Question

Which of the following pairs of atoms do not exhibit a covalent bond?

Cs and F
O and O
S and O
P and Cl

Answer 1

look for the pair with both metals and non-metals. The metals are on the left of the periodic table, and non-metals are to the right. If you look at the periodic table, it should be obvious. In this case the only pair that has both a metal and a non-metal is A, so that's your answer.

Why is that the case? Bond characteristics are related to the element's electronegativity value. The Pauling's scale (another "version" of the periodic table) is a table that shows you these values. In a diatomic molecule (or ionic compound), just take the more electronegative one and subtract it with the lesser one. The difference will tell you what kind of bonding it is, generally speaking. The scale is like this:

Ionic bond = difference in about 1.7 between the elements
Covalent bond = difference less than 1.7

Notice I said "generally speaking." This is a useful trend that you can use to determine if it's ionic or not, but there's always exceptions