Trends In Periodic Table

Within a group, atomic size increases as new shells are added, increasing atomic radius.

The solubility of sulphates in water generally decreases as we move down the group in the periodic table.
Among the alkaline earth metal sulphates, the solubility decreases in the order .
This trend is due to the increasing size and decreasing lattice energy as we move from beryllium to barium.
Thus, the correct answer is (a).

The atomic radius of neon (Ne) is indeed greater than that of argon (Ar).
This is because neon belongs to the noble gases in period 2, while argon is in period 3, and the atomic size decreases from left to right across a period due to increased nuclear charge pulling electrons closer to the nucleus.
Therefore, Ne has a larger atomic radius than Ar.

Sodium (Na) is an alkali metal, which belongs to Group 1 of the periodic table. Alkali metals share similar chemical properties, such as being highly reactive with water and forming strong bases (alkalies) when combined with water. Cesium (Cs), another alkali metal, is located in the same group as sodium, though it is further down the periodic table. Due to their similar chemical characteristics, cesium is the closest chemically similar element to sodium.

Therefore, sodium's closest chemically similar element is cesium (Cs), both being alkali metals in the same group.