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What is an Atom?

An atom is smallest unit of matter which is made up of particles called protons, neutrons, and electrons. These smallest particles of atoms are responsible for the mass and charge of atoms. Atom is called the basic building block of chemistry. Most of the space of an atom used to be empty.  It is consists of a positively charged and small nucleus of protons and neutrons, these inner particles surrounded by electrons. Atom examples are Hydrogen (H), Iron (Fe), Copper (Cu) and Calcium (Ca) etc.  The atom diagram is given below.

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Atomic Structure

Atoms are consists of 3 subatomic particles, protons, neutrons and electrons. The nucleus of an atom is formed by protons and neutrons and remain in the nucleus. Electrons orbit in a cloud form around the nucleus. In an atom, the number of protons used to be equal to the number of electrons and the number of protons is almost the same as neutrons but not always. An atomic number of an atom is defined as the number of protons present in the nucleus of an atom.  The atoms are electrically neutral hence the number of protons is equal to the number of electrons in an atom. Atomic weight indicates the relative weight with the atom of hydrogen (H) whose atomic weight is taken as unity.

Atomic Model

Many scientists have worked on the atomic structure from earlier times and they gave atomic structures and atomic models on the basis of their experimental observations. Some famous and important atomic models are as follows.

  • Dalton's Atomic Model

According to this model,

  1.  Elements are made of small and indivisible particles called atoms.

  2.  All atoms of the same element are identical and different elements have different types of atoms. 

  3. Atoms can neither be created nor destroyed.

  4. The formation of compounds took place when atoms of different elements join in simple proportions or ratios to form molecules.

  • Thomson Model of Atom 

Thomson model of an atom is also known as the watermelon model or plum pudding model of an atom. As per the model, the atom is made of electrons surrounded by a soup of positive charges to balance negative charges of the electrons in the same manner as “plums” (negatively charged)  surrounded by “pudding” (positively charged). In the mathematical model of Thomson, the corpuscles (which is now called electrons) were arranged in rotating rings non-randomly.

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  • Rutherford Model of Atom

The Rutherford model of the atom is considered the classic model of the atom In many ways, despite the fact that it is no longer considered an accurate representation. The atomic model of Rutherford represents that an atom is a mostly empty space where electrons orbits around a fixed and with electrons, positively charged nucleus in the set, predictable paths.

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  • Atomic Model of Bohr

Neils Bohr’s model of the atom was introduced in the year 1915 with the modification of Rutherford’s model of an atom. In his researches and experiments on atomic structure, he gave the following conclusions.

  1. Electrons In an atom move around the nucleus which is charged positively in a definite circular path called orbits or shells.

  2. These circular orbits are referred to as orbital shells and have fixed energy.

  3. The energy levels are shown by an integer number (n=1, 2, 3…) which is called the quantum number. The quantum numbers start from the nucleus side with n=1 being the lowest energy level. The orbits n=1, 2, 3, 4… are termed as K, L, M, N…. shells and an electron is said to be in the ground state when an electron attains the lowest energy level.

  4. The electrons in an atom move from a lower level of energy to a higher level of energy after attaining the required amount of energy and by loss of energy when an e- moves from a higher energy level to a lower level of energy.

  • Quantum Mechanical Model of Atom

As per the Quantum mechanical model of the atom these points have been concluded. They are as follows.

  1. The energy of an electron (e-) is quantized which means it can only have certain specific values of energy.

  2. The electron’s quantized energy is the allowed solution of the Schrödinger wave equation and quantized energy is the result of wave properties of the electron.

  3. According to the Uncertainty principle of Heisenberg, the exact position and momentum of an electron cannot be determined. So the only probability of finding an electron at a position can be determined and it is |ψ|² at that point where ψ represents the function of a wave of that electron.

  4. The orbital of an atom is defined as the wave function of an electron. Every wave function or atomic orbital have some energy and shape associated with it. 

  5. |ψ|² is probability density and have a positive value.

History and Facts of Atom

  • The matter is made of building blocks that are not divisible. This idea was considered in early the 5th century BCE by Leucippus and Democritus. These particles were called “atomos” by the Greeks which means indivisible and in modern times the word “atom” is derived from this term.

  • The concept that everything is made from atoms was firstly given by John Dalton in the year 1808.

  • The atoms consist of two major parts. These are a nucleus of an atom and a cloud of electrons.

  • The chemical behaviour of atoms is a result of the number of electrons and their arrangement in the electron cloud.

  • The nuclear properties such as atomic mass, atomic number, nuclear cross-sections etc. of the element are determined by the number of protons and the number of neutrons.

  • The number of atoms present in 1 mole (e.g. 12 grams of carbon) of a substance is equal to the constant of Avogadro that is equal to 6.022 x 10²³.

  • The approximate size of atoms is 10⁻¹⁰ meters or 10⁻⁸ centimetres. The volume of an atom is around fifteen orders of magnitude larger than the volume of a nucleus.


Atomic structure is the basic chapter of chemistry which help to understand other complicated concepts of chemistry. We have learnt the basic properties of atom, atomic structures as well as different atomic models proposed by different scientists based on their observations and experiments. 

Last updated date: 28th May 2023
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FAQs on Atom

1. What is an atom?

An atom is the smallest unit of matter that took formation in a chemical element. Every liquid, solid, gas, and plasma is made from ionized or neutral atoms. Every atom of this world is consists of a very small nucleus and one or more electrons. One or more protons and a number of neutrons participate in the formation of the nucleus of an atom. The most common type of hydrogen atom has zero neutrons. It is calculated by experiments that around 99.94% of the mass of an atom consists in the nucleus. When the number of protons and electrons are equal, then the atom is electrically neutral.

2. Who discovered atom?

The idea that everything is made of atoms was firstly introduced by John Dalton in his book which he published in the year 1808. He was an English chemist and physicist He is referred to as the "father" of atomic theory. It was speculated by him that all atoms of an element are identical in size and mass. It was reasoned by him that elements were composed of smaller atoms. He told that the mass of one atom of hydrogen (H) is the atomic mass unit. The unit became the Dalton.