A heliumlike arrangement with two electrons filling the first shell around the nucleus is a particularly favorable situation, as we saw previously. Now let us imagine constructing a series of atoms by adding electrons to helium one at a time in the second electron shell, making the corresponding addition of protons in the nucleus to keep the atoms electrically neutral. The atomic structures of the first ten atoms are diagramed at the top of the previous page.

The number of protons is shown within each nucleus (gray) and the number of electrons is indicated outside. The diagram at the top of this page, focuses attention on the inner two electron shells, showing how they are gradually filled from hydrogen to neon. Electrons are represented by black dots (labelled with a 'minus' in white) with a maximum of two in the inner shell and eight in the second shell. Partially filled shells are white and filled shells are colored.