Question

The electronegativity is 2.1 for H and 1.8 for Si. Based on these electronegativities, `SiH_4` would be expected ?A) be ionic and contain H+ ions (B) be ionic and contain H- ions (C) have polar covalent bonds with a partial negative charges on the H atoms

Answer 1

The answer is (C) have polar covalent bonds with a partial negative charge on the hydrogen atoms.

If you only go by the options given, the answer will be obvious because the difference in electronegativity between the two atoms is very small, which eliminates the possibility of their bond having a predominat ionic character.

In order for a bond to be considered ionic, the difference in electronegativity between the two atoms must be greater than 1.7, which is out of the question for your example.

However, there's an interesting comparison to be made with the famous `"C-H"` bond.

The difference in electronegativity for both bonds is approximately 0.3, but the `"C-H"` bond is considered to be nonpolar covalent, while the `"Si-H"` bond is considered to be polar covalent.

Because silicon is a larger atom than carbon, it will also have a larger electron cloud. This means that it has a greater polarizability than carbon does.

In the case of the `"Si-H"` bond, this will allow the more electronegative hydrogen to distort silicon's electron cloud and produce two partial charges, a positive one on the silicon and a negative one on the hydrogen.

As a result, the bond between silicon and hydrogen will indeed be considered polar covalent with a negative partial charge on the hydrogen atom.