What change occurs in the electronic configuration of atoms forming a covalent bond?

The electronic configurations of four elements are given below:
$(I) \, 1s^2 \, (II) \, 1s^2 2s^2 2p^2 \, (III) \, 1s^2 2s^2 2p^5 \, (IV) \, 1s^2 2s^2 2p^6$
Which element has the ability to form both ionic and covalent bonds?

Which set contains only covalently bonded molecules?

Silicon has $4$ electrons in the outermost orbit. In forming the bonds,

The electronic configuration of $X$ and $Y$ are given below:
$X: 1s^2 2s^2 2p^6 3s^2 3p^3$
$Y: 1s^2 2s^2 2p^6 3s^2 3p^5$
Which of the following is the correct molecular formula and type of bond formed between $X$ and $Y$?

Which of the following contains both polar and non-polar bonds?