<P> Similar to an electron in an inner shell, a valence electron has the ability to absorb or release energy in the form of a photon . An energy gain can trigger an electron to move (jump) to an outer shell; this is known as atomic excitation . Or the electron can even break free from its associated atom's valence shell; this is ionization to form a positive ion . When an electron loses energy (thereby causing a photon to be emitted), then it can move to an inner shell which is not fully occupied . </P> <P> Valence energy levels correspond to the principal quantum numbers (n = 1, 2, 3, 4, 5 ...) or are labeled alphabetically with letters used in the X-ray notation (K, L, M, ...). </P> <P> The number of valence electrons of an element can be determined by the periodic table group (vertical column) in which the element is categorized . With the exception of groups 3--12 (the transition metals), the units digit of the group number identifies how many valence electrons are associated with a neutral atom of an element listed under that particular column . </P> <Table> <Tr> <Th> Periodic table group </Th> <Th> Valence electrons </Th> </Tr> <Tr> <Td> Group 1 (I) (alkali metals) </Td> <Td> </Td> </Tr> <Tr> <Td> Group 2 (II) (alkaline earth metals) </Td> <Td> </Td> </Tr> <Tr> <Td> Groups 3 - 12 (transition metals) </Td> <Td> 3--12 </Td> </Tr> <Tr> <Td> Group 13 (III) (boron group) </Td> <Td> </Td> </Tr> <Tr> <Td> Group 14 (IV) (carbon group) </Td> <Td> </Td> </Tr> <Tr> <Td> Group 15 (V) (pnictogens or nitrogen group) </Td> <Td> 5 </Td> </Tr> <Tr> <Td> Group 16 (VI) (chalcogens or oxygen group) </Td> <Td> 6 </Td> </Tr> <Tr> <Td> Group 17 (VII) (halogens) </Td> <Td> 7 </Td> </Tr> <Tr> <Td> Group 18 (VIII or 0) (noble gases) </Td> <Td> 8 </Td> </Tr> </Table>

The number of valence electrons in an atom of an element is equal to the group number