Revision Notes on Structure of Atom

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  • Atoms are the basic building blocks of matter.

  • Different kinds of matter exist because there are different kinds of atoms present in them.

Charged Particles in Matter

  • Whenever we rub two objects together, they become electrically charged. This is because atoms contain charged particles in them. Therefore, atoms can be divided further into particles i.e proton, electron and neutron.

Protons were discovered by Ernest Rutherford, in his famous gold foil experiment.

Electrons were discovered by J.J. Thomson, in his cathode ray tube experiment.

Neutrons were discovered by James Chadwick.

Charged Particles in Matter

  • Atoms consist of protons and electrons in a balanced proportion.

  • Protons exist in the interiors of the atom and electrons exist in the exteriors of the atom. Therefore, electrons can be removed from an atom.

Failure of Dalton’s Atomic Theory

The postulates of the atomic theory by John Dalton

  • The matter is made up of tiny particles called Atoms that cannot be divided.

  • Atoms are never formed or destroyed during a chemical reaction. 

  • Atoms of an element exhibit same nature. They have the same size, mass, and character.

  • Atoms of different elements exhibit variant nature. They do not have same characteristics.

  • Atoms form compounds by combining in a ratio of whole numbers.

  • A compound contains a constant number and kinds of atoms

Dalton suggested that atoms can neither be created nor destroyed and are indivisible. But the discovery of electrons and protons in atoms lead to failure of this aspect of Dalton’s theory.

Thomson’s Model of an Atom

According to J.J. Thomson, the structure of an atom can be compared to Christmas pudding where electrons are present inside a positive sphere.

Thomson’s Model of an Atom

An atom is composed of a positively charged sphere in which electrons are embedded.

Atom is neutral as the positive and negative charged are equal in proportion.

Rutherford’s Model of an Atom

Rutherford’s Experiment

  • He experimented with thin gold foil by passing alpha rays through it.

  • He expected that the gold atoms will deflect the Alpha particles.

Rutherford’s Model of an Atom

Observations Inferences

Alpha particles which had high speed moved straight through the gold foil

Atom contains a lot of empty space

Some particles got diverted a by slide angles

Positive charges in the atom are not occupying much of its space

Only one out of 12000 particles bounced back

The positive charges are concentrated over a particular area of the atom.

Thus, Rutherford gave the nuclear model of an atom based on his experiment which suggests that -

  • Atoms contain a lot of unoccupied space

  • There is a heavily positively charged substance present in the center of the atom which is called the nucleus

  • The nucleus contains an equal amount of positive and negative charge.

The Nucleus of an Atom

  • The nucleus id located at the center of the atom.

  • All the mass of the atom is because of the nucleus.

  • The electrons revolve around the nucleus in circular parts which are called Orbits

  • If we compare the size of the atom and nucleus, the nucleus is much smaller than the atom.

The Nucleus of an Atom

Drawbacks of the Nuclear Model of an Atom

The Nuclear Model of the Atom failed to explain how an atom remains stable despite having positive and negative charges present in it. Maxwell has suggested a theory according to which if any charged particle moves in a circular motion it radiates energy. So, if electrons start moving in a circular motion around the nucleus they would also radiate some energy which would decrease at the speed of the electrons. As a result, they would fall into the nucleus because of its high positive charge.

What are nucleons? –  Protons and Neutrons are collectively called as Nucleons.

Bohr's Model of an Atom

Bohr suggested that –

  • Electrons spin around the nucleus in an individualized separate path or unattached orbit.

  • The electrons do not emit any energy while moving Indies special orbits.

  • These orbits are also called as Energy Levels.

  • They are represented using letters or numbers as shown in the figure below –

Bohr's Model of an Atom

The Neutrons

J. Chadwick discovered that there is another sub-atomic particle present in the atom. This particle carries no charge and is known as a Neutron. Therefore, we can conclude that atom consists of three types of particles -


which carry a negative charge


which carry a positive charge


they are neutral 

The distribution of electrons in different shells or orbits

  • If Orbit number = n

  • Then number of electrons present in an Orbit = 2n2

  • So, for n =1

  • Maximum electrons present in shell – K = 2 * (1)2 = 2

  • The outermost shell can contain at most 8 electrons.

  • The shells in an atom are filled in sequence.

  • Thus, until the inner shells of an atom are filled completely the outer shells cannot contain any electrons.


  • Valence Electrons – Electrons existing in the outermost orbit of an atom are called Valence Electrons.

  • The atoms which have completely filled the outermost shell are not very active chemically.

  • The valency of an atom or the combining capacity of an atom is given by the number of elements present in the outermost shell.

  • For Example, Helium contains two electrons in its outermost shell which means its valency is two. In other words, it can share two electrons to form a chemical bond with another element.

  • What happens when the outermost shell contains a number of electrons that are close to its maximum capacity?

Valency in such cases is generated by subtracting the number of electrons present in the outermost orbit from octet (8). For example, oxygen contains 6 electrons in its outermost shell. Its valency is calculated as: 8 – 6 = 2. This means oxygen needs two electrons to form a bond with another element.

Atomic Number of an Element

Atomic Number (Z) = Number of protons in an atom

Mass Number of an Element

Mass Number = Number of protons + Number of neutrons

Atomic symbol Notation


  • The atoms of an element can exist in several forms having similar atomic numbers but varying mass numbers.

  • Isotopes are pure substances.

  • Isotopes have a similar chemical nature.

  • Isotopes have distinct physical characteristics.

Isotopes of Hydrogen

Where can we use Isotopes?

1. The fuel of Nuclear Reactor – Isotope of Uranium

2. Treatment of Cancer – Isotope of Cobalt

3. Treatment of Goiter – Isotope of Iodine

Example: Consider two atomic species namely U and V. Are they isotopes?

  U V
Protons 5 5
Neutrons 5 6
Mass Number 5 + 5 = 10  5 + 6 = 11
Atomic Number 5 5

From the above example, we can infer that U and V are isotopes because their atomic number is the same.


The atoms of several elements can have a similar mass number but distinct atomic masses. Such elements are called Isobars



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