MONOATOMIC IONS

An ion is a chemical species with a net electrical charge. It can be composed of a single atom (monoatomic ion) or a group of atoms (polyatomic ion).

When the net charge is positive, the ion is called an ANION. In the presence of a net positive charge, the ion is called a CATION.

Example:

  • Na+; Cl–; Ca2+; O2- (monoatomic ions).
  • NH4+, VO2+, CO32-, SO42-, PO43- (polyatomic ions).

CATIONS

Let’s focus on the following monoatomic cation:

The positive charge +1 indicates that the number of protons exceeds the number of electrons by one.

This is obtained by subtracting one electron from the neutral atom, NOT by adding one proton.

Li+   3 protons; 2 electrons.

From the current information, it’s not possible to determine the number of neutrons.

(If the calculation of subatomic particles is unclear, click here).   

                                 

A positive ion, obtained by the removal of one or more electrons, is called a CATION

ANIONS

Let’s focus on the anion F– obtained by the following neutral element:

Fluorine is composed of 9 protons, 10 neutrons, 9 electrons. The addition of one electron, results in the anion F– . 

F–      9 protons; 10 neutrons 10 electrons                             

A negative ion, obtained by the addition of one or more electons, is called an anion.

A mistake that must be absolutely avoided, is adding or subtracting a proton instead of an electron. This mistake results in a change of the atomic number.

KEY POINTS:

  • A monoatomic cation is a positive ion, composed of a single atom, obtained by the removal of one or more electrons from a neutral element.
  • A monoatomic anion is a negative ion, composed of a single atom, obtained by the addition of one or more electrons from a neutral element.

SOLVED EXERCISE:

Calculate the number of protons, neutrons and electrons of the ion S2- from the following neutral atom:

The atomic structure of the neutral atom is composed of 16 protons, 16 neutrons e 16 electrons;

It’s necessary to add or subtract a number of electrons consistent with the charge of the ion;

Therefore, the structure of the ion is made up of di 16 protons, 16 neutrons e 18 electrons.

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