2. How can you relate the molecular geometries of these hydrides to the identities of the central atoms in these compounds

Hello, please, the question is not complete. Attached is the complete question.

1. Their bond strength

2. Their acidic strength

3. the size of their central atom

Explanation:

CH4 and Si4 compounds belong to the same molecular geometry and group and they do have the same bond angle and they do not contain lone pair and they have no repulsion. we can conclude that they are from the carbon family.

NH3 and PH3 have the same molecular geometry  an bond angle which we can say they are 3 bond and 1 lone pair. they are from the nitrogen family and lone pair.

NOTE: Bond angle shows that first atomically is f second period and PH3 is from third period (This is the Drego rule)  

HF and HCL have the same molecular geometry and bond angle.