These electrons contribute to the shape and reactivity of the molecule, but do not
directly bond the atoms together. Most Lewis structures are based on the concept that
eight valence electrons (corresponding to s and p electrons outside the noble gas core)
form a particularly stable arrangement, as in the noble gases with s2p6 configurations. An
exception is hydrogen, which is stable with two valence electrons. Also, some molecules
require more than eight electrons around a given central atom. Simple molecules such as
water follow the octet rule, in which eight electrons surround the oxygen atom. The
hydrogen atoms share two electrons each with the oxygen, forming the familiar picture
with two bonds and two lone pairs: