What are some examples of covalent compounds?

The following are classic examples of : H2O CO2 CH4

Covalent bonds are formed when two share electrons. This typically happens between two non-metals.

The following compounds are not covalent: NaCl CaCl2, because they are between a metal and a non-metal.

Its also important to note that all covalent bonds are not equal. Covalent bonds can have a polarity, meaning even though two elements are sharing electrons, they are not shared equally. This polarity is determined by (the strength of an atom to pull electrons towards itself)

So for instance, oxygen has an electronegativity value of 3.5 and hydrogen has an electronegativity value of 2.1. Oxygen is quite a bit stronger at pulling electrons. So even though they share electrons, oxygen gets the lion share of them. This, along with molecular shape, is why water is considered to be a polar molecule. (Polar meaning opposite) Water has a partially negative side near the oxygen atom because it has more electrons than it normally would by itself and a partially positive side near the hydrogens because it has less electrons than it normally has.

It is important to note that electrons have a negative charge. So if an atom has more electrons, its negative; and if an atom has less electrons, its positive.

Its a gray line as to when a bond is Ionic or Covalent. I've seen different cut off values, but I typically use a difference of 1.67 as the cut off between Covalent and Ionic bonds.

So for instance the electronegativity value of Na is 0.9 and the value of Chlorine is 3.0. The difference between those two elements is 2.1 which is greater than 1.67; therefore, that compound is Ionic. Its ionic because chlorine is so much stronger at pulling electrons than sodium, so it is able to steal its electron rather than sharing it.