Variation along the period
The number of valence electrons increase from left to right in a period. So, the valency of the elements first increase from 1 to 4 and then decreases to zero.
Variation down the group
On moving down the group the number of valence electrons remain the same and therefore all the elements in the group exhibit the same valency.
Melting and boiling points
The melting and boiling points of elements are determined by the inter-atomic or inter-molecular forces between the constituent particles in the lattice and liquid state respectively. It is difficult to predict the variation in melting and boiling points of elements in the periodic table because of lot of variation in the nature of intermolecular forces and the mode of packing.
Variation along the period
In case of representative elements the melting and boiling points increase along the period and reach the maximum value in the middle of the period. For example in the second period the melting point increases from Li to C and then decreases.
Variation down the group
The melting and boiling points decrease as we go down the group. In case of alkali and alkaline Earth metals, due to the decreasing attractive forces between the atoms with increasing size, the melting and boiling points decrease. However the melting and boiling points of halogens increase down the group because the inter-molecular forces between the molecules of halogens are Van der Waals type.
