carbon-atomAtom is the smallest building block of an element which can’t be further broken by using any chemical process. A typical atom contains electrons, protons and neutrons. Neutrons and protons are held together in the center of an atom called nucleus and electrons orbit around the atom at fixed distances called orbits or shells. All neutral atoms contain an equal number of electrons and protons.


Elements are distinguished from each other by the number of protons. If somehow number of protons is changed in the atoms of an element as it happens during radioactivity than the original element will be converted into another element. But changing the number of neutrons will give different isotope of the same element. An isotope of an element has the same number of protons but the different number of neutrons.


The total mass of the atom is concentrated in its nucleus, which is measured in a.m.u (atomic mass unit). One a.m.u is about 1.6 × 10^-27 kilogram. Mass of the atom is calculated as number of nucleons or sum of number of protons and neutrons. One nucleon (either a proton or a neutron) has mass equal to 1 a.m.u.

There 118 different kinds of atoms based on the number of protons that builds 118 different elements. These elements are classified into “Periodic Table” on the basis of similarities between them.



Electrons are negatively charged particles that orbit around the nucleus in certain energy levels. Their orbiting speed is so fast that it is almost impossible to predict 100% accurately where they located. For an atom to be neutral it must contain equal number of electrons and protons. Electrons are 1800 times smaller than neutron and proton.


Protons are positively charged particles held together with neutron in the nucleus of an atom. Their mass is 1 a.m.u. Elements are distinguished from each other by the number of proton. Hydrogen is a unique element containing only a proton in its nucleus and one electron orbiting around it. If electron is somehow removed from it than only proton will remain or you can say a proton is a positively charged hydrogen.



Neutrons are chargeless particles, which are also located in nucleus of an atom together with proton. Number of neutron predicts the isotope of an element. An isotope of an element is the same element with different mass. Changing the number of neutrons will change the isotope of an element. For example: Hydrogen have three isotopes (Protium, Deuterium, and Tritium) each containing one proton but different number of neutrons.


History of Atom

Democritus developed the idea of the atom in 460 B.C. He thought that if you break a piece of matter in half and again break that in half and continue this process. At some point, you will get the smallest piece of matter which will not divide further. He called that piece of matter “Atom”.

In 1800’s, John Dalton (An English chemist) performed some experiments on different chemicals and showed that atoms are made up of elementary particles. He didn’t know about their structure, but only some evidence which showed that there are fundamental particles than the atom.

In 1897, J.J Thomson presented model of the atom. He knew that electrons have negative charge so there must be equal positive charge to make the atom neutral so, He thought that electrons are stuck on the protons like raisin in the pudding.


In 1911, Earnest Rutherford performed an experiment by bombarding alpha particles on gold foil and he came up with the result that most of the volume of the atom is empty and nucleus containing positive charge lies in the center of the atom and electron are orbiting around the nucleus in that empty volume like planets around the sun. But there was a problem with his model because according to theory of electricity and magnetism, opposite charges attract each and electrons gradually losing its energy must fall in the nucleus spirally, giving continuous spectrum of light. But in reality it was not happening.

In 1912, a physicist Neils Bohr came up with the idea and rectified the error from atomic model of Rutherford. He said that electrons don’t spiral into the nucleus and gave two rules about the motion of the electron.
Rule 1: Electrons orbit around the nucleus at certain allowed distances called “shells.”
Rule 2: Atoms radiate energy only when Electrons jump from higher energy orbit to lower energy orbit and absorb radiation when jumps from lower energy orbit to higher energy orbit.

Different people came with different ideas and the atomic model was developing until it reached the present day model of the atom.

Cite this Page

Abdul Wahab, "What is Atom," in science4fun - Learn Science with Experiments and Topics, June 20, 2018,