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Atoms are tiny particles that comprise all the things in the known universe. Atoms of an element are responsible for all chemical reactions occurring in nature. We know that atoms are made up of three fundamental particles namely,

  • Electrons
  • Protons
  • Neutrons

These particles are also called subatomic particles as they are found inside atoms. An atom is the basic building block of matter and all the thing around us is made up of combing atoms in different proportions. Let’s learn more about Atom, its constituents, and its structure in detail in this article.

Atom Definition

Atoms are defined as the smallest particle that can exist independently in nature and can participate in a chemical reaction. Atoms are the building block of all the matter in the universe. The word term “atom” is derived from the Greek word “Atmos” meaning indivisible. It was viewed that if we break the matter into smaller parts the smallest part so obtained is called the atom. But we know today that atoms can be further divided into fundamental particles.

Structure of Atom

Atoms are the smallest part of an element and between the 18th and 19th centuries, many scientists proposed their theories regarding the structure of atoms based on their studies. These theories were published to elaborate on the structure of atoms, and its constituent.

The most appreciated theories about the atomic model were proposed by,

These theories give us an idea about the structure of the atom and the general output of all the theories is that,

“An atom consists of two parts a nucleus which lies at the center and carries a proton (positively charged particle) and neutron (negatively charged particle) and the other part is the outer shell that carries electrons (negatively charged particles). The electrons present in the outer shells continuously revolve around the nucleus without losing any energy.” 

The image shown below gives the general structure of the atom.

Structure of Atom


Learn more about, Structure of Atom

Now let’s learn about these fundamental particles, Electrons, Protons, and Neutrons in detail.

What are Electrons?

Electrons are negatively charged particles that are present in the energy shells of an atom surrounding the nucleus. In the year 1897, J.J Thomson discovered electrons with his cathode ray tube experiment. The charge of an electron is equal and opposite to that of a proton held by the atom. Both charges neutralize each other and hence, atoms are neutral in nature.

Properties of Electrons

The basic property of the electrons are,

  • Charge of Electron: An electron is a negatively charged particle. It carries a negative unit.

Charge on an electon= -1.602 × 10-19 Coulombs

  • Mass of an electron: The mass of an electron is negligible in an atom. Its mass is 1/1837 of a proton.

Mass of a proton = 9.1093 × 10 -31 Kg


What are Protons?

Protons are the positively charged particles that are present in the nucleus of an atom. In the year 1886 Goldstein found that the charge and mass ratio of these positively charged particles depends on the nature of the gas. This concludes that the charge-to-mass ratio(e/m) is different for different gases.

Properties of Protons

The basic property of the protons are,

  • Charge of Proton: Proton is positively charged. The charge of a is equal to the number of charges in an electron(negative charge).

Charge on a proton = +1.602 × 10-19 Coulombs

  • Mass of proton: The mass of a proton is considered to be equal to a hydrogen atom. As a hydrogen atom consists of one electron and a proton in which the mass of an electron is negligible therefore it can be said that the mass of a proton is equal to a hydrogen atom.

Mass of a proton = 1.676 × 10 -27 Kg


What are Neutrons?

Neutrons are subatomic particles that are neutral in nature. They do not carry any of the charges. Neutrons are the major constituent of the nucleus and lie along with protons.  The mass of a neutron ranges slightly greater than a proton in magnitude. The protons and neutrons present in the nucleus are responsible for the chemical properties of an atom.

The concept of the neutron was proposed by Ernest Rutherford in 1920. And was discovered by the British physicist James Chadwick in 1932.

Properties of Neutrons

The basic property of neutrons are,

  • Charge on a neutron: As neutrons are neutrally charged subatomic particles the charge carried by them is 0.

Charge on Neutron = 0

  • Mass of a neutron: The mass of a neutron can be calculated by subtracting the mass of the proton from the mass of the deuterium nucleus. The mass of a neutron is 1.008 atomic mass units (amu).

Mass of a neutron = 1.676 × 10-27 Kg

  • Although a neutron is a neutral subatomic particle yet it gets affected by the presence of the magnetic field. Due to this its magnetic moment does not equal zero.
  • Neutrons are not affected by the electric fields.

History of Atom

According to some scientists, all the atoms in the universe are created 13.7 billion years ago after the Big Bang. Initially, only electrons and quarks are created and millions of years later as the universe cools down quarks aggregated to form protons and neutrons which on further engagement created atoms. These atoms after combining form all the matter in the universe. Early Indian philosophers were the first to think that matter is made up of smaller particles and they call them “Anu” and “Parmanu”. The Greeks were also of the same view. But the modern thinking of atoms was first pioneered by John Dalton.

What is the Size of an Atom?

Atomic Size of an atom is explained in terms of its radius. The distance from the center of the nucleus to the outermost shell of an atom is called its atomic radius. The atoms are very small particles and it is impossible to see atoms through the naked eye. Using a microscope also it is very difficult to see the atom only very powerful electrons microscope are able to see the electrons. The exact size of atoms is not easily calculated, as the location of the electron with respect to the nucleus of the atom can not be found. However can estimate the atomic radius as size to be, 10-9 m.

We can visualize the size of atoms as,


Radii (in m)

Atom of Hydrogen10-10 m
Grain of Sand10-4 m
A football10-1 m

From the above table, we can observe that a grain of sand has, 106 atoms and a football has 109 atoms.

What is Atomic Mass?

The sum of the number of protons and neutrons is called the atomic mass of the element. The atomic mass as the name suggests is the mass of the atoms. As we know that atoms are made up of electrons, protons, and neutrons and the mass of protons and neutrons is exponentially higher than the mass of electrons so for finding the mass of the atom we find the number of protons and neutrons in the atom.

Atomic Mass is a relative concept and is measured in Atomic Mass Units. We define 1 amu as the 1/12 mass of the carbon (C-12) atom. Hence if we say the mass of a Sodium atom is 23 amu it means it is 23 times heavier than 1/12 of the C-12 atom.

Atomic Mass of Some Common Elements

The atomic masses of some common elements are discussed below in the table:


Atomic Mass (amu)


What is Atomic Number?

The number of protons in an atom is called the Atomic Number of an element. We know that there are various elements and observing all the properties of all the elements individually is tedious work. So we arrange various elements into some group that resembles similar properties. These atoms are arranged on their atomic number. It is the number of protons that any atom has and it is denoted by “Z”.

Atomic Number of Some Common Elements

The atomic numbers of some common elements are discussed below in the table:

ElementsAtomic Number

Read More,

FAQs on Atom

Q1: Who Discovered Atom?


atom was first used by the famous Greek philosopher Democritus in 400 B.C. 

Q2: What is an Atomic Radii?


The atomic radii is defined as the distance between the nucleus and the electrons in the outermost shell of an atom.

Q3: Why are Atoms Neutral?


As we know that atoms consist of electrons (negatively charged particles), protons (positively charged particles) and neutrons, and it is electrically neutral because it has an equal number of electrons and protons.

Q4: What determines the Mass of an Atom?


The mass of the atom is determined by the nucleons of that atom, i.e. the number of protons and neutrons that the nucleus of the atom has. It is also explained using the mass number of the atom.

Q5: Why do Atoms Combine?


Atoms combine to form molecules which are more stable in nature than atoms.

Q6: What is the Structure of an Atom?


An Atom is composed of Electrons, Protons and neutrons. The nucleus of the atom consists of protons that are positively charged and neutrons that are neutral. The outer region of atom consists of electrons that are negatively charged.

Last Updated : 17 Dec, 2023
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