Select the correct statements below.
a. An atom of chlorine is larger than sulfur because the addition of an electron increases the repulsions without an increase in attraction for the nucleus resulting in less screening.
b. An atom of oxygen is smaller than an atom of carbon because the distance of the valence electrons remains the same while the attraction for the nucleus increases with increasing number of protons.
c. An atom of potassium is smaller than an atom of rubidium because both effective nuclear charge increases down a group and the principal quantum number of the valence orbitals increases.
d. An atom of sodium is smaller than an atom of magnesium because both effective nuclear charge increases down a group and the principal quantum number of the valence orbitals increases.
e. An atom of chlorine is smaller than an atom of bromine because the principal.

On moving across the period, successive electrons gets add up to the orbitals in the same shell and the shielding of the nuclear charge by the core electrons do not change to a large extent so as to decrease the attraction of electrons to the nucleus. This results in the increase of effective nuclear charge.

Trend down the group:

On moving down the group, the outermost electron gets further away from the nucleus. The shielding effect by the core electrons get increased. This result in the decrease of effective nuclear charge. As, the principal quantum number increases, the effective nuclear charge gets decreased and screening effect increases.

Atomic radius of an atom is defined as the total distance from the nucleus to the outermost shell of the atom.

Trend down the group:

On moving down the group, new shell gets add up around the nucleus of an atom and the outermost shell gets far. The distance between the nucleus and outermost shell increases. This results in the increase of atomic radii of the atom.

Trend across a period:

On moving across a period, more and more electrons get add up in the same shell. The attraction between the last electron and nucleus increases, which results in the shrinkage of size of an atom. So, the atomic radii gets decreased.

For the given options:

Option a: Chlorine atom is smaller than sulfur atom because of the increase in the attraction of electrons by the nucleus.

Option b: Oxygen atom is smaller than Carbon atom because of the increase in effective nuclear charge in the atom.

Option c: Potassium atom is smaller than rubidium atom because of the addition of new shell around rubidium atom and also potassium has more effective nuclear charge as compared to rubidium.

Option d: Sodium atom is larger than magnesium atom because of less effective nuclear charge in sodium.

Hence, the correct answer is Option b.