Atomic Theory Notes for NCERT Class 11 Chemistry

Posted on July 26th, 2019

Atomic Theory NCERT Class 11 Chemistry


A theory of the structure and behavior of atoms has taken more than two million years to evolve, from the introspection of ancient Greek philosophers to the high-tech experiments of modern scientists. However, ideas about the atom were mainly theoretical, before the scientific revolution and the development of the scientific method that was started in the 16th century,. It wasn’t until the very end of the 19th century that technology became advanced enough to allow scientists a glimpse of the atom’s constituent parts: the electron, nucleus, proton, and neutron. It was John Dalton, in the early 1800s, who determined that each chemical element consists of a novel form of an atom and that the atoms are different by their masses. He developed a system of chemical symbols and, having ascertained the relative weights of atoms, arranged them into a table. In addition, he formulated the theory that a chemical combination of different elements occurs in simple numerical ratios by weight, which led to the development of the laws of definite and multiple proportions.


Matter:- Everything that surrounds us is a matter. The matter is anything that occupies space and has mass. All the buildings we see, the bridges, the atomic particles all are matter. Even our DNA, the air, the molecules inside our bodies, everything is matter.  We know that matter comprises of particles. These particles are atoms and molecules. In this section, we will cover the nature of matter.

Based on its physical state, we can divide the nature of matter into three major categories.




Solids: These are the substances in which the particles held very close to each other. Strong intermolecular forces subsist between these particles. All particles are firmly held in their positions. These particles have only vibratory motion. Solids have a definite shape and volume. Examples include iron, aluminum, stone, etc.

Liquids: These are the substances with weak intermolecular forces. The particles are capable of minimum movement. They have a definite volume but do not have a definite shape. They usually take the shape of the container in which we place them. Examples are water, milk, etc.

Gases:  These are the forms of matter having very weak intermolecular forces between their molecules. In gases, the molecules are free to move. The distance between molecules is large as compared to solids and liquids. Gases do not have a fixed shape as well as a definite volume. They tend to completely occupy the container in which they are placed. Examples are air, oxygen, hydrogen, methane, etc.

NOTE: – We can change the state of matter from one form to another by changing the conditions of pressure and temperature. We must also note that the nature of matter depends on its composition as well. If the matter consists of more than one type of particles then it is a mixture. On the other hand, if it consists of a single type of particles then it is a pure substance.


The atomic theory was proposed by the scientist John Dalton (1766-1844). This theory explains many of the concepts that are relevant and acceptable in the visual world i.e. the composition of a pure gold necklace, what makes the pure gold necklace different than a pure silver necklace, and what occurs when pure gold is mixed with pure copper. Before discussing the atomic theory, this article explains the theories that Dalton used as a basis for his theory: the law of conservation of mass and the law of constant composition.

LAW OF CONSERVATION OF MASS OR LAW OF CHEMICAL COMPOSITION : This law states that the total mass present before a chemical reaction is the same as the total mass present after the chemical reaction. In other words, mass is conserved. Antoine Lavoisier (1743-1794) was the one who formulated the law of conservation of mass by performing his combustion experiment in which he observed that the mass of his original substance i.e a glass vessel, tin, and air—was equal to the mass of the substance that produced—the glass vessel, “tin calx”, and the remaining air.

Atomic Theory, Class 11 chemistry, Bohr Model, Dalton’s atomic theory

Historically, this was a difficult concept for scientists to grasp. If this law was true, then how could a large piece of wood be reduced to a small pile of ashes? The wood clearly has a greater mass than the ashes. From this observation, scientists concluded that mass had been lost. However, the illustration below shows that the burning of coal does follow the law of conservation of mass. Scientists did not take into account the gases that play a critical role in this reaction.

LAW OF CONSTANT COMPOSITION: This law is also called the law of definite proportions. The law was formulated by Joseph Proust (1754-1826). This law states that if a compound is broken down into several integral elements, then the masses of the constituents will always have the same proportions, regardless of the quantity or source of the original substance. Joseph Proust did his experiments with basic copper carbonate. The illustration below depicts this law; 31 grams of H2O and 8 grams of H2O are made up of the same percent of hydrogen and oxygen.



Atomic Theory, Class 11 chemistry, Bohr Model, Dalton’s atomic theory


1) All matter is made of atoms.

2) Atoms are indivisible and indestructible.

3) All atoms of a given element are identical in mass and properties

4) Compounds are formed by a combination of two or more different kinds of atoms.

5) A chemical reaction that occurs is a   result of rearrangement of the atoms.


a. Dalton’s atomic theory proposed that all matter was composed of atoms, indivisible and indestructible building blocks. While all atoms of an element were identical, different elements had atoms of differing size and mass.

b. Dalton’s atomic theory also stated that all compounds were composed of combinations of these atoms in defined ratios.

c. Dalton also postulated that chemical reactions resulted in the rearrangement of the reacting atoms.

Atomic theory has been revised over the years to incorporate the existence of atomic isotopes and the inter conversion of mass and energy. In addition, the discovery of subatomic particles has shown that atoms can be divided into smaller parts. However, Dalton’s importance in the development of modern atomic theory has been recognized by the designation of the atomic mass unit as a Dalton.

Drawbacks of Dalton’s Atomic Theory of Matter

1. It was proved that an atom is not indivisible. As an atom can be subdivided into electrons, protons, and neutrons. But remember that atom is the smallest particle that takes part in a chemical process.

2. According to Dalton Atomic Theory, atoms of an element are identical in mass, size and many other chemicals or physical properties. But, practically we observe that atoms of different elements differ in their densities and masses. These atoms with the different masses are known as isotopes. For example, Chlorine (Cl) has 2 isotopes with the mass numbers of 35 and 37.

3. Also, according to this Theory, atoms of two-different elements differ in mass, size as well as many other chemical or physical properties. However, this is not correct in all situations. For example, Argon (Ar) and Calcium (Ca) atoms, each have an atomic mass of 40 AMU. These atoms are known as isobars having same atomic masses.

4. According to the Theory, when atoms of two or more elements combine in simple whole-number ratios, we get chemical compounds. But this is not true in case of complex organic compounds.

5. Dalton Atomic Theory fails to explain the existence of allotropes. This implies that the Dalton atomic theory fails to explain the differences in the properties of charcoal, graphite, and diamond (allotropes of carbon).

Discovery of subatomic particles: Thomson plum pudding model

Atomic Theory, Class 11 chemistry, Bohr Model, Dalton’s atomic theory

The Plum Pudding Model is a model of atomic structure proposed by J.J. Thomson in the late 19th century. Thomson had discovered that atoms are complex objects that are made of pieces with a positive and negative charge and the negatively charged electrons within the atom were very small as compared to the entire atom. He, therefore, proposed that atoms have a structure similar to plum pudding, with tiny, negatively charged electrons in which positively charged substrate are embedded. This was later shown to be incorrect.


a. This model failed to explain the emission of the electron spectrum

b. It also failed to explain the existence of the light spectrum.

c. It fails to provide a satisfactory mechanism to explain the deflection of α – particle. In the year of 1911, British Physicist Ernest Rutherford gave an atomic model which is capable of explaining the above phenomenon such as hydrogen spectrum consisting of different frequencies, light spectrum consisting of different frequencies, deflation of α – particles in the external field. Therefore the Ernest Rutherford’s nuclear model of the atom in 1911 replaced the Thomson’s Plum Pudding Model.


Discovery of the nucleus:- Planetary model of Rutherford model of an  atom

This theory was proposed by the Nobel Prize-winning chemist Ernest Rutherford in 1911 and is sometimes called the Rutherford model. Based on observations of experiments that showed the atom seems to contain a small core of positive charge. Rutherford postulated that the atom consisted of a small, dense and positively charged nucleus, around which electrons orbited in circular rings. This model was one of the first to propose the odd idea that atoms are mostly made up of empty space through which the electrons move.

Atomic Theory, Class 11 chemistry, Bohr Model, Dalton’s atomic theory


Bohr Model

A Danish physicist named Neil Bohr in 1913 proposed the Bohr atomic model. He modified the problems and limitations that were associated with Rutherford’s model of an atom and then proposed a model. According to this model, a small positively charged nucleus is surrounded by revolving negatively charged electrons in fixed orbits. He concluded that electron will have more energy if it is located away from the nucleus whereas the electrons will have less energy if it located near the nucleus.

Atomic Theory, Class 11 chemistry, Bohr Model, Dalton’s atomic theory

Postulates of Bohr Atomic Model

a. Electrons revolve around the nucleus in a fixed circular path termed “orbits” or “shells” or “energy level.”

b. The orbits are termed as “stationary orbit.”

c. Every circular orbit will have a certain amount of fixed energy and these circular orbits were termed as orbital shells. The electrons will not radiate energy as long as they continue to revolve around the nucleus in the fixed orbital shells.

d. The different energy levels are denoted by integers such as n=1 or n=2 or n=3 and so on. These are called quantum numbers. The range of quantum number may vary and begin from the lowest energy level (nucleus side n=1) to the highest energy level.

e. The different energy levels or orbits are represented in two ways such as 1, 2, 3, 4…  or K, L, M, N….. Shells.  The lowest energy level of the electron is called the ground state.

f. The change in energy occurs when the electrons jump from one energy level to others. In an atom, the electrons move from lower to higher energy level by acquiring the required energy. However, when an electron loses energy it moves from higher to lower energy level.




1. The article consists of electrons, protons, and nucleus.

2. We also discussed that the electrons are particles smaller than atom revolving around the nucleus in fixed orbits .they are negative charged particles their number count is equal to the atomic number. They are discovered by Thomson

3. Protons are positively charged particles founds in nucleus

.no. of electrons=no. of protons. They are discovered by Goldstein

4. The nucleus is a particle which carries the mass of an atom in which protons found .it is discover by Rutherford.

5. Thomson, Goldstein, Rutherford gave their own theories everyone has their limitations. That Thomson was not able to describe the location of electrons. Then Neil Bohr told that atom found in a fixed orbit which is slightly away from the nucleus.

Read another article for class 11 chemistry Covalent bonds and its types

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