Question

Density of elements across a period:
A.Increases
B.Decreases
C.Constant
D.None of these

Answer 1

Hint:To answer this question, you must recall the trends properties in the modern periodic table. We know that density is the mass present per unit volume of a substance. Thus, we need to study the trend of atomic volume or the atomic radius across a period.
Formula used:
 $\text{Density}={\displaystyle \frac{\text{atomic weight}}{\text{atomic volume}}}$

Complete step by step answer:
From the formula of density we can see that density is inversely proportional to the volume of an atom. We also know that the volume of an atom is directly proportional to its radius. Thus, we can conclude that the density is inversely proportional to the atomic radius.
Atomic radius is the most common way of expressing the size of an atom. It can be termed as half of the distance between the nuclei of two identical atoms bonded together.
When we move from left to right in a period, protons and electrons are added to the same energy level. As the nuclear charge increases, the pull on the electrons increases and thus the size decreases. But ultimately in the last elements of the period, the size increases due to inter electronic repulsions.
Since atomic radius decreases across a period we can say that the density across a period increases as the change in mass is not as significant as the change in volume.

Hence, the correct answer is A.

Note:
The trend in the atomic radius is also affected by several other factors. Shielding effect of the inner shells tend to increase the size of the atoms and if the shielding is poor, like in the cases of shells containing d and f orbitals, the atomic size doesn’t increase significantly and may also decrease in some cases.