Dependence of electron affinity on atomic number. Values generally increase across each period, culminating with the halogens before decreasing precipitously with the noble gases. Examples of localized peaks seen in hydrogen, the alkali metals and the group 11 elements are caused by a tendency to complete the s-shell (with the 6s shell of gold being further stabilized by relativistic effects and the presence of a filled 4f sub shell). Examples of localized troughs seen in the alkaline earth metals, and nitrogen, phosphorus, manganese and rhenium are caused by filled s-shells, or half-filled p- or d-shells.
Mendeleev's periodic table from his book An Attempt Towards a Chemical Conception of the Ether
A version of Mendeleev's 1869 periodic table: An experiment on a system of elements based on their atomic weights and chemical similarities. This early arrangement presents the periods vertically, and the groups horizontally.