Which of the following shows variable valencies ?

The correct option is C

$\mathrm{P}$

The explanation for the correct answer

Option C

• The combining capacity of an element to form chemical bonds is called its valency.
• The valency of an element depends on the valence electrons.
• For elements having valence electrons 1,2 or 3, valence is equal to the number of valence electrons.
• For elements having valence electrons 4,5,6, or 7, valence is equal to the number of an electron to be added to complete the octet.

Valence = 8 - Number of valence electrons

• Some elements show more than one valency, which is called variable valency.
• Phosphorous ( Z=15) has the electronic configuration 2,8,5.
• So, to complete its octet, one P atom can gain 3 electrons from the other. Hence the valency is 3
• It also expands its valency to 5 because of the availability of vacant d orbit.
• Hence Phosphorous show variable valencies.

The explanation for the incorrect answers

Option A

• Name of the chemical symbol $\mathrm{Na}=\mathrm{Sodium}$
• Atomic number $=11$
• Electronic configuration = $2,8,1$
• Number of valence electrons = $1$
• Valency of Sodium ( $\mathrm{Na}$ ) = $1$

Option B

• Name of the chemical symbol $\mathrm{Mg}$ = $\mathrm{Magnesium}$
• Atomic number = $12$
• Electronic configuration = $2,8,2$
• Number of valence electrons = $2$
• Valency of Magnesium ( $\mathrm{Mg}$ ) = $2$

Option D

• Name of the chemical symbol $\mathrm{Cl}$ = $\mathrm{Chlorine}$
• Atomic number = $17$
• Electronic configuration = $2,8,7$
• Number of valence electrons = $7$
• Valency of Chlorine ( $\mathrm{Cl}$ ) = $8-\mathrm{valence}\mathrm{electrons}$= $8-7=1$

Conclusion: Hence option C is correct.