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What is an Atom? Definition, Structure, Parts

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Atoms are the tiny particles of an element that are responsible for chemical reactions. In the modern periodic table, all the known atoms are grouped into different groups and periods. Among them, some are stable and can exist independently whereas some of them are unstable due number of subatomic particles.

We have studied that atoms are made up of three subatomic particles that are electrons, protons, and neutrons. Their stability depends on the number of these subatomic particles. Therefore, to attain stability they share their valence electrons with the atoms of other elements. Atoms try to obtain octet states by sharing their electrons and gathering eight valence electrons on their outermost shell.

  • Atoms are the tiny particles of elements that get involved in a chemical reaction.
  • An atom was first discovered by John Dalton in1808.
  • Atoms are assumed to be created about 13.7 billion years ago.

This article provides a detailed study of the structure of the atom and its subatomic particles including electrons, protons, and neutrons.

Structure of Atom

Atoms are the smallest part of an element and between the 18th and 19th centuries, the study of the atomic models became a common topic to study. 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, its constituent.

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

  • John Dalton,
  • J.J Thomson,
  • Niels Bohr, and
  • Ernest Rutherford.

The proposed by all these scientists came up with a new concept and better explanations. It also made the atom an interesting topic for study and research to other scientists. Every theory had its own brief explanation and merits along with its limitations and exceptions.

An atom consists of two parts a nucleus which lies at the centre and carries 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 in a spiral motion.

Let’s study these subatomic particles in detail.

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:

  • 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

Electrons (-)

Electrons are negatively charged particles that are present at the outer shells of an atom. In the year 1897, J.J Thomson discovered electrons with his cathode ray tubes 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:

  • 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

Neutrons (n)

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 lies along with protons.  The mass of 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:

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

Charge on a Neutron = 0

  • Mass of a neutron: The mass of a neutron can be calculated by subtracting the mass of 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 it is a neutral subatomic particle it is affected by the presence of the magnetic field. Due to which its magnetic moment does not equal zero.
  • Neutrons are not affected by the electric fields as being a neutral charge.

Sample Questions

Question 1: Why are electrons and protons present in equal proportion in an atom?


Protons and electrons are found in equal proportion because they have equal and opposite charges in an atom.

Question 2: Why are atoms electrically neutral?


The numbers of protons and electrons is equal in an atom. Hence, these charged particles neutralizes each other and atom becomes neutral in nature.

Question 3: What does proton consist of?


The proton consist of basic particles called quarks and gluons.

Question 4: What are the symbols used for neutron?


The symbols n or n0 are used for the representation of nucleus.

Question 5: What determines the mass of an atom?


The nucleus consist of neutrons and protons at the center of atom determines its mass. As the mass of electron is negligible. 

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Last Updated : 12 Oct, 2021
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