118 different chemical elements are known to modern chemistry. 92 of these elements can be found in nature and the others can only be made in laboratories. Chemical elements are commonly arranged in the periodic table according to their atomic number.
Specific regions of the periodic table can be referred to as blocks in recognition of the sequence in which the electron shells of the elements are filled. A block of the periodic table is a set of elements unified by the orbitals their valence electrons or vacancies lie in. The term appears to have been first used by Charles Janet. Each block is named after its characteristic orbital: s-block, p-block, d-block, and f-block.
The s-block comprises the first two groups (alkali metals and alkaline earth metals) as well as hydrogen and helium. The p-block comprises the last six groups, which are groups 13 to 18 and contains all of the metalloids and non-metals. The d-block comprises groups 3 to 12 and contains all of the transition metals. The f-block, often offset below the rest of the periodic table, has no group numbers and comprises most of the lanthanides and actinides.
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