According to Lewis and Langmuir,how and why is a covalent bond formed?

The interatomic distances in $H_2$ and $Cl_2$ molecules are $74 \, pm$ and $198 \, pm$ respectively. The bond length of $HCl$ is $.............$ $pm$.

Which of the following is an electron deficient molecule?

Number of electrons in the valence orbit of nitrogen in an ammonia molecule are

Which of the following is a Lewis acid?

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?