The atomic number of an element is the number of protons it has in its nucleus. Electron affinity is the energy released when an electron is added to an isolated gas-phase atom to produce a neutral atom.
The alkaline earth metals (group 2) in the periodic table are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). For these elements, electron affinity increases down the group, becoming more positive.
This increase occurs because the size of the atomic nucleus and the number of valence electrons in the atomic orbitals increase down the group. This means that as the nucleus gets bigger and more positive, it becomes more effective at attracting electrons, making it easier for an electron to be added to the atom. Therefore, electron affinity increases down the group.
