<P> The electron shells are labeled K, L, M, N, O, P, and Q; or 1, 2, 3, 4, 5, 6, and 7; going from innermost shell outwards . Electrons in outer shells have higher average energy and travel farther from the nucleus than those in inner shells . This makes them more important in determining how the atom reacts chemically and behaves as a conductor, because the pull of the atom's nucleus upon them is weaker and more easily broken . In this way, a given element's reactivity is highly dependent upon its electronic configuration . </P> <P> Each shell is composed of one or more subshells, which are themselves composed of atomic orbitals . For example, the first (K) shell has one subshell, called 1s; the second (L) shell has two subshells, called 2s and 2p; the third shell has 3s, 3p, and 3d; the fourth shell has 4s, 4p, 4d and 4f; the fifth shell has 5s, 5p, 5d, and 5f and can theoretically hold more but the 5f subshell, although partially occupied in actinides, is not filled in any element occurring naturally . The various possible subshells are shown in the following table: </P> <Table> <Tr> <Th> Subshell label </Th> <Th> l </Th> <Th> Max electrons </Th> <Th> Shells containing it </Th> <Th> Historical name </Th> </Tr> <Tr> <Td> s </Td> <Td> 0 </Td> <Td> </Td> <Td> Every shell </Td> <Td> sharp </Td> </Tr> <Tr> <Td> p </Td> <Td> </Td> <Td> 6 </Td> <Td> 2nd shell and higher </Td> <Td> principal </Td> </Tr> <Tr> <Td> d </Td> <Td> </Td> <Td> 10 </Td> <Td> 3rd shell and higher </Td> <Td> diffuse </Td> </Tr> <Tr> <Td> f </Td> <Td> </Td> <Td> 14 </Td> <Td> 4th shell and higher </Td> <Td> fundamental </Td> </Tr> <Tr> <Td> g </Td> <Td> </Td> <Td> 18 </Td> <Td> 5th shell and higher (theoretically) </Td> <Td> (next in alphabet after f, excluding j) </Td> </Tr> </Table> <Tr> <Th> Subshell label </Th> <Th> l </Th> <Th> Max electrons </Th> <Th> Shells containing it </Th> <Th> Historical name </Th> </Tr>

The maximum number of electrons that the p subshell can hold is