Atoms and Molecules


History Of Atoms

Thomson’s Atomic Model

Rutherford’s Atomic Model

Atomic Number

Atomic Mass



History of Atom

The journey of atoms starts around 500 BC, an Indian philosopher Maharishi Kanad postulated that if we go on dividing matter, we shall get smaller smaller particles ultimately a time willl come when we shall come across the smallest particle beyound which furthet division is not possible. He named this particle as “parmanu”

Around the same era ancient Greek philosopher Democritus and his mentor Leucippus suggested a similar hypothesis and called this indiviscible particle as “atom”. But all this based on their philosophical consideration they don’t have any experimental proof to validate their ideas.

Later in the year 1803 a british chemist John Dalton picked up the idea of divisibility of matter, which was till then just a philosophy. He took the name for the smallest particle of matter as “atom”. His theory was based on the “laws of chemical combination”. Dalton”s TOMIC THEORY POSTULATES that all matter wheather it is element or compound or a mixture is composed of small particles called atom. Atoms are indivisible and cannot be created or distroyed in a chemical reaction. Atoms are identical in mass and chemical properties for a given element and have different properties for different element.

According to John Dalton’s theory atoms are considered as the smallest indivisible particle but later experiment shows that that’s not true instead atoms are also made upof different particle.

The next breakthrough come in the year 1897 by a british physicist Joseph John Thomson. JJ thomson through his vaccum tube experiment concluded that the atom was not indiviscible as previously claimed. In his experiment he supplied a high voltage in a vaccum tube consists of a two metallic plates. The plate which connect with the positive terminal was called as “anode” whereas which connects with negative terminal as “cathode”. He observed that at the anode end current starts flowing even through the circuit is not complete which let him to think that their is something which flows between cathod to anode. In order to get to know the direction of flow he applied a photosensitive substance behind both the plates and observe lighting behind the anode plate which led him to conclude that the direction of flow is from cathode to anode and called them as cathod rays. Later he applied positive and negative plate along the sides of the tube he observe that the lighting shifts toward the positive plate. This shows that cathod rays are negatively charged and instead of rays they are actually particles. His experiment also shows that the negative particles were alike andsmaller then atoms. Thomson also concluded that the negative particles were couldn’t be the fundamental unit of atom because atoms are electrically neutral.

Thomson Atomic Model:

Based on his finding Thomson put forward his model on the structure of atom which is simillar to that of a christmas pudding. He states that electrons are arranged in a spherical positive sphere as like dry fruits were arranged in a christmas pudding. For better understanding you can think of a watermelon and consider watermelon as a positive sphere and its seeds inside the watermelon were the electrons. He also added that the negative and positive charges were equal in magnitude which makes the atom electrically neutral.

The next major discovery on atom was done by Ernest Rutherford. He was a physicist from New Zealand and studied at Cambridge University under J.J. Thomson. He performed an experiment which is known as Rutherford’s gold foil experiment.

Rutherford’s Atomic Model:

On the basis of this experiment he discovered the nucleus and put forward the nucleus model of an atom, which had the following features:

  • Atom is positively charged at the center and called it as nucleus.
  • All the mass of the atom resides in the nucleus.
  • The electrons revolved around the nucleus as planets revolve around the sun.
  • Most of the part of the atom is empty.
  • Size of the nucleus is very small as compared to the size of the atom.

He later discovered that nucleus consists of a positively charged particle and named it as “proton”.He also predicted about the existence of a neutrally charged particle but failed to find them. Later one of his student named James Chadwick went on to discover neutron in 1932.

Drawbacks of Rutherford’s Model:

His model unable to explain the reason which makes the electrons to keep revolving in a orbit instead of falling into the nucleus.

Entry of Niels Bohr:

The drawbacks of Rutherford atomic model was overcome by a Danish physicist named Niels Bohr. He postulates that electrons are allowed to revolved only around a certain special orbit known as discrete orbits of electrons.These orbits are called energy level. Electrons do not radiate energy while revolving at a certain energy level. If an electron loses energy it demotes to lower energy level and if electron gains energy it promotes to higher energy level, and hence the electron does not fall in the nucleus.


Valency can be defined as the combining capacity of an atom. From the Bohr-Bury model model we know that electrons in the outermost orbit tries to complete their octet (i.e. they always try to have 8 electrons in their outermost orbit). In order to complete their octet atoms lose, gain or share their electrons to other atoms of same or other element. And the number of electrons exchange during the process gives us the valency.

For example hydrogen has one electron in its outer most orbit so in order to complete its octet he has two option either he can gain ‘7’ electron or share his ‘1’ electron with another atom in such a way that he had ‘8’ electrons in its outermost orbit. So first he share his electron with another hydrogen atom and have 2 electrons in its outermost orbit. Now he looks for an atom who had 6 electron in its outermost orbit and ready to share his electrons. hydrogen find oxygen who had 6 electron in its outermost orbit and by sharing their electrons they complete their octet.

Atomic Number:

Atomic number is defined as the total number of proton present in the nucleus of an atom.

Atomic Mass:

Atomic Mass or Mass Number is the sum of the total number of protons and neutron present in an atom. We didn’t considered electrons here because their mass is negligible as compared to proton and neutron.


Isotopes are atoms of the same element which have the same atomic number but different mass number.


Isobars are atoms having the same mass number but different atomic number.