Question

The atomic number of third alkali metal is:
A. $3$
B. ${\text{11}}$
C. ${\text{19}}$
D. ${\text{37}}$

Answer 1

Hint: The alkali metals belong to the group 1 of the periodic table. There are in all six alkali metals. The third alkali metal is potassium $\left( {\text{K}} \right)$.
The atomic number is the number of protons in the nucleus of the atom of the element.

Complete step by step answer:
Step 1:
The periodic table is an arrangement of elements in an increasing order of their atomic numbers. The periodic table consists of groups (vertical columns) and periods (horizontal rows). Alkali metals belong to the group 1 of periodic table.
The alkali metals are lithium $\left( {{\text{Li}}} \right)$, sodium $\left( {{\text{Na}}} \right)$, potassium $\left( {\text{K}} \right)$, rubidium $\left( {{\text{Rb}}} \right)$, caesium $\left( {{\text{Cs}}} \right)$ and francium $\left( {{\text{Fr}}} \right)$.
Step 2:
The atomic number is the number of protons in the nucleus of the atom of the element.
The atomic numbers of the alkali metals are lithium $\left( {{\text{Li}} - 3} \right)$, sodium $\left( {{\text{Na}} - 11} \right)$, potassium $\left( {{\text{K}} - 19} \right)$, rubidium $\left( {{\text{Rb}} - 37} \right)$, caesium $\left( {{\text{Cs}} - 55} \right)$ and francium $\left( {{\text{Fr}} - 87} \right)$.
The third alkali metal is ${\text{K}}$ (potassium).
The atomic number of ${\text{K}}$ (potassium) is ${\text{19}}$.
Thus, the atomic number of third alkali metal is ${\text{19}}$.
So, the correct answer is “Option A”.

Additional Information: The elements of group 1 of periodic table are highly reactive in water and air. Thus, they are stored under oil or kerosene.
The elements of group 1 on reaction with water form an alkaline solution and thus, they are known as alkali metals.

Note: In the periodic table, the elements are arranged in an increasing order of their atomic numbers. The horizontal rows are known as periods while the vertical columns are known as groups. The alkali metals are group 1 elements of the periodic table.