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1.BASIC CONCEPTS eLearn.Punjab

A sufficient amount of energy is to be provided to a neutral atom to ionize it.

A → A+ + e-

This A+ is called a cation. A cation may carry +1, +2, +3, etc.charge or charges. The number of charges
present on an ion depends upon the number of electrons lost by the atom. Anyhow, energy is always
required to do so. Hence the formation of the positive ions is an endothermic process. The most
common positive ions are formed by the metal atoms such as Na+, K+, Ca2+, Mg2+ , Al3+, Fe3+, Sn4+, etc.
The chapter on chemical bonding will enable us to understand the feasibilities of their formation.
When a neutral atom picks up one or more electrons, a negative ion is produced, which is called an
anion.

B +e− → B-

Energy is usually released when an electron is added to the isolated neutral atom, Therefore, the
formation of an uninegative ion is an exothermic process.The most common negative ions are
F − ,Cl− ,Br − , S 2− etc.
The cations and anions possess altogether different properties from their corresponding neutral
atoms. There are many examples of negative ions which consist of group of atoms like OH-, CO32-,
SO42-, PO43-, MnO41-, Cr2O72- etc. The positive ions having group of atoms are less common e.g. NH4+
and some carbocations in organic chemistry.

1.1.4 Molecular Ion

When an atom loses or gains an electron, it forms an ion. Similarly, a molecule may also lose or
gain an electron to form a molecular ion, e.g., CH4+, CO+, N2+
Cationic molecular ions are more abundant than anionic ones.
These ions can be generated by passing high energy electron
beam or α-particles or X-rays through a gas. The break down
of molecular ions obtained from the natural products can
give important information about their structure.

Animation 1.4: Molecules
Source & credit: wikimedia

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