1.1 The structure of the atom

When scientists started exploring matter, they realized that matter can be divided into smaller and still smaller particles. They called the smallest particle an ‘atom’. The name ‘atom’ was derived from the Greek word ‘atoms’, meaning ‘indivisible’. They discovered that the ‘atom’ maintains its chemical identity through all chemical and physical changes.


Dalton’s Atomic Theory

John Dalton provided a simple theory of matter to provide theoretical justification to the laws of chemical combinations in 1805. The basic postulates of the theory are:

  1. All substances are made up of tiny, indivisible particles called atoms.
  2. Atoms of the same element are identical in shape, size, mass and other properties.
  3. Each element is composed of its own kind of atoms. Atoms of different elements are different in all respects.
  4. Atom is the smallest unit that takes part in chemical combinations.
  5. Atoms combine with each other in simple whole number ratios to form compound atoms called molecules.
  6. Atoms cannot be created, divided or destroyed during any chemical or physical change.


Nature of Atom

At present we know that the atom is the smallest particle of an element. It is made up of sub-atomic particles like electrons, protons and neutrons. Atoms of one type of element differ from those of the other due to different number of sub-atomic particles. The protons and neutrons are in the nucleus (centre) of the atom and the electrons orbit round the outside in shells (energy levels or layers). The picture below represents an atom of lithium. Lithium has 3proton, 4 neutrons and 3 electrons as shown. Notice that the number of electrons and that of electrons are always equal in neutral atoms.


Properties of sub-atomic particles

  1. Electrons
  • Move around the nucleus in electron shells or energy levels
  • Are negatively charged (-1)
  • Have negligible mass
  • Are equal to the number of neutrons in neutral atom
  • Are lost or gained in chemical reactions
  1. Protons
  • Are present in the nucleus
  • Have a positive charge (+1)
  • Have a relative mass of 1 and contribute to the mass number(A) of an atom
  • Are equal to the number of electrons if an atom is neutral
  • Do not get lost or gained during chemical reactions
  • Are equal to the atomic number(Z) of an element
  1. Neutrons
  • Are present in the nucleus of all atoms, except hydrogen
  • Have no charge, i.e. they are electrically neutral
  • Have relative mass of 1, equal to protons
  • Do not get involved in chemical reactions


Electron energy levels in atoms

The electrons revolve rapidly around the nucleus in fixed circular paths called energy levels or shells. The ‘energy levels’ or ‘shells’ or ‘orbits’ are represented in two ways: either by the numbers 1, 2, 3, 4, 5 and 6 or by letters K, L, M, N, O and P. The energy of the K shell is the least while those of L, M, N and O shells increases progressively. The energy levels are counted from centre outwards.

1st energy level is K shell. It has a maximum of 2 electrons

2nd energy level is L shell. It has a maximum of 8 electrons

3rd energy level is M shell has a maximum of 8 electrons

4th energy level is N shell and so on. The 19th and 20th electrons go into the 4th shell


Electronic configuration of an element

The arrangement of electrons in the various shells/orbits/energy levels of an atom of the element is known as electronic configuration.

Important Rules: Number of electrons in a shell

  • Maximum number of electrons that can be accommodated in a shell is given by 2n2where n = shell number
  • For 1stenergy level, n = 1

Maximum number of electrons in 1st energy level = 2n2

2 x (1) 2 = 2

  • For 2nd energy level n=2

Maximum number of electrons in the 2nd energy level = 2n2

2 x 22 = 2 x 4 = 8

  • For 3rdenergy level n=3

Maximum number of electrons in the 3rd energy level = 2n2

= 2x (3)2

= 2×9=18

  • For 4thenergy level n=4

Maximum number of electrons in the 4th energy level = 2n2

= 2x (4)2

= 2×16=32

Shell number (n) 1 2 3 4

Letter K L M N

Maximum number of electrons (2n2) 2 x (1) 2

2 x 22 2 x (3)2 2 x (4)2 Total 2 8 18 32


The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has a capacity to accommodate more electrons. This is a very important rule and is also called the Octet rule. The presence of 8 electrons in the outermost shell makes the atom very stable.

Geometric Representation of Atomic Structure

Example 1: Magnesium atom


  • The first 2 electrons will go to the 1st shell = K Shell (2n2)
  • The next shell L takes a maximum of 8 electrons (2n2).
  • In this way 2 + 8 = 10 electrons have been accommodated. The next 2 electrons go to the M Shell.

Example 2: Oxygen


Special case of potassium and calcium elements

Atomic number of potassium is 19 and its electronic configuration is


2: 8: 8: 1

Atomic number of calcium is 20 and its electronic configuration is


2: 8: 8: 2


This abnormal behaviour can be explained as follows:

It is found that shells have sub shells. The smaller sub shells are termed s, p, d and f. The maximum numbers of electrons that can go into these are 2, 5, 10 and 14 respectively. These sub shells can overlap, resulting in energies that may differ from that predicted purely on the basis of n=1, 2, 3 etc. Therefore when electrons start filling, they may go to a new outer shell even before the inner shell is filled to capacity.


Atomic Number and Mass Number

The nuclei of atoms are made up of protons and neutrons. These two components of the nucleus are referred to as nucleons. The electrons occupy the space outside the nucleus. Since an atom is electrically neutral, the number of protons in the nucleus is exactly equal to the number of electrons.

This number is the atomic number given by the symbol Z. Atomic number represents the number of protons in an atom. As atoms are electrically neutral, an atom contains as many electrons as it has protons. The total number of protons and neutrons present in one atom of an element is known as its mass number.


Mass number (A) = number of protons (Z) + number of neutrons (n)

It can also be said that:

Mass number (A) = atomic number (Z) + number of neutrons (n)



The mass number (A) is written as a superscript on the top-left corner of the symbol of the atom. The atomic number (Z) is written as a subscript on the bottom-left corner.


The symbol represents an atom of sodium whose atomic mass is 23 and atomic number is 11. Calculate the number of protons, electrons and neutrons.

Atomic number Z =11

Atomic mass A = 23

No. of protons = Z = 11

No. of electrons = 11

No. of neutrons = A – Z

23 – 11 = 12


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