What is the Atomic Number of an atom ?

Definition of 'Atomic Number':

The Atomic Number of an atom is the number of protons it contains (which are located in the nucleus of the atom).

More about the Atomic Number of an atom ...

  • The Atomic Number uniquely identifies a chemical element.
  • The Atomic Number of an atom is represented by the symbol Z.
  • The number of protons in an atom is also (sometimes) called the Proton Number.
  • Each element has its own Atomic Number (number of protons).
  • Because atoms are neutral (that is neither positive '+ve' nor negative '-ve'), and protons have a charge of +1 while electrons have a charge of -1, the number of protons in an atom = the number of electrons in the atom.
    The Atomic Number is therefore also the number of electrons in an atom.

Examples of Atomic Numbers of Elements

The best place to look for examples of the Atomic Number of chemical elements is the Periodic Table - which is included in many chemistry textbooks, wall-charts in chemistry classrooms and laboratories and online.

In most cases the atomic number and mass number of each element is included. Obviously the atomic number is the smaller of the two figures, but can appear in either an upper- or lower- corner of the square or rectangle of its element.

Here are a few examples of the atomic numbers of common elements:

Name of Element

Symbol of Element

Atomic Number

Number of Protons

Number of Electrons


Carbon

C

6

6

6

Oxygen

O

8

8

8

Sodium

Na

11

11

11

Calcium

Ca

20

20

20

Copper

Cu

29

29

29

It is not necessary to memorize these examples for GCSE Chemistry.

For more examples see the Periodic Table and the List of Chemical Elements (with brief notes about each element).

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