Which statement best explains why, within a given atom, a 2s electron is lower in energy than a 2p electron?
a) Because a 2p electron has a larger angular-momentum quantum number, it is more stable and therefore lower in energy than a 2s electron.
b) A 2s electron penetrates closer to the nucleus, experiencing a larger effective nuclear charge than a 2p electron.
c) A 2s electron is, on average, farther from the nucleus, so it is more shielded and therefore lower in energy than a 2p electron.
d) Spin pairing in the 2s subshell increases electron–electron repulsion, so the unpaired 2p electrons lie lower in energy.