Glossary
valence electron
A valence electron sits in an atom's outermost occupied shell, and the count there decides which bonds the atom forms and what charge its ion carries.
A valence electron is one of the electrons in an atom's outermost occupied shell. That shell is where chemistry happens: the inner shells are full and stable, and the outer count decides how many bonds the atom forms, the charge of its ion, and its group on the periodic table. Sodium has one and gives it away, reaching the full shell beneath it; chlorine has seven and takes one. Both are moving toward the octet rule — eight electrons in the outer shell, which the noble gases already have, so they barely react.
Shell capacity is fixed for the elements a course like this covers: 2 in the first shell, 8 in the second, 8 in the third. Count electrons in from the inside out, using the atomic number; the last shell's count is the answer. So oxygen (8) fills 2 then 6 and has six valence electrons; magnesium (12) is 2, 8, 2 and has two.
Two traps. A full outer shell is not a full atom — helium has two electrons in total and a full shell. And valence count repeats down a group rather than across a period, which is why the alkali metals behave alike.
Taught in
Related terms
- ionic bond — An ionic bond is the attraction between two oppositely charged ions, formed when one atom gives electrons to another — a metal and a nonmetal, usually.
- covalent bond — A covalent bond is a shared pair of electrons between two atoms, the usual bond between nonmetals and the bond that holds molecules together.
Source: NIST periodic table and reference data for the atomic masses and half-life figures, NIH PubChem for the substance data, and NIST Special Publication 811 for the unit conventions — the same sources the chemistry guide cites. All three are US federal government material, not subject to copyright in the United States. Every definition, example and calculation above was written for this course; no dictionary or textbook text is reproduced. (NIST and PubChem accessed 2026-10-26; definitions written 2026-10-27)