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#1282
For main group elements, valence electrons can be defined as those in the electronic shell of highest principal quantum number n.[1] For example the electronic configuration of phosphorus (P) is 1s2 2s2 2p6 3s2 3p3 so that are 5 valence electrons (3s2 3p3), corresponding to a maximum valence for P of 5 as in the molecule PF5. This configuration is normally abbreviated to (Ne) 3s2 3p3, where (Ne) signifies the core electrons whose configuration is identical to the noble gas neon.
However this simple method does not work for transition metals, which have incomplete nd (i.e. 3d, 4d or 5d) subshells whose energy is normally comparable with that of the (n+1)s electrons. The valence electrons are instead defined as those outside a noble-gas core