IONS – Anions and Cations

Atoms are neutral; they contain the same number of protons (positive particles) and electrons (negative particles). When the numbers of these subatomic particles are not equal, the atom is electrically charged and is called an ion i.e. ions are electrically charged atoms with extra electrons or missing electrons. It means ions are formed when atoms lose or gain electrons. When an atom removes an electron or more, it will have a positive charge. When an atom have an extra electron or more, it will have a negative charge. When an ion is formed, the number of protons does not change.
“Ions are electrically charged particles formed when atoms lose or gain electrons. They have the same electronic structures as noble gases”.

The charge of an atom is defined as follows:

Atomic charge = Number of protons − Number of electrons

   
(a) If the atom has the tendency to give away electrons then the atom attains positivity. The positivity of the atom is represented as positive ion. A positive charged ion is known as cation.
For example, a neutral sodium atom, contains 11 protons and 11 electrons. By removing an electron from this atom we get a positively charged Na⁺ ion (11 protons and 10 electrons) that has a net charge of +1.

Na      –    1 electron              →      Na⁺
Sodium atom                                          Sodium cation
11 p and 11 e                                             11 p and 10 e
net charge = 0                                         net charge = +1

Mg      –    2 electron              →       Mg⁺²
Magnesium atom                                Magnesium cation
12 p and 12 e                                         12 p and 12 e
net charge = 0                                      net charge = +2              

Al      –    3 electron              →       Al⁺³
Aluminium atom                                   Aluminium cation
13 p and 13 e                                           13 p and 10 e
net charge = 0                                        net charge = +3
In normal conditions, it is not possible to remove more than 3 electrons from an atom because it requires very high energy. The electrons lost by a metal atom during the formation  of a cation transfers to the non-metal atom. The non-metal accepts the electron and becomes anion (negatively charged ion).

(b) If an atom has the tendency to accept electrons then it gains negative charge. This negativity results due to acceptance of negative charge (electron) and is represented as negative ion. A negative charged ion is known as anion. A neutral chlorine atom, for example, contains 17 protons and 17 electrons. By adding one more electron we get a negatively charged Cl^– ion (17 protons and 18 electrons) with a net charge of -1.

Cl      +    1 electron              →        Cl^–
Chlorine atom                                       Chlorine anion
17 p and 17 e                                            17 p and 18 e
net charge = 0                                         net charge = -1

O      +    2 electron              →        O^-2
Oxygen atom                                           Oxygen anion
8 p and 8 e                                               8 p and 10 e
net charge = 0                                         net charge = -2

N     +   3 electron                →        N^-3
Nitrogen atom                                         Nitrogen anion
7 p and 7 e                                                7 p and 10 e
net charge = 0                                         net charge = -3
In normal conditions, it is not possible to add more than 3 electrons to an atom because the coming electron experiences a great force of repulsion from the anion already having 3 negative charges.
The non-metal accepts the electron by a metal atom during a chemical reaction and becomes anion (negatively charged ion).

•  When any atom form ion, it tries to acquire the electronic configuration of the nearest noble gas in the periodic table. The noble gases follow octet rule. Sodium (Na – 2,8,1) loses an electron to form Na⁺ (2,8) and attains the electronic configuration of Neon (2,8 – noble gas). Chlorine (Cl – 2,8,7) gains an electron to form Cl^– (2,8,8) and attains the electronic configuration of Argon (2,8,8 – noble gas). In both the cases Na and Cl try to follow octet rule in the formation of an ion and further into a compound.
• Metal atoms and non-metal atoms go in opposite directions when they ionize.
  (a) Metal atoms lose the electron, or electrons, in their highest energy level and become positively charged ions or cations.
  (b) Non-metal atoms gain an electron, or electrons, from another atom to become negatively charged ions or anion.
  Na      +    Cl              →        Na⁺      Cl^–
  (2,8,1)            (2,8,7)                   (2,8)     (2,8,8)

---

The gain or loss of electrons by an atom to form negative or positive ions has an enormous impact on the chemical and physical properties of the atom. Sodium metal, for example, which consists of neutral sodium atoms, bursts into flame when it comes in contact with water. Neutral chlorine atoms instantly combine to form Cl₂ molecules, which are so reactive that entire communities are evacuated when trains carrying chlorine gas derail. Positively charged Na⁺ and negatively charged Cl^– ions are so unreactive that we can safely take them into our bodies whenever we salt our food.

---

•  Simple ions – The ions which are formed from single atom carrying a positive or negative charge are called simple ions or monoatomic ions. Example -Na⁺ , Cl^–, O^-2, Al⁺³, N^-3(nitride), Mg⁺², Ca⁺ , Cu⁺² , Fe⁺²etc.
• Polyatomic ions -Polyatomic ions or compound ions are molecules formed from groups of atoms bonded together that carry a positive or negative charge. Example – NH₄⁺ , CO₃^-2, OH^–, SO₄^-2, NO₃^–,
  PO₄^-3, SO₃^-2  etc.