Chemistry

Atoms and Molecules

1. A Positively Charged Ion is Known as Cation

Sodium ion,

Na

⁺,and magnesium ion,

Mg

²⁺, are cations because they are positively charged ions.A cation is formed by the loss of one or more electrons by an atom.For example, sodium atom loses 1 electron to form a sodium ion,

Na

⁺, which is a cation :
Since a cation is formed by the removal of electrons from an atom, therefore,a cation contains less electrons than a normal atom. We also know that a normal atom (or a neutral atom) contains an equal number of protons and electrons. Now, since a cation is formed by the loss of one or more electrons by an atom, therefore,a cation contains less electrons than protons. The ions of all the metal elements are cations. Only the hydrogen ion,

H

⁺,and ammonium ion,

NH

⁺₄,are the cations formed from non-metals. We will now give the reason for the positive charge on a cation.
We know that a proton has 1 unit positive charge and an electron has 1 unit negative charge. Now,a normal atom contains an equal number of protons and electrons, so it is electrically neutral. When a normal atom loses electrons to form a cation, then in this cation, the number of electrons becomes less than the number of protons. In other words, we can say that a cation has more protons than electrons.Due to more protons than electrons, a cation has a positive charge on it.This will become more clear from the following example.
Let us write down the number of protons and electrons in a sodium atom as well as a sodium ion as shown below :
Sodium atom,Na,contains 11 protons and 11 electrons. Since the number of protons and electrons in a sodium atom is equal, it is electrically neutral, having no overall charge. In the sodium ion, Na+, there are 11 protons but only 10 electrons. This means that there is 1 proton more than electrons. Due to 1 more proton (than electrons), a sodium ion has 1 unit positive charge and it is written as

Na

⁺
All the metal atoms can lose electrons easily, so all the metal elements form cations (or positive ions).Please note that:
(i) If an atom loses 1 electron, then the cation (positive ion) formed has 1 unit positive charge. For example, a sodium atom loses 1 electron to form a sodium ion,

Na

⁺, having 1 unit positive charge.
(ii) If an atom loses 2 electrons, then the cation (positive ion) formed has 2 units of positive charge.For example, a magnesium atom can lose 2 electrons to form a magnesium ion,

Mg

²⁺+,having 2 units of positive charge.
(iii) And if an atom loses 3 electrons, then the cation (positive ion) formed has 3 units of positive charge. For example,an aluminium atom can lose 3 electrons to form an aluminium ion,

Al

³⁺, having 3 units of positive charge.
It is, however, usually not possible to remove more than 3 electrons from an atom due to very high energy required for this purpose. Another point to be noted is that the electrons lost by a metal atom during the formation of a cation (positive ion) are accepted by a non-metal atom (which then turns into an anion or negative ion).

Notes

Atoms and Molecules - Notes

1. Drawbacks of Dalton’s Atomic Theory