valence electron
Technical term in chemistry and physics for an electron in the outermost shell of an atom that can participate in chemical bonding.
The number of valence electrons determines the chemical properties of an atom.
原子価 (valence) is the combining capacity of an atom, often determined by the number of 価電子.
Compound of 価 (ka, 'valence') and 電子 (denshi, 'electron'). A straightforward technical coinage.