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 -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 -particles passed straight through the atom.

Atom has a very small rigid, positively charged body called the nucleus. -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, ……… (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.

Note

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.

Radioactivity

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.