Question

Compare and contrast the compounds CH4 and PH3 in terms of covalent bonding, molecular geometry (shape), and intermolecular forces.

Answer 1

CH4 would be non polar. Remember, carbon makes four bonds to each of the hydrgoen atoms. and there are no lone pairs so it forms a tethedral structure. remember also although the bonds are covalent the molecule isn't polar because the dipoles cancel out. the intermolecular forces here would be weak, probably LDF.



Now remember let's take a look at phosphorus right? it has 5 valence electrons. now it forms bonds with 3 Hydrogen atoms to fill its octet. but wait, you see that phosphrous has a lone pair of electrons right? well it's going to be trigonal pyramidal. and also phosphrous is more electronegative than hydrogen so it's going to be polar. you'd expect maybe some dipole dipole interactions, and LDF. weak IMF.

another thing to point out: when we have a lone pair of electrons what happens is that it actually shrinks the bond angles. because this is all based off of VESPR meaning that when we determine molecular shape we want to ensure that our bond angles are far apart as possible, to minimize repulsions, fancy term for like charges in this case electrons not wanting to be too close to each-other. the more lone pairs we have, meaning electrons that don't participate in bonding, the bond angles actually get smaller. so we would expect that the bond angles would be a bit smaller than the ones in CH4.