Electrons orbit around the nucleus of an atom. Each element has a different configuration of electrons, as the number of orbitals and energy levels varies between types of atoms. An orbital is a space that can be occupied by up to two electrons, and an energy level is made up of sublevels that sum up to the quantum number for that level. There are only four known energy levels, and each of them has a different number of sublevels and orbitals.
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 Periodic table of the elements

Determine the energy level of the atom. Denote it as "n." The row number in the periodic table is equal to the energy level for all types of elements, except lanthanides, actinides and transition metals. For example, carbon has an energy level of n=2, and calcium has an energy level n=4. Transition metals have an energy level of 1 less than the row number; for example, tungsten has an energy level of n=5.

Find the sum of the sublevels in the energy level. The number of sublevels an energy level has is equal to the energy level, so energy level n=3 has 3 sublevels. Denote the first sublevel as "s," the second sublevel as "p," the third sublevel as "d" and the fourth sublevel as "f." Thus, the first energy level, n=1, has only sublevel, s, while the third sublevel, n=3, has the sum of s, p and d. For example, oxygen has an energy level of n=2, and it has 2 sublevels, s and p.

Add up the number of orbitals in each sublevel. S has 1 orbital, p has 3 orbitals, d has 5 orbitals and f has 7 orbitals. Thus, the fourth energy level has 16 orbitals, since it has all four sublevels, s, p, d and f, which equals 1 + 3 + 5 + 7. Energy level n = 2 would have only two sublevals, s and p  or 1 + 3 = 4 orbitals.
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