Pick two elements from the table. A metal + nonmetal transfer electrons and become oppositely charged ions: an ionic bond. Two nonmetals share pairs of electrons: a covalent bond.
+→
Metal (loses electrons)Nonmetal (gains or shares electrons)Can bond with your first pickNot included here
What's left out, and why
Only pairs that follow the simple rules are included: metals lose all their valence electrons, nonmetals gain or share until they have 8 (hydrogen: 2), and every compound shown really exists with that formula.
Transition metals, metalloids, noble gases, and the radioactive Fr and Ra are greyed out.
Beryllium is a metal, but its compounds (like BeCl₂) are covalent, so it's left out.
Aluminum pairs only with oxygen and fluorine. Its chloride, bromide, iodide, sulfide, and nitride are largely covalent.
Nitrides appear only for lithium and the group 2 metals; the other alkali metals don't form stable ones. Metals with H, C, or P (hydrides, carbides, phosphides) are left out.
Nonmetal pairs are left out when the octet rule doesn't predict the real molecule: N + O and S + O (NO, NO₂, SO₂…), P + O, C + N, sulfur fluorides (SF₆), and unstable ones like NBr₃ and NI₃. C, P, S, and Se bonded to themselves form chains and rings, not simple molecules.