Question

1. justify why atomic radius decreases across a period and increases down a group using the ideas of attraction and repulsion.

trend
atomic radius decreases
across a period.
atomic radius increases
down a group.
justification
a
b

Explanation:

• Atomic radius decreases across a period:

As we move across a period, the number of protons in the nucleus increases (so the positive charge of the nucleus increases). The electrons are added to the same principal energy level. The increased nuclear charge exerts a stronger attractive force on the electrons. This stronger attraction pulls the electrons closer to the nucleus, reducing the atomic radius.

• Atomic radius increases down a group:

When moving down a group, new electron shells are added. Each new shell is farther from the nucleus. The outermost electrons are in higher - energy levels. The distance between the nucleus and the outermost electrons increases. Also, the inner electron shells shield the outermost electrons from the full nuclear charge. The net result is that the attractive force of the nucleus on the outermost electrons is reduced, and the atomic radius increases.

Answer:

• A: As we move across a period, the nuclear charge increases while electrons are in the same shell. Stronger nuclear attraction pulls electrons closer, decreasing atomic radius.
• B: Down a group, new electron shells are added. Distance from nucleus increases and shielding occurs, so atomic radius increases.