posted by Steve .
How do you tell what type of solid (atomic, molecular, ionic, covalent network) will any substance form (i.e. PH3, H2, acetate)? I know that its mainly determined by what is at the lattice points but still have no clue how to go about it.
In general you go by the difference in electronegativity. For exmple, H2 is bonded H-H and since both have the same EN, then the difference is zero and the gas is covalent. In CsF, the EN for Cs is about 0.8 and that of F is 4.0, the difference is 3.2 and CsF is ionic. Few compounds are purely covalent or ionic; however, the 50% point is about 1.9 or so. That is H2O is EN of 2.1 for H and 3.5 for O for a difference of 1.4. For practical purposes we usually call this a covalent bond since it is less than 1.9; actually, it is polar covalent. NaCl has EN of 0.9 for Na and 3.0 for Cl for 2.1 difference. We usually call this ionic but you can see its in the neighborhood of 50% mark but well over the half-way point.
For atomic or molecular, there are a few molecules, the others are atomic. H2, N2, O2, F2, Cl2, Br2, I2 are diatomic. The other atoms we write as single atoms. Most metals have a "metallic" bond. You can look that up in your text. I hope this gets you started on the right path. (With acetate ion, that is a polyatomic ion of C, H, and O and you can see there is no huge ionic character although it is polar.)