Question

among the elements of the main group, the first ionization energy increases from left to right across a period. from right to left across a period. when the atomic radius increases. down a group.

Explanation:

The first - ionization energy is the energy required to remove the outermost electron from a neutral atom in the gaseous state. Across a period (from left to right), the nuclear charge increases while the shielding effect remains relatively constant. This results in a stronger attraction between the nucleus and the outermost electrons, making it more difficult to remove an electron and thus increasing the first - ionization energy. Down a group, the atomic radius increases, the outermost electrons are farther from the nucleus and more shielded, so the first - ionization energy decreases. Also, as the atomic radius increases, the first - ionization energy generally decreases.

Answer:

A. from left to right across a period.