Question

Two elements X and Y are starting elements of $3^{rd}$ and $4^{th}$ periods. One with lower density is:

• A. X
• B. Y
• C. both have same
• D. none of these

Answer 1

The correct option is B Y

$3^{rd}$ and $4^{th}$ periods start with sodium and potassium is less than that of sodium. This is because sodium has no vacant d subshell and is small in size. Though an increase in nuclear charge from $11$ to $19$ increases mass increase in atomic volume is more than that of atomic mass. Since density is mass per unit volume, it decreases.