Final answer:
The ionization energy of alkali metals decreases with increasing atomic number because larger atomic size leads to valence electrons being more distantly located from the nucleus and hence less energy is needed to remove them. Further, increased reactivity is associated with the ease of electron loss from the outer shell. Alkali metals are also very soft and have low densities.
Step-by-step explanation:
The ionization energy of alkali metals decreases with the increase in atomic number because, as you go down a group in the periodic table, the atoms increase in size. An increase in atomic size means that the first electron removed is farther from the nucleus, despite an increase in nuclear charge due to more protons. The outer electron is less strongly attracted to the nucleus and therefore requires less energy to be removed, corresponding to the decrease in ionization energy. This pattern is because the electrons in the outer shell are less effective at screening each other from the nuclear charge, resulting in the valence electrons being farther away and more easily removed.
Another factor is that reactivity increases with increasing atomic number, which is linked to the ease of losing the lone valence electron, highlighting the decreasing ionization energy. For this reason, it's generally harder to reduce alkali metals to isolate them in their elemental form, owing to their propensity for oxidation. It's also notable that alkali metals tend to be very soft with low densities, such as lithium which is the lightest solid metal.