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.