Why do all the elements of the (a) same group have similar properties, (b) same period have different properties? answer soon

(a) Elements in the same group have similar properties because they have the same number of electrons in their outermost shell. Those with 3 or fewer electrons in that shell tend to be metals and those with 4 or more non metals. Metals lose their outer electron(s) when they react and non metals gain (or share) electrons when they react. The fewer electrons that need to be lost or gained the more reactive the element. So groups 1 and 7 are the most reactive. With metals the reactivity increases as you go down the group whereas with non metals the most reactive are towards the top of the group. The noble gases have complete outer electron shells and so are unreactive.

(b) The elements in a period of periodic table have only one common factor that is same no of shells but the no of valence electrons are different for each and every atom so they have different chemical and physical properties.