Allotropes Some elements exist in several different structural forms, called allotropes. Each allotrope has different physical properties.

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For more information on the Visual Elements image see the Uses and properties section below.

GroupA vertical column in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.

PeriodA horizontal row in the periodic table. The atomic number of each element increases by one, reading from left to right.

BlockElements are organised into blocks by the orbital type in which the outer electrons are found. These blocks are named for the characteristic spectra they produce: sharp (s), principal (p), diffuse (d), and fundamental (f).

Atomic numberThe number of protons in an atom.

Electron configurationThe arrangements of electrons above the last (closed shell) noble gas.

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Melting pointThe temperature at which the solid–liquid phase change occurs.

Boiling point The temperature at which the liquid–gas phase change occurs.

SublimationThe transition of a substance directly from the solid to the gas phase without passing through a liquid phase.

Density (g cm−3)Density is the mass of a substance that would fill 1 cm3 at room temperature.

Relative atomic massThe mass of an atom relative to that of carbon-12. This is approximately the sum of the number of protons and neutrons in the nucleus. Where more than one isotope exists, the value given is the abundance weighted average.

IsotopesAtoms of the same element with different numbers of neutrons.

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CAS numberThe Chemical Abstracts Service registry number is a unique identifier of a particular chemical, designed to prevent confusion arising from different languages and naming systems.