Question

What are the valencies of $\text{Si}$ and $\text{Cl}$ with atomic numbers 14 and 17 respectively?
A) \[\text{ }3,2\]
B) \[\text{ 4},2\]
C) \[\text{ 2},3\]
D) \[\text{ 4},1\]

Answer 1

Hint: The valency is defined as the number of electrons which are said to be gained, shared or lost by an element to complete its octet.The group 14 elements have the 4 electrons in the outermost shell. Since it is difficult to lose or gain the 4 electrons to complete the octet. They share their electrons to attain stability. The group 17 elements can
complete octet by accepting an electron.
Complete step by step answer:
The valency of an element is the combining or reactivity property of an element. The valency is the number of electrons of an element that are used to combine with the other elements or atoms to form a bond.
The valence electrons of an element are used to combine with the other elements however not every valence electron is available for bonding.
The valency of the element can be also given as the number of valence electrons gained or lost by an atom or element to attain the noble gas configuration.
noble gas has the completely filled outermost shell. They are less reactive. The reactivity of elements depends on the ability of an element to attain the noble gas configuration.
For example the valency for the alkali metal is 1.For sodium the outer electronic configuration is $\text{1}{{\text{s}}^{2}}\text{2}{{\text{s}}^{2}}\text{2}{{\text{p}}^{6}}\text{3}{{\text{s}}^{1}}$ .
The last shell which is $\text{3s}$holds an electron. The sodium element loses its $\text{3s}$ shell electron to attain the noble gas configuration of $\text{Ne}$ gas. Thus the valency for sodium is one.
Let’s have a look at the elements given in question.
a) The silicone $\text{(Si)}$ is group 14 element. The electronic configuration of silicon as follows:
$\text{S}{{\text{i}}_{\text{14}}}\text{= 1}{{\text{s}}^{\text{2}}}\text{ 2}{{\text{s}}^{\text{2}}}\text{ 2}{{\text{p}}^{\text{6}}}\text{ 3}{{\text{s}}^{\text{2}}}\text{ 3}{{\text{p}}^{\text{2}}}$
The last shell which is the third shell contains the 4 electrons in it.To attain the nearest noble gas configuration of argon it has to either lose or gain the 4 electrons. Since it is very difficult to lose or gain the four electrons as it required higher energy to break the bonds. Thus the group 14 elements prefers to share the electrons to attain the Noble gas configuration. The $\text{Si}$forms four covalent bonds with the other element.
Thus the valency of silicon is 4.
\[\begin{matrix}
   {} & \text{ }\!\!|\!\!\text{ } & {} \\
   \text{-} & \text{Si} & \text{-} \\
   {} & \text{ }\!\!|\!\!\text{ } & {} \\
\end{matrix}\]
b) The chlorine is a group 17 elements. The electronic configuration of chlorine is as follows:
$\text{C}{{\text{l}}_{\text{17}}}\text{= 1}{{\text{s}}^{\text{2}}}\text{ 2}{{\text{s}}^{\text{2}}}\text{ 2}{{\text{p}}^{\text{6}}}\text{ 3}{{\text{s}}^{\text{2}}}\text{ 3}{{\text{p}}^{5}}$
The third shell contains the total of seven electrons .To attain the nearest noble gas configuration of $\text{A}{{\text{r}}_{\text{18}}}$ it requires the one electron. Chlorine cannot lose its seven electrons. It is hard to lose 7 electrons so the chlorine completes its octet by accepting a 1 electron. It accepts 1 electron hence its valency is 1.
For example in$\text{NaCl}$, the sodium has the valency of one. Sodium loses its third shell electrons which take part in bonding. The chlorine has seven electrons in last shell electronic configuration. Since it can accept an electron from the sodium to attain stability .Thus chlorine holds the eight electrons in the last shell.

Hence, (D) is the correct option.

Note: in periodic table, the valency of the elements increases from the group 1 elements to the group 14 elements. It then decreases from group 15 to group 18. The valency of group 18 is zero.