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
The following conclusions can be drawn.
Most part of atom is empty space because most of
Atom has a very small rigid, positively charged body called the nucleus.
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, ………
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.
Alpha Rays (
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 (
They are deflected towards positive plate in electric field.
They produce lesser ionization in gases than
Emission of one
Gamma Rays (
They are not deflected from their path in electric or magnetic field.
They produce minimum ionization in gases.
They posses maximum penetration power.
Notes
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.
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
Cu and Cr are exceptions to Aufbau’s principle
Energy of an electron in
Velocity of e – in nth orbit =
Z = atomic number