Concept:Density is a measure of how tightly packed mass is within a given space.
Formula:$$ \text{Density} = \frac{\text{Mass}}{\text{Volume}} $$
Solution:- Alkali metals (Group I-A) are located at the extreme left of their respective periods.
- Because effective nuclear charge is at its lowest on the left side of the table, alkali metals possess the largest atomic volumes in their periods.
- Since volume is the denominator in the density equation, a massively large volume results in a very low overall density. (Lithium, Sodium, and Potassium are even less dense than water and will float).
Why other options are incorrect:- Option A & D: While weak forces and the \( ns^1 \) configuration explain their low melting points and softness, density is a direct mathematical consequence of their inflated atomic volume.
- Option C: They are the largest elements in their periods, not the smallest.
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