Why do all elements of the same group have similar properties?

Question

(i) Why do all elements of the same group have similar properties?
(ii) How does the atomic size vary in a period with increase in atomic number?
(iii) Atomic number of Magnesium and Calcium are 12 and 20 respectively. Write their electronic configuration and indicate the period to which they would belong.

Answer 1

(i) Elements of the same group have similar properties because of the equal number of valence shell electrons.
(ii) Atomic size decreases in a period with increase in atomic number. This is due to increase in nuclear charge or increase in force of attraction between nucleus and electrons.
(iii) Mg: 2,8,2 (KLM Shell). It belongs to 3rd period. Ca: 2,8,8,2 (KLMN Shell). It belongs to 4th period.