Ions

Ions

In an atom, the number of protons is equal to the number of electrons and so the atom is electricallyneutral. But, during chemical reactions atoms try to attain stable electronic configuration (duplet or octet) either by gaining or losing one or more electrons according to valency. When an atom gains an electron it has more number of electrons and thus it carries negative charge. At the same time when an atom loses an electron it has more number of protons and thus it carries positive charge. These atoms which carry positive or negative charges are called ions. The number of electrons gained or lost by an atom is shown as a superscript to the right of its symbol. When an atom loses an electron, ‘+’ sign is shown in the superscript and ‘–’ sign is shown if an electron is gained by an atom. Some times, two or more atoms of different elements collectively lose or gain electrons to acquire positive or negative charge. Thus we can say, an atom or a group of atoms when they either lose or gain electrons, get converted into ions or radicals.

1. Types of Ions

Ions are classified into two types. They are cations and anions.

Cations

If an atom loses one or more electrons during a chemical reaction, it will have more number of positive charge on it. These are called cations (or) positive radicals. Sodium atom loses one electron to attain stability and it becomes cation. Sodium ion is represented as Na⁺.

Anions

If an atom gains one or more electrons during a chemical reaction, it will have more number of negative charge on it. These are called anions or negative radicals. Chlorine atom attains stable electronic configuration by gaining an electron. Thus, it becomes anion. Chlorine ion is represented as Cl^–.

2. Different valent ions

During a chemical reaction, an atom may gain or lose more than one electron. An ion or radical is classified as monovalent, divalent, trivalent or tetravalent when the number of charges over it is 1,2,3 or 4 respectively. Based on the charges carried by the ions, they will have different valencies.

Valency of Anions (negative radicals) and Cations (positive radicals)

The valency of an anion or cation is a number which expresses the number of hydrogen atoms or any other monovalent atoms (Na,K,Cl….) which combine with them to give an appropriate compound. For example, two hydrogen atoms combine with one sulphate ions (SO^2-₄) to form sulphuric acid (H₂SO₄).

Activity 2

Classify the following ions into monovalent, divalent and trivalent.

Ni²⁺, Fe³⁺, Cu²⁺, Ba²⁺, Cs⁺, Zn²⁺, Cd²⁺, Hg²⁺ Pb²⁺, Mn²⁺, Fe²⁺, Co²⁺, Sr²⁺, Cr³⁺, Li⁺, Ca²⁺, Al³⁺

Answer:

Monovalent ions : Li⁺, Cs⁺

Divalent ions : Ni²⁺, Cu²⁺, Ba²⁺, Zn²⁺, Cd^2+, Hg²⁺, Pb²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ca²⁺, Sr² Trivalent ions : Fe³+, Cr³⁺, Al³⁺ 

So, the valency of SO²₄^- is 2. One chlorine atom (Cl) combines with one ammonium ion (NH⁺₄) to form NH₄Cl. So, the valency of NH+₄ is 1. Valencies of some anions and cations and their corresponding compounds are given below.