8. The Machinery Behind The      Periodic Table   Previous PageNext Page
     Quantum Levels and Atomic Buildup

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)


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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