Question

The correct order of density of the following elements $\mathrm{Be},\mathrm{Mg},\mathrm{Ca},\mathrm{Sr}$ is

• A. $\mathrm{Be}>\mathrm{Mg}>\mathrm{Ca}>\mathrm{Sr}$
• B. $\mathrm{Ca}>\mathrm{Mg}>\mathrm{Be}>\mathrm{Sr}$
• C. $\mathrm{Mg}<\mathrm{Ca}<\mathrm{Sr}<\mathrm{Be}$
• D. $\mathrm{Ca}<\mathrm{Mg}<\mathrm{Be}<\mathrm{Sr}$

Answer 1

The correct option is D

$\mathrm{Ca}<\mathrm{Mg}<\mathrm{Be}<\mathrm{Sr}$

The explanation for the correct option:

Option (D): $\mathrm{Ca}<\mathrm{Mg}<\mathrm{Be}<\mathrm{Sr}$

1. Density is defined as the ratio between mass and volume or mass per unit volume.
2. The periodic table does not have a clearly defined density trend because the densities do not follow a consistent pattern in relation to any element properties.
3. The force of attraction in group $2$elements is proportional to the element's size. As the size of the element grows larger, the volume of the atom increases, and the density decreases.
4. The molecular mass of the bottom elements is now quite large in comparison to their volume, and the density is increasing due to the strong attraction force.
5. Therefore, for moving down the group the density decreases first and then increases.
6. Hence, it is the correct option

The explanation for the incorrect option:

Option (A): $\mathrm{Be}>\mathrm{Mg}>\mathrm{Ca}>\mathrm{Sr}$

1. For moving down the group the density decreases first and then increases as there is no specific trend for density.
2. Strontium is the last element among the group of Be, Mg, Ca and Sr Hence it will have a high density.
3. Therefore, it is an incorrect option.

Option (B): $\mathrm{Ca}>\mathrm{Mg}>\mathrm{Be}>\mathrm{Sr}$

1. As there is no specific trend in the periodic table for density, the density decreases from Beryllium to Magnesium and then increases from Calcium to Strontium.
2. Strontium having the highest molecular mass will have a high density.
3. Hence, it is an incorrect option.

Option (C): $\mathrm{Mg}<\mathrm{Ca}<\mathrm{Sr}<\mathrm{Be}$

1. The volume of the atom increases as the size of the element grows larger, while the density decreases. The density of the bottom elements is increasing due to the strong pull force, and their molecular mass is now fairly enormous in contrast to their volume.
2. Beryllium will have a density less than Strontium as its molecular mass is less than Strontium. Also, from Beryllium to magnesium density decreases and then increases from Calcium to Strontium.
3. Hence. it is an incorrect option.

Therefore, the correct order of density of the following elements $\mathrm{Be},\mathrm{Mg},\mathrm{Ca},\mathrm{Sr}$ is $\mathrm{Ca}<\mathrm{Mg}<\mathrm{Be}<\mathrm{Sr}$, option (D) is the correct answer.