What is monatomic ions? definition, list, examples

In this article you will know the definition of monatomic ions and list as well as example, so let's start with-

What is a monatomic ion?

Definition of  monatomic ions

A chemical entity is a monatomic ion if it is composed of a single atomic nucleus whose charge is not compensated by the electron cloud that accompanies it.

This condition is met if the number of protons in the nucleus (each carrying a positive elementary charge) is different from the number of electrons (each carrying a negative elementary charge)

Cations and anions

There are two kinds of ions:

  • cations are globally positive ions, so they have shortage of electrons (they have less electrons than protons)

  • anions are globally negative ions, so they have excess of electrons (they have more electrons than protons)

Chemical formula

The chemical formula of a monatomic ion consists of:

  • an atomic symbol
  • an exhibitor

The exponent makes it possible to quickly differentiate an atom from an ion, it corresponds to the number and the sign of the elementary charges in excess, it always includes a sign:

  • positive (+) for cations, when the number of electrons (negative) is less than the number of protons (positive). 
  • negative (-) for anions, when the number of electrons exceeds that of protons .

This sign is preceded by the number of excess elementary charges and corresponds to the difference between the number of protons and electrons .

Note: If the ion has only one excess charge (positive or negative) its number (1) is not noted. The exponent is then only a + or a- sign.

Example of monatomic ions

The magnesium ion has the atomic number Z = 12, so its nucleus has 12 protons or 12 positive elementary charges. On the other hand, it only has 10 electrons which carry a total of 10 negative elementary charges.

The magnesium ion therefore has 2 excess positive charges, therefore:

  • It is a cation
  • the exponent of its formula is 2+

The chemical formula of the magnesium ion is therefore Mg2+

Number of electrons in an monatomic ion

A monatomic ion consists of a nucleus around which electrons rotate , the number of which can be determined from its chemical formula:

  • The atomic symbol allows us to find its atomic number Z (thanks to the periodic table) which corresponds to the number of protons in the nucleus as well as to the number of electrons of the atomic form.
  • The exponent makes it possible to determine the number n of electrons in excess or in default. By subtracting the number of excess electrons or by adding the number of defective electrons to the atomic number we obtain the number of electrons in the ion.


The ferric ion has the formula Fe 3+ . The symbol Fe is that of the element iron with atomic number Z = 26, therefore the iron atom has 26 electrons .

The exponent 3+ indicates that the ion has a defect of 3 electrons therefore its total number of electrons is 26-3 = 23

Note: For an element belonging to one of the first three periods of the periodic table it is possible to find the number of electrons in the ionic form using the stability rules .

Electronic configuration of a monatomic ion

The electronic configuration of an ion is written according to the same rules as for an atom, i.e. by placing the electrons in priority on the lowest electronic layers .

Instead of writing the electronic configuration of an ion entirely, it is also possible to deduce it from that of its atomic form. 

It suffices to add the excess electrons or to remove the defective ones from the valence shell (the highest shell occupied by electrons ) in order to pass from the electron configuration of the atom to that of the ion.


The aluminum atom has the atomic number Z = 13, therefore electronic configuration of aluminium is 1s2 2s2 2p6 3s2 3p1

The aluminum ion of formula Al3+ has a defect of 3 electrons , to obtain its electronic structure there It is therefore sufficient to remove the 3 electrons from the valence layer (layer n ° 3) which then empties entirely, which gives 1s2 2s2 2p6

Prediction of monatomic ion formed by an atom

For all the elements we can consider the existence of multiple different ionic forms (presenting a short or an excess of 1 to 4 electrons ) however, among these different possibilities only certain (most often only one) forms are stable and therefore really exist.

For the elements of the first three lines of the periodic table it is possible to predict the electronic configuration of stable ions using the duet and byte rules according to which an atom, when it transforms into an ion, seeks to acquire the same electronic configuration. (in duet or in byte) than the noble gas to which it is closest.

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