Chemistry (Part-II)

Elements in the same group (vertical column) of the periodic table have similar chemical properties because they have the same number of valence electrons. Looking at the options: 

(a) Ne and Ar are noble gases (Group 18), but Na (alkali metal, Group 1) and Mg (alkaline earth metal, Group 2) are not.

(b) He is a noble gas (Group 18), Li is an alkali metal (Group 1), Mg is an alkaline earth metal (Group 2), and K is an alkali metal (Group 1).

(c) H is in Group 1, Li is in Group 1, Mg is in Group 2, and Ca is in Group 2.

(d) H, Li, Na, K are all alkali metals and belong to Group 1.

Therefore, the correct answer is H, Li, Na, K.

The correct statement regarding atomic radius trends in the periodic table is Atomic radius decreases across a period and increases down a group in the periodic table.

Across a period, the number of protons and electrons increases, leading to a stronger effective nuclear charge that pulls the electrons closer to the nucleus, thus decreasing the atomic radius.

Down a group, the number of electron shells increases, causing the valence electrons to be farther from the nucleus, thus increasing the atomic radius.

Density generally increases with increasing atomic mass and decreasing atomic radius. Among the given options:

Hydrogen and Fluorine are gases with very low densities.

Tungsten is a dense metal.

Osmium is known to be one of the densest naturally occurring elements.

Therefore, the answer is Osmium.