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Electron affinity is defined as the energy released when electrons are added to the gas phase atom. It increases from left to right across periods and upwards for groups. For groups, this occurs because as principal quantum number increases down a group, the size of the orbital increases (atomic radius increases) so the affinity for the electron is less. Moving from left to right across a period, the forces of attraction from the protons become stronger because of their greater number so the electron comes closer to the nucleus, hence increasing affinity. Overall, the trend is the same as electronegativity and ionization energy.
To add on, electron affinity is basically the likelihood of an atom to gain an electron. Electron affinity increases upward for the groups and from left to right across periods of a periodic table. Hopefully this image helps you visualize the trend.
Electron affinity increases upward for the groups and from left to right across periods of a periodic table because the electrons added to energy levels become closer to the nucleus, thus a stronger attraction between the nucleus and its electrons.
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