Predict the maximum number of valence electrons possible for atoms in the first period of the periodic table.

An element belongs to the third period and second group of the periodic table:
$(a)$ State the number of valence electrons in it.
$(b)$ Is it a metal or a non-metal?
$(c)$ Name the element.
$(d)$ Write the formula of its oxide.

Given below are some elements of the modern periodic table. Atomic numbers of the elements are given in the parentheses: $A(4), B(9), C(14), D(19), E(20)$.
$(a)$ Select the element that has one electron in the outermost shell. Also,write the electronic configuration of this element.
$(b)$ Which two elements amongst these belong to the same group? Give reason for your answer.
$(c)$ Which two elements amongst these belong to the same period? Which one of the two has a bigger atomic radius?

In Mendeleev's Periodic Table,gaps were left for elements to be discovered later. Which of the following elements found a place in the periodic table later?

Calcium is an element with atomic number $20$.
$(i)$ Will it be a metal or a non-metal?
$(ii)$ What will be its valency?
$(iii)$ What would be the formula of its chloride?
$(iv)$ Will it be larger or smaller than $K$ (Potassium)?

In Mendeleev's Periodic Table,the elements were arranged in the increasing order of their atomic masses. However,cobalt with an atomic mass of $58.93 \ amu$ was placed before nickel having an atomic mass of $58.71 \ amu$. Give the reason for the same.