Chemical structure

This section covers:

See also:

Terms to watch out for:

mass, weight

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Elements

Did you know? Chemical elements are pure chemical substances made up of a single type of atom (eg oxygen, carbon, mercury, gold). 

The periodic table lists 118 named elements, and more are being added as researchers synthesise new ones. Each chemical element is denoted by a symbol, which is either the first letter or a combination of the first letter and another letter from the element’s name, in English (eg carbon is C, calcium is Ca, argon is Ar, arsenic is As), or in Greek or Latin (eg gold is Au from the Latin aurum; mercury is Hg from the Greek-derived word hydrargyrum).

Write the names of chemical elements in roman type without a capital, but use an initial capital for the symbols:

calcium (Ca)     carbon (C)     helium (He)     oxygen (O)

Some synthetic elements are named after people; however, unlike other eponymic names (such as disease names), they are written without a capital:

lawrencium (Lr)     meitnerium (Mt)

In the periodic table, elements are listed by group (columns), period (rows), blocks (s, p, d, f) and unique numbers. Use lower case for all terms associated with the table, and hyphenate block names:

Calcium is element 20 in group 2, period 4 and s-block of the periodic table.

In general texts, it is best to write out chemical element names in full, wherever possible (eg sulfur, carbon); however, in technical reports and scientific journal papers, it might be appropriate to use the symbol (especially in tables and figures). If abbreviations for chemical elements are used, consider identifying them in full at first use in text or in notes under tables and figures:

phosphorus (P)     silver (Ag)

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Atoms and isotopes

Did you know? An atom is made up of protons, neutrons and electrons. The position of an element in the periodic table is based only on the number of protons in its nucleus, which is known as the atomic number.

Isotopes, molecules and ions are indicated using various superscript and subscript notations.

Diagram showing how to write an isotope and a molecule with superscript and subscript details explained

The mass of an atom includes both the protons and neutrons, and is usually the same for each atom of an element; however, some variants (isotopes) occur, which have different numbers of neutrons. A familiar example is the carbon-14 isotope of carbon, which is used for carbon dating of ancient organic remains (the common form of carbon is carbon-12).

When referring to an isotope in general text, use the full name with a hyphen and the mass number: 

carbon-14     radium-226    

In technical texts, tables and figures, use the element symbol with the mass number of the atom in the left superscript position:

12C     14C     226Ra     32S

When the isotopic symbol is used as a descriptor, place it immediately before the part of the chemical name it describes in square brackets, with no space before the chemical name:

[14C]glucose

Include any modifiers in the square brackets before the isotopic symbol:

l-[methyl-14C]methionine     [2,3-3H]serine

Brackets are not used when the isotopic mass symbol is attached to the name of a compound that does not normally contain the element concerned:

32S-ATP

or to a word that is not a specific chemical compound:

14C-amino acids     125I-labelled protein

If quoting both the mass number and the atomic number, use the following format (note that the numbers are right-aligned):

12
6C     14
7C     32
16S

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Ionic charge

Ions are atoms that have a positive or negative electric charge: the number of electrons is either more or less than the number of protons. The difference between the number of electrons and the number of protons is called the valency (charge). Show the ionic charge in the right-hand superscript position; do not use multiple positive or negative signs:

Na+     Ca2+   not   Ca++

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Oxidation state

Did you know? The German chemist Alfred Stock originally, in 1919, proposed the use of arabic numbers to show oxidation state with the style iron(2)-oxide. In 1924, a German Commission recommended the use of roman numerals with no hyphen: iron(II) chloride. Stock apparently approved the use of roman numerals but wanted to keep the hyphen – his suggestion was not followed.

The oxidation state of an atom in a chemical compound is its number of electrons relative to the number it has as a free atom. In inorganic chemical formulas, roman numerals are used for atoms that can exist in more than 1 oxidation state. Usually, the roman numeral is set in parentheses with no space before, a single (nonbreaking) space after and no hyphens:

cobalt(II) oxide 
not
cobalt (II) oxide   or   cobalt(II)-oxide

The roman numeral designating the oxidation state can also be shown using a superscript with no brackets:

cobaltII

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Radicals

Did you know? An atom, molecule or ion that has unpaired valence (outer) electrons is called a free radical.

In chemical equations, free radicals are frequently denoted by a dot placed immediately to the right of the atomic symbol or molecular formula:

Cl∙     HO∙

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