Question

The valency of gold in $AuCl$ is
A. $1$
B. $2$
C. $3$
D. $4$

Answer 1

The valency is defined as the capacity of an element to form bonds with other elements. The element has to combine with other elements to form bonds.

Complete answer:
According to Aufbau’s rule and Hund’s rule the electrons are arranged in the orbitals in specific order. The electrons that are present in the outermost orbital/shell are labeled as valence electrons.
The outermost electrons are the ones which take in chemical bonding upon excitation. The count of the number of electrons on an atom required during bond formation with other atoms is called the valency of that atom.
Thus a chemical compound is formed by combining two or more atoms and in the process the valency of each atom is satisfied. The valency is obtained by considering the atom to reach the nearest noble gas electronic configuration or by filling up the octet.
For example if an atom like carbon has $4$ electrons in its outermost shell, the valency of carbon is $4$. But if an atom has more than $4$ electrons like oxygen which has $6$ electrons, the valency is obtained by subtracting the number of valence electrons from the octet number of electrons. Thus the valency of oxygen is $2$.
The given compound is gold chloride. It has a single bond between gold atom and chlorine atom. Thus one electron from each of gold and chlorine is involved in the bond in $AuCl$. The chlorine atom has $7$ electrons in its outermost shell. So the valency of chlorine is $8 - 7 = 1$ .
Since only one bond is formed between gold and chlorine in $AuCl$, so the chlorine with valency $1$ will combine with one gold atom.
Hence the valency of gold in $AuCl$ is $1$ , i.e. option A is the correct answer.

Note:
The valency of an element is not fixed. Elements tend to show variable valency depending on the compound formed or the element it combines with. For example gold shows two valencies 1 and 3 and hence it combines with chlorine to form $AuCl$ and $AuC{l_3}$ .