Valence, also spelled valency, in chemistry, the property of an element that determines the number of other atoms with which an atom of the element can combine. Introduced in 1868, the term is used to express both the power of combination of an element in general and the numerical value of the power of combination.
The explanation and the systematization of valence
was a major challenge to 19th-century chemists. In the absence of any
satisfactory theory of its cause, most of the effort centred on devising empirical rules for determining the valences of the
elements. Characteristic valences for the elements were measured in terms of
the number of atoms of hydrogen with
which an atom of the element can combine or that it can replace in a compound. It became evident, however, that the valences of
many elements vary in different compounds. The first great step in the development of a
satisfactory explanation of valence and chemical combination was made by the
American chemist G.N. Lewis (1916) with the identification of the
chemical bond of organic compounds with a pair of electrons held jointly by two
atoms and serving to hold them together. In the same year, the nature of the
chemical bond between electrically charged atoms (ions) was discussed by German
physicist W. Kossel. After the development of the detailed electronic theory of
the periodic system of the elements, the theory of valence was reformulated in
terms of electronic structures and interatomic forces. This situation led to
the introduction of several new concepts—ionic valence, covalence, oxidation number, coordination number, metallic valence—corresponding to different
modes of interaction of atoms.
Q1. Why do Helium, Neon
and Argon have zero valency?
Q2. Helium has 2
electrons in its valence shell but its valency is not 2. Do you agree?
Q3. Ramesh answered that
35Cl and 37Cl have different valences. Do agree with his
answer, justify.
Q4. An atom with 3
Protons and 4 Neutrons will have a valency ………………
ANSWER KEY-
(1)Full credit: Helium has 2 electrons in its only energy shell, while Argon and Neon have 8 electrons in their valence shells. As these have maximum number of electrons in their valence shells, they do not have any tendency to combine with other elements. Hence they have a tendency equal to zero.
No credit: Other responses and missing.
(2). Full credit:No. Its duplet is complete. Hence the valency is zero.
No credit: Other responses and missing.No credit: Other responses and missing
Full credit: ANS-1.
No credit: Other responses and missing
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