In this post, you will find a bunch of important formulas related to atomic structure.

## formulas related to atomic number & mass number

**(a) Atomic number (Z) **= number of electrons = number of protons

**(b) Mass number (A) **= number of protons + number of neutrons = atomic number + number of neutrons

**(c) Number of protons **= atomic number = mass number – number of neutrons

**(d) Number of electrons **= atomic number = mass number – number of neutrons

**(e) Number of neutrons **= mass number – atomic number

**Bohr and Bury Scheme** – Maximum number of electrons in energy levels

**Bohr and Bury Scheme – **Maximum number of electrons in energy levels

(i) Maximum number of electrons that can be accommodated in a shell is given by 2n^{2 }where n = shell number

(ii) **For 1st energy level (K shell), n = 1 **

Maximum number of electrons in 1st energy level = 2n^{2} = 2 x (1)^{2} = 2

(iii) **For 2nd energy level (L shell), n = 2 **

Maximum number of electrons in the 2nd energy level = 2n^{2} = 2 x (2)^{2} = 2 x 4 = 8

(iv) **For 3rd energy level (M shell), n = 3 **

Maximum number of electrons in the 3rd energy level = 2n^{2} = 2 x (3)^{2}= 2 x 9 = 18

(v) **For 4th energy level (N shell), n = 4 **

Maximum number of electrons in the 4th energy level = 2n^{2} = 2 x (4)^{2} = 2 x 16 = 32

The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has the capacity to accommodate more electrons. This is a very important rule and is also called the * Octet rule*. The presence of 8 electrons in the outermost shell makes the atom very stable.