How many energy levels (shells) does an atom of oxygen have? How do you know this by looking at the periodic table?

Explanation:

An atom of oxygen has two energy levels (shells) because that is the principal quantum number of the outermost electrons.

An atom of oxygen has atomic number 8, which is equal to the number of protons and electrons (for neutral atoms).

Thus the electron configuration is:

There you see that there are 2 electrons in the first energy level (n = 1) and 6 electrons in the second energy level (n = 2).

You can know this by looking at the periodic table because the number of energy levels (shells) is equal to the period of the atom, i.e. the row in the periodic table. Oxygen is the the second row of the periodic table; therefore it has two energy levels.