Question

Answer the following questions:

(a) What are halogens?

(b) Which Group and Subgroup are they placed in?

(c) What is their valency?

(d) Why are they called halogens?

(e) Why do they not occur free in nature?

Answer 1

(a)

• Elements present in Group 17 or VIIA of the periodic table are termed as halogens. It includes Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), Astatine (At) and Tennessine (Ts).

(b)

• Elements present in Group 1, 2, 13 to 17 have a valence shell (outermost shell) not completely filled and are termed as normal elements or representative elements. These Groups lie in Subgroup A.
• Since halogens are present in Group 17 of the periodic table, they lie in the category of Subgroup A.

(c)

• Valence electrons of an element is defined as the number of electrons present in the outermost shell of that element.
• In the case of Group 13 to 18, the formula to calculate the number of valence electrons is:

$\mathrm{Number}\mathrm{of}\mathrm{valence}\mathrm{electrons}=\mathrm{Group}\mathrm{number}-10$

Substitute 17 for Group number to calculate the number of valence electrons in halogens.

$\begin{array}{rcl}\mathrm{Number}\mathrm{of}\mathrm{valence}\mathrm{electrons}& =& 17-10\\ & =& 7\end{array}$

• Valency of an element is the tendency of an element to accept or lose electrons to form a molecule with other element. The valency of elements with valence electrons 5, 6, 7 can be calculated by subtracting valence electrons from 8.
• Valency of T is calculated as:

$\begin{array}{rcl}\mathrm{Valency}\mathrm{of}\mathrm{T}& =& 8-7\\ & =& 1\end{array}$

(d)

• Since halogens undergo a reaction with the metal to form a salt, they are termed as halogens. Example, Sodium ($\mathrm{Na}$), a metal, reacts with Chlorine gas (${\mathrm{Cl}}_2$) to form a salt Sodium chloride ($\mathrm{NaCl}$).
• $2\mathrm{Na}\left(\mathrm{s}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)\to 2\mathrm{NaCl}\left(\mathrm{s}\right)$

(e)

• Since halogens have seven valence electrons, they will readily gain one electron from the other element to complete their octet (eight electrons in the valence shell). Hence, halogens generally occur in the combined state.
• Example: $2\mathrm{Na}\left(\mathrm{s}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)\to 2\mathrm{NaCl}\left(\mathrm{s}\right)$
• In this reaction each $\mathrm{Cl}$ atom will gain one electron from each Na atom to form two molecules of $\mathrm{NaCl}$.