Question

The electronegativity is $2.1$ for H and $1.8$ for Si. Based on these electronegativities, $Si{H_4}$ would be expected?
A) be ionic and contain ${H^ + }$ ion
(B) be ionic and contain ${H^ - }$ ions
(C) Have polar covalent bonds with partial negative charges on the $H$ atoms.

Answer 1

In ionic bond, one atom completely donates its electron to another atom whereas in covalent bond, one atom bonded with another by sharing electrons. The bond form between a metal and a nonmetal is an ionic bond whereas the bond form between two nonmetals is a covalent bond.

Complete answer:
Ionic bonds are those in which complete transfer of electrons takes place between the two atoms. Substances such as $KCl$ and $CaC{l_2}$ are examples. For the bond to be ionic, the difference in electronegativity should be greater than $2.0$ . Here the difference is very much less than $2.0$. So, $Si{H_4}$ does not have an ionic bond.
So, both option (A) and (B) are not right.
Covalent bonds are those in which the sharing of electron pairs between atoms takes place. These electron pairs are known as bonding electron pairs or shared electron pairs. For a bond to be covalent, if the difference in electronegativity is greater than $0.4$ then it is polar covalent bond and if it is less than $0.4$ then it is nonpolar covalent bond.
To check whether the compound is polar or nonpolar, we have calculate the electronegativity difference which is:
$= 2.1 - 1.8$
$\Rightarrow 0.3$
Here we find that its electronegativity is less than $0.4$ but it has polar covalent bonds. This is because of the larger size of silicon. Due to its larger size it has a large number of electron clouds. This means greater polarizability. In the $Si - H$ bond, the hydrogen distorts silicon’s electron clouds more resulting in two partial charges, the positive charge on silicon and the negative on the hydrogen.
Thus, the bond between silicon and hydrogen atoms are polar covalent with partial negative charge on the $H$ atoms.

So, option (C) is correct.

Note:
Covalent compounds form bonds when electron pairs are shared between atoms. Hence, because of sharing of electrons, they have characteristic physical properties that have lower melting and electrical conductivity as compared to that of ionic compounds.