Question

what specific name is given to the number of hydrogen atoms that can combine with or be displaced by one atom of an element

Answer 1

Valency is the number of hydrogen atoms that can combine with [or displace] one atom of the element [or radical] to form a compound. For example, one atom of hydrogen combines with one atom of chlorine to form hydrogen chloride [HCl]; so, the valency of chlorine [chloride] is one. Similarly, the valency of the nitrate radical [NO3] in the compound nitric acid [HNO3] is 1, and the valency of the sulfate radical in the compound sulfuric acid [H2SO4] is 2.

For elements that do not combine with hydrogen, the valency is the combining power of the element with another element whose valency is known. Valency may also be defined as the number of electrons that an atom donates or accepts to form the duplet state (i.e., 2 electrons in outermost shell) or octet state (i.e., 8 electrons in outermost shell).

The valency of an element [or radical] is always a whole number. Elements [or radicals] with valency one are monovalent, those with valency two are divalent, and those with valency three are trivalent.