Why the element of second period shows a number of difference in properties from other members of their respective families?

Why the elements of the second row (first short period)? exhibit a number of differences in properties from othero members of their respective families ?

Oxygen differs from other member of irts family due to

Why the elements in a period posses different chemical properties ?

Both boron and aluminium show difference in properties from the remaining members of group 13. This because

The first element of the group 1 and 2 are different from other members of the respective group.Their behaviour is more similar to the second element of the following groups,What is this relationship known as ?