Chemistry

Atoms and Molecules

2. A Negatively Charged Ion is Known as Anion

Chloride ion,

Cl

^-, and oxide ion,

O

^2-, and anions because they are negatively charged ions.An anion is formed by the gain of one or more electrons by an atom.For example, a chlorine atom gains (accepts)1 electron to form a chloride ion,

Cl

^-, which is an anion :
Since an anion is formed by the addition of electrons to an atom, therefore,an anion contains more 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 an anion is formed by the addition of one or more electrons to an atom, therefore, an anion contains more electrons than protons.The ions of all the non-metal elements are anions (except hydrogen ion and ammonium ion). We will now give the reason for the negative charge on an anion.
A normal atom contains an equal number of protons and electrons, so it is electrically neutral. When an atom gains electrons to form an anion, then in this anion the number of electrons becomes more than the number of protons.Due to more electrons, than protons,an anion has a negative charge on it.This will become more clear from the following example.
Let us write down the number of protons and electrons in a chlorine atom and a chloride ion as shown below :
Chlorine atom, Cl, contains an equal number of protons and electrons (17 each), so it is electrically neutral. In the chloride ion,

Cl

^-,there are 17 protons but 18 electrons. That is, a chloride ion has 1 electron more than protons. Due to 1 more electron than protons, a chloride ion has 1 unit of negative charge and it is written as

Cl

^-. From this discussion we conclude that an anion contains more electrons than protons.
Most of the non-metal atoms can gain (or accept) electrons easily, so most of the non-metal elements form anions (or negative ions).Please note that:
(i) If an atom gains 1 electron, then the anion (negative ion) formed has 1 unit of negative charge. For example, a chlorine atom accepts 1 electron to form a chloride ion,

Cl

^-, having 1 unit negative charge
(ii) H an atom gains 2 electrons, then the anion (negative ion) formed has 2 units of negative charge.For example, an oxygen atom accepts 2 electrons to form an oxide ion,

O

^2-, having 2 units of negative charge.
(iii) And if an atom gains 3 electrons, then the anion (negative ion) formed will have 3 units of negative charge. For example, a nitrogen atom can gain 3 electrons to form a nitride ion,

N

^3-, having 3 units of negative charge.
It is, however, usually not possible to add more than 3 electrons to an atom because of the great force of repulsion experienced by the subsequent electrons from the anion (negative ion) already having 3 negative charges. Please note that the electrons which are gained by non-metal atoms are given by the metal atom during chemical reactions.

Notes

Atoms and Molecules - Notes

1. Drawbacks of Dalton’s Atomic Theory