikan.

Proton (1P1 or 1H1)

The existence of positively charged particles in an atom was discovered by E. Goldstein in 1886.

The magnitude of charge on proton is 1.6 clip_image003[3] 10 – 19 coulomb.

Mass of proton 1.6 clip_image003[4] 10 – 24 gram.

When a high voltage is supplied across the electrodes a new kind of rays emerges from the hole in the cathode opposite to the anode. These rays are anode rays.

Properties of Anode Rays

Anode rays travel in a straight line and cast shadow.

Anode rays are deflected by the magnetic and electric field and bent in the opposite direction of cathode rays.

The anode rays can also rotate the wheel placed in their path and also have heating effect.

The charge to mass ratio (e/m) for these rays is considerably smaller than electrons and is equal to 0.58clip_image003[5]104 coulomb / g and this ratio e/m for positive rays depends upon the nature of the gas taken in the tube.

Neutron (0n1)

The neutrons were discovered by James Chadwick.

These particles are neutral having charge 0.

Their mass is same as that of a hydrogen atom.

So, its mass = 1.66 clip_image003[6] 10 – 27 Kg.

Atomic Number

It was given by Moseley. The number of protons present in the nucleus or the number of electrons revolving around the nucleus is called Atomic number. Atomic number = number of protons in the nucleus = number of electrons.

Mass Number

The sum of number of protons and neutrons present in the nucleus is called mass number. It is a whole number.

Mass number (A)

= number of protons + number of neutrons

= number of electrons + number of neutrons

= Atomic number + number of neutrons.


Isotopes were discovered by F. Soddy.

Isotopes have the same atomic number but differ in their mass numbers.

Isotopes have identical chemical properties but differ in physical properties.

e.g., 1H1 (Protium), 1H2 (Deuterium) and 1H3 (Tritium), 8O16, 8O17 and 8O18


Isotones are the atoms of different elements possessing the same number of neutrons and have different mass numbers :

e.g., 1H3 and 2He4, 15P31 and 16S32, 19K39 and 20Ca40