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

We now can return to the shell models discussed in the first six chapters, and reinterpret them in terms of quantum levels of the atom. The first ten elements are diagramed across the top of this page (and pages 10 and 11). The hydrogen atom has only a single electron, and in its lowest-energy state, or ground state, the electron occupies the 1s quantum level.

The helium atom has two electrons, and in its ground state both will occupy the 1s orbital as shown.


Electrons have one more property that has not been mentioned so far, spin. Electrons behave magnetically as if they were tiny spinning spheres of negative charge, with the north pole pointing either up or down. Two electrons can occupy the same orbital if, and only if, they have opposite spin directions. Hence we say that the 1s orbital in helium is occupied completely by a pair of electrons of opposite spins.

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