Question

On moving top to down in a group, the atomic density:
A.Decreases
B.Remains same
C.Can’t be said
D.Generally increases.

Answer 1

Hint:Density of a substance is defined as the ratio of mass per unit volume. If mass will increase then density will increase and if volume will increase then its density will decrease. If both the quantities are increasing then change in density will depend on the quantity which is changing at a faster rate.

Complete step by step answer:
Now let us first talk about periodic tables.
Periodic table is a table made to place the elements in a systematic order (i.e. according to the increasing atomic number). There are two components in a periodic table i.e. periods (horizontal lines) and groups (vertical lines).
Periods: The horizontal lines in the periodic table are known as periods. There are a total seven horizontal i.e. seven periods in a periodic table. And they are named as period one, period two and so on. In period first there are a total two elements i.e. hydrogen and helium. In periods $2$ and $3$ there are eight elements in each. In periods $4$ and $5$ there are $18$ elements in each and in periods $6$ and $7$ there are $32$ elements in each period. Hence are total $112$ elements. Elements of the same period have different properties.
Groups: The vertical lines in the periodic table are known as groups. There are a total eighteen groups in the periodic table. And they are named as groups $1,2,3,$ and so on. The elements of the same group have almost the same chemical and physical properties. As we down in a group then its size increases and also volume of the atom increases as the number of electrons and protons increases. But the increase in mass is greater than that of increase in volume so the density which is defined as mass per unit volume will increase generally. But if increase in volume is greater than that of increase in mass then density will decrease in that case.
So, on moving top to down in a group, the atomic density generally increases.

Hence option D is correct.

Note:
Volume of the elements on moving down in a group increases because when we move down in a group then the number of shells, in which electrons move, increases. And hence the effective nuclear charge increases. So their volume increases.