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Periodicity

Classifying the elements

The modern periodic table is a chart that shows the known elements. These are organised into periods and groups. There are four blocks of elements in the periodic table:

  • s-block
  • p-block
  • d-block
  • f-block

The diagram below shows a periodic table with these blocks marked.

The d-block contains the transition metals. IUPAC defines a transition metal as:

An element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell.

Using this definition, some d-block elements are not transition metals. Zinc is one of these elements:

  • zinc atoms have the electronic configuration: [Ar] 3d10 4s2
  • and zinc always forms Zn2+ ions, which have a complete 3d sub-shell: [Ar] 3d10

s-block

The outer electrons in the atoms of s-block elements are in an s sub-shell.

p-block

The outer electrons in the atoms of p-block elements are in a p sub-shell. There are six columns because a maximum of six electrons can occupy a p sub-shell.

d-block

Atoms of d-block elements have one or more outer electrons in the d sub-shell, but no electrons in the corresponding p sub-shell. There are ten columns because a maximum of ten electrons can occupy a d sub-shell. Notice that lanthanum [Ar] 5d1 6s2 and actinium [Rn] 6d1 7s2 are d-block elements. They are the first members of the lanthanoids and actinoids.

f-block

Atoms of f-block elements have one or more outer electrons in the f sub-shell, but no electrons in the corresponding p or d sub-shells. There are 14 columns because a maximum of 14 electrons can occupy an f sub-shell.

Hydrogen is often placed above lithium in the periodic table (but not in group 1). When this is done, all the elements in that column have electronic configurations ending in s1. In the same way, helium might be placed above beryllium. If this were to be done, all the elements in that column would have electronic configurations ending in s2.

These two changes would give two columns in the s-block, matching the maximum of two electrons that can occupy an s subshell. However, hydrogen has very different properties from the alkali metals and, based on its properties, helium clearly belongs in group 18 with the other noble gases.