Valence Electrons

valence electrons:  the outer electrons of an atom that are available to participate in chemical reactions.

In most atoms, these are the electrons in the s and p sub-levels of the highest (numbered) energy level.

For example,  phosphorus (P) has the electron configuration:  1s2 2s2 2p6 3s2 3p3, or [Ne] 3s2 3p3.  The highest numbered energy level is level 3.  The 3s2 3p3 at the end of its electron configuration tells us that magnesium has 2 electrons in the 3s sub-level and 3 in the 3p sub-level, for a total of 5 valence electrons.  This means that phosphorus has 5 electrons available to participate in chemical reactions.


Noble gases have “full” valence shells.  This means that helium has 2 valence electrons (because it has only a 1s sub-level), and all other noble gases have 8 valence electrons (because their highest-numbered s sub-level has 2 electrons, and their highest-numbered p sub-level has 6 electrons, for a total of 8.)

Phosphorus ([Ne] 3s2 3p3) needs 3 more electrons to fill its 3p sub-level.  This would give it 8 valence electrons, just like argon (the nearest noble gas on the periodic table).  This means that phosphorus is most likely to gain 3 electrons, which means that it’s most likely to form an ion with a −3 charge.

Potassium ([Ar] 4s1) has only one valence electron.  If it loses that one valence electron, it will have the same electron configuration as argon, the nearest noble gas on the periodic table.  This means that potassium is most likely to lose 1 electron, which means that it’s most likely to form an ion with a +1 charge.

Because the energy of an s sub-level is so close to the energy of the d sub-level of the next lower energy level, transition metals can easily shift electrons between these s and d sub-levels.  This means they can have different numbers of valence electrons, depending on the situation.  For example, copper can have the electron configuration [Ar] 4s2 3d9, or [Ar] 4s1 3d10, meaning that copper can have either one or two valence electrons.  This explains why copper is observed to sometimes have a +1 charge, and other times a +2 charge.


Group Numbers

You can read the number of valence electrons that an element has directly from the periodic table, using the “old-style” group numbers (Roman numeral plus either the letter “A” or “B”).  For the “A” elements (representative elements, in the s and p blocks of the periodic table), the Roman numeral is the number of valence electrons.  For the “B” elements (transition metals, in the d and f blocks of the periodic table), the Roman numeral is the number of apparent valence electrons that might be expected for elements in the group.  (This prediction works best for groups III B, IV B, I B and II B, and less well for groups V B through VIII B.)