In two to three sentences, explain the trend for atomic size. Why does this trend occur?

The number of energy levels increases as you move down a group as the number of electrons increases. Each subsequent energy level is further from the nucleus than the last. Therefore, the atomic radius increases as the group and energy levels increase. 2) As you move across a period, atomic radius decreases.

Explanation:

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.