Question

What is the valency of:
i) Fluorine in ${\text{Ca}}{{\text{F}}_2}$
j) Sulfur in ${\text{S}}{{\text{F}}_6}$
k) Phosphorous in ${\text{P}}{{\text{H}}_3}$
l) Carbon in ${\text{C}}{{\text{H}}_4}$
m) Nitrogen in the following
(v) ${{\text{N}}_2}{{\text{O}}_3}$
(vi) ${{\text{N}}_2}{{\text{O}}_5}$
(vii) ${\text{N}}{{\text{O}}_2}$
(viii) ${\text{NO}}$
n) Manganese in ${\text{Mn}}{{\text{O}}_2}$
o) Copper in ${\text{C}}{{\text{u}}_2}{\text{O}}$
p) Magnesium in ${\text{M}}{{\text{g}}_3}{{\text{N}}_2}$

Answer 1

Hint: Valency of an atom or a molecule is the measurement of its combining capacity i.e., its ability to combine with atoms of other elements. Valency can also be defined as the ability of an element to add or displace hydrogen atoms.

Complete step by step solution:
i. We know the valency of Ca is +2 (as by losing two electrons Ca attains the noble gas configuration). If we consider the valency of fluorine as x, we can write;

2 + 2x = 0 (since ${\text{Ca}}{{\text{F}}_2}$ is a neutral molecule, the valencies of Ca and F will be equal and opposite, the 2x is written because there are two fluorine atoms attached to one Ca atom)
$ \Rightarrow $ 2x = -2
$ \Rightarrow $x = $\dfrac{{ - 2}}{2}$
$ \Rightarrow $x = -1

Therefore, In ${\text{Ca}}{{\text{F}}_2}$ the fluorine has a valency of -1, almost in every case halogens have the -1 valency.

j. In the previous solution, we have found that the valency of F is -1. And we know that the valency of an element remains the same in most cases.

If we consider the valency of sulfur in ${\text{S}}{{\text{F}}_6}$ to be x, then we can write;

X + 6(-1) = 0 (${\text{S}}{{\text{F}}_6}$ is also a neutral molecule, therefore the valencies of F and S will be equal and opposite)

$ \Rightarrow $x = 6
Therefore, the valency of sulfur is +6 in ${\text{S}}{{\text{F}}_6}$

k. Using the definition of valency given in the hint, in this case, ${\text{P}}{{\text{H}}_3}$ we will consider the number of hydrogen-bonded with the phosphorus atom. 3 hydrogens are attached to P
Hence, we can conclude that the valency of phosphorus is +3 in phosphine or ${\text{P}}{{\text{H}}_3}$
l. As solved in the previous solution, considering that the 4 hydrogens are bonded with the carbon we can say that the valency of carbon ${\text{C}}{{\text{H}}_4}$ is +4
m.
v. In ${{\text{N}}_2}{{\text{O}}_3}$ if we consider that nitrogen has a valency of x, and we know the valency of oxygen is -2 (by gaining 2 electrons oxygen will attain the noble gas configuration of Ne), then we can write;
2x + 3(-2) = 0 (now by using simple arithmetic equation we will solve the rest part)
$ \Rightarrow $2x = 6
$ \Rightarrow $x = 3
Therefore, the valency of nitrogen ${{\text{N}}_2}{{\text{O}}_3}$ is +3
vi. In ${{\text{N}}_2}{{\text{O}}_5}$, if we again consider the valency of nitrogen as x, and -2 as the valency of O, then then we can write;
2x + 5(-2) = 0
$ \Rightarrow $2x = 10
$ \Rightarrow $x = 5
Therefore, we can say that valency of nitrogen as +5 for ${{\text{N}}_2}{{\text{O}}_5}$
vii. In case of ${\text{N}}{{\text{O}}_2}$ again by considering the valency of nitrogen as x, and oxygen as -2, then we can write;
x + 2(-2) = 0
$ \Rightarrow $x = 4
Therefore, the valency of nitrogen in ${\text{N}}{{\text{O}}_2}$ is + 4

viii. As solved in the previous cases, we will take the valency of nitrogen as x, and valency of oxygen as -2 in NO. therefore, we can write;
x + (-2) = 0
$ \Rightarrow $x = 2
Hence, in case of NO the valency of nitrogen is + 2
As said before valency of an atom is not always the same, and nitrogen is a classic example of multiple valencies.
The valency of an atom in a compound also depends on the atom to which it is bonding.
n. In ${\text{Mn}}{{\text{O}}_2}$ we will again consider that oxygen has -2 valency and taking the valency of Manganese (Mn) as x, we can write that;
x + 2(-2) = 0
$ \Rightarrow $x = 4
Therefore, the valency of Mn in ${\text{Mn}}{{\text{O}}_2}$ is +4

o. We will be using the same valency of oxygen i.e., (-2) in${\text{C}}{{\text{u}}_2}{\text{O}}$ and consider that the valency of copper to be x, then we can write;
2x + (-2) = 0
$ \Rightarrow $x = 1
Therefore the valency of copper in ${\text{C}}{{\text{u}}_2}{\text{O}}$ is +1
p. Now, this is an interesting case, we have found that nitrogen can exist in multiple valencies. Therefore, we cannot consider the valency of nitrogen to calculate the valency of Mg.
In such a case, we will consider the group of the element (because elements belonging to the same group exhibit similar valencies). Magnesium or Mg is a 2nd group element, usually, 2nd group elements have + 2 valencies.
Therefore, the valency of Mg is +2

Note: Valency and oxidation state are not the same. The oxidation state of an atom gives the number of electrons an atom has gained or lost, in contrast to its valency.