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A block of the periodic table of elements is a set of adjacent groups. The respective highest-energy electrons in each element in a block belong to the same atomic orbital type. Each block is named after its characteristic orbital; thus, the blocks are:

• s-block
• p-block
• d-block
• f-block
• g-block (no elements belonging to the g-block have been observed)

The block names (s, p, d, f, g, h,...) are derived from the quality of the spectroscopic lines of the associated atomic orbitals: sharp, principal, diffuse and fundamental, the rest being named in alphabetical order. Blocks are sometimes called families.