Which property decreases within a group as atomic number increases?

As you move down a group in the periodic table, ionization energy generally decreases. Ionization energy refers to the energy required to remove an electron from a neutral atom in its gaseous state. This decrease occurs because, as the atomic number increases, the number of electron shells also increases. Each additional shell places the outermost electrons further away from the nucleus, which leads to a weaker electrostatic attraction between the positively charged nucleus and the negatively charged electrons. As a result, it becomes easier to remove an electron from these atoms, thus reducing the ionization energy.

This trend of decreasing ionization energy down a group is a crucial concept in understanding elemental behavior and reactivity. Comparatively, atomic size increases, electronegativity tends to decrease, and reactivity of metals increases down a group; hence ionization energy demonstrates a distinct downward trend.