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I know that valence is defined in the Gold Book, and valency, electrovalency, and covalency are not defined. However, there are some terms that are confusing me. According to the given definitions, a metal shows positive electrovalency and a nonmetal shows negative electrovalency in the bonding of a metal and a nonmetal. For example, for $\ce{MgO}$, electrovalency of $\ce{Mg}$ is $+2$ and that of $\ce{O}$ is $-2$. I am not comfortable assigning a sign to valence:

Valence: The maximum number of univalent atoms (originally hydrogen or chlorine atoms) that may combine with an atom of the element under consideration, or with a fragment, or for which an atom of this element can be substituted.

According to IUPAC, valence or valency (which is not defined) should always be positive.

Further the textbook says that valency of $\ce{H}$ is always $+1$. What confuses me further is the concept of covalency, where the shared electrons result in a positive valency regardless of the electronegativity of the atoms involved.

References:

Valence, Variable Valence, and Valency

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    $\begingroup$ The whole concept of valence, with whatever pre- or suffix, is old and flawed and has probably changed meaning throughout the centuries a couple of times. There is a reason for why there are no official definitions: the terms are ambiguous and no longer useful in scientific communication. See them as prose that helps a paper come together for your reading pleasure. What dies it matter what hydrogen's valence is when it bond to two fluoride ions? $\endgroup$ Nov 18, 2023 at 17:41
  • $\begingroup$ Electrovalency or electrovalence were two synonynous terms used in the first half of the 20th century to describe the charge of a metallic cation. Some chemists used electrovalence, and some electrovalency. Today both terms have disappeared. $\endgroup$
    – Maurice
    Nov 18, 2023 at 21:13
  • $\begingroup$ @Martin-マーチン you raise a vital issue regarding the concept of valence, I wonder why valence is still defined in the Gold Book. I guess it only works as a rule of thumb. $\endgroup$
    – ananta
    Nov 20, 2023 at 1:36
  • $\begingroup$ @Maurice clearly, since it is still part of textbooks and the Gold Book, the terms and related concept has not disappeared and is used regularly by chemists to explain several ideas. $\endgroup$
    – ananta
    Nov 20, 2023 at 1:37
  • $\begingroup$ It's used by teachers to waste students' time ;> $\endgroup$
    – Mithoron
    Nov 21, 2023 at 17:52

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