Class 9 Science Chapter 4 Structure of Atom NCERT Notes

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The purpose of Class 9 Science Chapter 4 Structure of Atom NCERT Notes is to provide you with concise, step-by-step important points that will help clarify complex concepts. Students can check their knowledge of the topic.

Structure of Atom Class 9 Science NCERT notes are also an excellent source of information for students preparing for exams. It is also important for students to learn new things so that they can develop themselves.

Chapter 4 Structure of Atom Class 9 Science CBSE NCERT Notes

Atoms and molecules are the fundamental building blocks of matter.

By 1900, it was established that the atom was an indivisible particle, but it was known to contain at least one sub-atomic particle called the electron, which was identified by J.J. Thomson.

In 1886, E. Goldstein discovered the presence of new radiations in a gas discharge, which he called canal rays. These rays were positively charged radiations, and they eventually led to the discovery of another sub-atomic particle. This newly discovered sub-atomic particle had a charge equal in magnitude but opposite in sign to that of the electron. It was named the proton and was found to have a mass approximately 2000 times that of the electron.

Generally, an electron is represented as ‘e-‘ and a proton as ‘p+’.

The Structure of an Atom

Atomic Models

From the knowledge of existence of subatomic particles viz., electron, proton and neutron in an atom, various atomic models were proposed by different scientists.

Following are some of the atomic models:

  • Thomson’s Model of Atom
  • Rutherford’s Model of Atom
  • Bohr’s Model of Atom

The most trusted and scientifically established model of atom which is adopted these days is ‘Quantum Mechanical Model of Atom’. It will be dealt in higher classes.

Thomson’s Atomic Model

This model is often called the ‘Water Melon Model’.

In this model, Thomson predicted the presence of electrons inside positive sphere (made up of protons), just same as seeds of watermelon are embedded in red edible part of watermelon.

Although this model explained neutrality of atom but couldn’t able to explain other scientific experiments conducted on atom. Hence it was discarded.

Rutherford’s Atomic Model

In his famous ‘ɑ-ray Scattering Experiment’, Rutherford bombarded ɑ-ray (Helium nucleus, He) upon thin gold foil.

Rutherford made following observations from this experiment:

  • Most of ɑ-particles passed through gold foil undeflected.
  • Some of the ɑ-particles deflected by foil by small angles.
  • One out of every 12000 particles appeared to rebound.

From his observation, Rutherford draw following conclusions: Atom consists of predominantly empty space as most of

  • a-particles passed through gold foil undeflected.
  • Atom contains centrally placed positively charged nucleus (carrying positively charged particles), because few alpha particles were deflected and very few i.e., one in 12000 bounced back.
  • Since a minute fraction of a-particles suffered deflections and very few bounced back, this lead to conclusion that most of the space an atom is empty and the space occupied by nucleus is negligible compared to this empty space. Size of nucleus was about 10 times that of size of atom.
  • Whole of the atomic mass is concentrated in the nucleus.

On the basis of his experiment, Rutherford proposed model of atom having following features:

  • There is positively placed nucleus in an atom. Nearly all the mass resides in nucleus (Proton + Neutron).
  • Electrons revolves round the nucleus in well defined orbits.
  • Size of nucleus is very small compared to the size of atom.

Drawbacks of Rutherford’s Model (Instability of Atom)

According to Rutherford, electrons revolve round the nucleus in well-defined orbits, but electrons being charged particles will lose their energy and finally will fall into the nucleus. This will make atom highly unstable. This was the major drawback of Rutherford which was unexplained by him.

Bohr’s model of atom

To overcome drawbacks of Rutherford’s Model, Neil Bohr in 1912 proposed modified model of structure of atom. He made following assumptions:

Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
While revolving in discrete orbits, the electrons do not radiate energy.

Energy is emitted or absorbed by an atom only when an electron moves from one orbit to another.

These orbits or shells are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,….

Discovery of Neutrons

In 1932, James Chadwick discovered another sub-atomic particle that had mass, nearly equal to that of proton but no net charge. These particles were named as ‘Neutrons’ (being chargeless it was neutral and so was named as such).

Atomic Number

The total number of proton lying in the nucleus of any atom is called the atomic number.

An atomic number is the identity of an atom, changing atomic number means changing the atom.

Atomic number is denoted by ‘Z’. (Z=no. of Proton). For a neutral atom, no. of protons and electrons are equal.

Mass Number

It is the sum of total number of protons and neutrons lying in the nucleus of an atom.

Mass Number = Number of Proton + Number of Neutron

It is denoted by ‘A’. In the notation for an atom, the atomic number, mass number and symbol of the element are to be written as:

For example, nitrogen is written as:


Isotopes are atoms of same elements having same atomic number and different mass numbers e.g. Isotope of Hydrogen. protium (11H), deuterium (21H or D), tritium (13H or T). For example, Chlorine has two isotopes of mass number 35 and 37 respectively. 3517Cl and 3717Cl.

Uses of isotopes

  • Uranium isotope is used as fuel in nuclear reactor.
  • Isotope of cobalt is useful in treatment of cancer.
  • An isotope of iodine is used in the treatment of goiter.
  • Carbon-14 is used in carbon dating.

Relative atomic mass is an average of the masses of all the isotopes of the element.

In any mixture of pure chlorine, 75% of Cl35 and 25% of Cl37 is present.
∴ Relative atomic mass = 75% of Cl35 +25% Cl37
Relative atomic mass of chlorine


Isobars are the atoms of those elements which have the same mass number but different atomic numbers are called isobars. 4020Ca and 4018Ar have same mass number and different atomic number. 2411Na and 2412Mg are another examples.

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