noun, used as a suffix, counter
Chemistry suffix/counter used to indicate the combining power of an element or ion, as in 原子価 (atomic valence) or イオン価 (ion valency).
See also: 原子価 (げんしか) · イオン価 (イオンか)
炭素の原子価は4です。
The valence of carbon is 4.
このイオンの価は2です。
The valency of this ion is 2.
原子価 is the full compound meaning 'atomic valence', while 価 alone is the suffix/counter used in such compounds.
From Chinese 價 (jià), meaning 'value' or 'price', adopted in Japanese chemistry terminology to denote valence.