What is atom made up of ?
An atom consists of two parts :
(a) Nucleus :
Nucleus is situated in the centre of an atom. All the protons & neutrons are situated in the nucleus, therefore, the entire mass of an atom is almost concentrated in the nucleus. The overall charge of nucleus is positive due to the presence of positively charged protons (neutrons have no charge). The protons & neutrons are collectively called nucleons. The radius of the nucleus of an atom is of the order of 10-15 m.
(b) Extra nuclear region :
In extra nuclear part or in the region outside the nucleus, electrons are present which revolve around the nucleus in orbits of fixed energies. These orbits are called energy levels. These energy levels are designated as K, L, M, N and so on.
i) The maximum number of electrons that can be accommodated in a shell is given by the formula 2n2 (where n = number of shell i.e. 1, 2, 3—)
|
Shell |
n |
2n2 |
max. no. of e- |
|
K |
1 |
2(1)2 |
2 |
|
L |
2 |
2(2)2 |
8 |
|
M |
3 |
2(3)2 |
18 |
|
N |
4 |
2(4)2 |
32 |
Diagramatical representation of Atom
(ii) In the outermost shell of any atom, the maximum possible number of electrons is 8, except in the first shell which can have at the most 2 electrons.
(iii) The arrangement of the electrons in different shells is known as the electronic configuration of the element. The pictorial representations of Bohr’s model of hydrogen, helium, carbon, sodium and calcium atoms having 1, 2, 6, 11 and 20 electrons respectively are shown in the figure where the centre of the circle represents the nucleus