text: Although E varies greatly across the periodic table, some patterns emerge. Generally, nonmetals have more positive E than metals. Atoms whose anions are more stable than neutral atoms have a greater E. Chlorine most strongly attracts extra electrons; neon most weakly attracts an extra electron. The electron affinities of the noble gases have not been conclusively measured, so they may or may not have slightly negative values.
  E generally increases across a period (row) in the periodic table prior to reaching group 18. This is caused by the filling of the valence shell of the atom; a group 17 atom releases more energy than a group 1 atom on gaining an electron because it obtains a filled valence shell and therefore is more stable. In group 18, the valence shell is full, meaning that added electrons are unstable, tending to be ejected very quickly.
  Counterintuitively,  E does not decrease when progressing down most columns of the periodic table. For example, E actually increases consistently on descending the column for the group 2 data. Thus, electron affinity follows the same "left-right" trend as electronegativity, but not the "up-down" trend.
  The following data are quoted in kJ/mol.