3 The four blocks of the periodic table

The table is divided into four main regions or blocks, named after the sublevel which receives the last electron.

• 1.
• s-block: on the left.
• The outermost electrons occupy the s sublevel: group 1A (ns¹) and group 2A (ns²), where n is the period number and the number of the last energy level.

• 2.
• p-block: on the right.
• The outermost electrons occupy the p sublevel: groups 3A (ns² np¹), 4A (np²), 5A (np³), 6A (np⁴), 7A (np⁵) and the zero group (noble gases, np⁶), where p is saturated.
• (Helium is 1s².)

• 3.
• d-block: in the middle.
• The d sublevel is filled successively; as d holds up to 10 electrons, the block has ten vertical columns: seven belong to the B groups (3B to 7B, then 1B and 2B in the table's numbering) and three columns together form the eighth group (Fe, Co, Ni and their analogues).

• 4.
• f-block: placed separately at the bottom so the table does not become too long.
• The f sublevel (up to 14 electrons) is filled successively; it has two series:

• Lanthanides: the 4f sublevel is filled, 14 elements; the outer sublevel of all is 6s².
• Actinides: the 5f sublevel is filled, 14 elements, all radioactive (unstable nuclei).