The classification of chemical elements as blocks is based on the most energetic orbitals of their electron shell. A block groups several groups of the periodic table together.
The s-subshell of the respective current main shell is filled. The block includes the 1st and 2nd group as well as the element helium. All s-elements except helium are highly reactive, metals of the s-block are strongly electropositive and have low melting and boiling points.
The p-block – p-subshells of the current main shell are filled – includes the 3rd to 8th main group (excluding helium). The reactivity of elements in a group generally decreases downwards.
Transition elements (excluding lanthanides and actinides) are grouped in the d-block. d-subshells hold 10 electrons each, all elements in this block are metals and they are referred to as “outer transition metals”.
Lanthanides and actinides (inner transition metals) are defined by the f-block. They have two valence electrons in the outermost main shell, their chemical differences are correspondingly small.