The atomic number of the atom that does not usually form compounds is 2, which corresponds to helium (He).

Here's why:

* Noble Gases: Helium is a noble gas, a group of elements known for their inertness.

* Full Valence Shell: Helium has a complete outer shell of electrons (2 electrons). This stable configuration makes it very difficult for helium to gain or lose electrons and form chemical bonds.

* Limited Reactivity: Helium's lack of reactivity is why it is rarely found in compounds.

While there have been some reports of helium compounds forming under extreme conditions, these are extremely rare and unstable.