Which best explains why ionization energy tends to decrease from the top to the bottom of a group?
the number of orbitals decreases.
the number of neutrons decreases.
electrons get closer to the nucleus.
electrons get farther from the nucleus.

The answer would be that the Electrons get farther from the nucleus.  As you go down a group, the Period is increasing, which means that each time a shell is being added on as such the valence electron are moving further away from the nucleus.  It is therefore easier for the valence electron to be removed thus why less energy is needed to remove that electron, thus a decrease in ionization energy.

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