The remaining eight elements shown
are built up by adding more electrons, one at a time, and placing
them in the lowest-energy orbital that is still unfilled. The 2s
orbital is slightly lower in energy than the 2p, so it is
filled with two electrons (Be) before the third electron goes into
the first of three 2p orbitals in boron. Because placing
two electrons in the same orbital means placing them close to one
another in space, there is an electron-electron repulsion between
them. This means that the first three electrons in 2p orbitals
(B through N)
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go into the three different 2p
orbitals, before a pairing of two electrons in one orbital (in an
O atom) is forced by the lack of more empty orbitals of the same
energy.
The electronic configuration is given by listing the orbitals in
order of increasing n and I values,
and indicating the number of electrons in each orbital with a superscript.
The electronic configurations proceed in a systematic way, and are
shown at the bottom of the diagrams across the top of these pages.
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