The number which designate and distinguish various atomic orbitals and electrons present in an atom are called quantum numbers.

Electronic configuration of Magnesium is 2, 8, 2. So the valency is 2.
Electronic configuration of Sulphur is 2, 8, 6. So the valency is 2(8 - 6).

Atoms of different elements with different atomic numbers and different mass numbers, but with the same number of neutrons are called isotones.
Example: Boron and Carbon have same number of neutrons.

Uses of radioactive isotopes:
1. Uranium 235 is used as fuel in nuclear reactor.
2. Cobalt 60 is used in the treatment of cancer.
3. Iodine 131 is used in the treatment of goitre.
4. C-14 is used in carbon dating.

When two elements A and B combine together to form more than one compound, the masses of A which separately combines with the fixed mass of B are in simple ratio. This law was proposed by John Dalton.

Atomic number = Number of protons = Number of electrons
Number of protons = Number of electrons = 11
∴ Atomic number = 11
Name of the element is Sodium.

CH_4(g) +2O_2(g) → CO_2(g) +2H₂O_(g)
1             2           1              2
volume volumes  volumes volumes
1x           2x          1x          2x
50cm³ 50cm³  50cm³    50cm³
50cm³ 100cm³ 50cm³ 100cm³ 
Volume of oxygen used = 100 cm³
Volume of carbon dioxide formed = 50 cm³