can someone explain a diatomic molecule, monatomic gas and what would happen if a metal atom bond with a nonmetal atom?
I'm not sure what you are looking for. A diatomic gas is one (H is an example) in which the electrons from each atom helps fill the shells for both. That is H has an electron which we may write as H. and the other H likewise but I will turn it around like so .H to make H:H. By sharing electrons, each atom now has the required two electrons in its shell.
What can I say about a monatomic gas. I assume this is something about why monatomic gases are not diatomic. The answer is that the noble gases, He, Ne, etc have outside shells that are already filled; therefore, they do not need to share electrons with another atom. How do metals and non-metals combine. Take Na combining with Cl.
Na has 1 electron in its outside shell. Cl has 7 electrons in its outside shell. The Na atom loses the 1 outside shell (which makes it an ion, +1 charge, AND the outside shell that is left is full (remember Na is 2,8,1 electrons in 1st, 2nd, and 3rd shells). The Cl atom, having 7 electrons in its outside shell, needs one more to complete its octet; therefore, it takes the 1 electron from the Na atom, incorporates it into its own electron shell, and becomes a negative ion. The attractive force is now between a Na^+ ion and a Cl^- ion and that is an ionic bond.