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Language of Chemistry — CHEMISTRY STD 8 — Question

ICSE BoardEnglish MediumSTD 8CHEMISTRYLanguage of Chemistry3 Marks
Question
Define the term 'valency'. With reference to water $\&$ ammonia as compounds respectively, state the valency of oxygen $\&$ nitrogen. Magnesium $[2,8,2]$ has valency $2^{+}$. Give reasons.
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Answer

Valency is the number of hydrogen atoms which combine with $($or displace$)$ one atom of the element forming a compound. Compound water $\ce{H_2O}$, two atoms of hydrogen combine with one atom of oxygen to form water. Hence valency of oxygen atom is $2$ .
Compound ammonia $\ce{NH_3}$
Valency of atom nitrogen is $3$ , as it combines with $3$ atoms of hydrogen.
Valency of magnesium is $2^{+}, \ce{Mg [2,8,2]}$ Electronic configuration of $Mg$ is $2,8,2$ i.e. there are $2$ valence electrons which $Mg$ can lose and achieve stable configuration of nearest noble gas Neon. Hence is cation $Mg ^{2+}$ has valency $2^{+}$

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