What is Atomic Structure ?

Atimic Structure notes for upcoming government exams

John Dalton, in the early nineteenth century gave his atomic theory. This theory has undergoes several changes owing to new discoveries. According to the modern atomic theory-atoms consists of electron, proton and neutron.

Atomic Strucutre, Particles present in atom


Electron is a subatomic particle that revolves around the nucleus in the atom. It’s mass is nearly negligible compared to that of the proton and neutron. It is 1/1837 of the mass of a hydrogen atom.


Like electron it is also a fundamental subatomic particle that present in the nucleus of the atom. It determine e the atomic number of the electron.


Along with the proton it constitutes the nucleus. It has no charge and Mass is slightly greater than a proton.

DiscovererJ.J. ThomsonLord RutherfordJ.Chandwick
ChargeNegatively Charged
Particles (-1.6*10⁻¹⁹) C
Positively charged
Particles (1.6*10⁻¹⁹) C
Neutral particles, zero charge
Mass9.1*10⁻³¹ kg1.67*10⁻²⁷ kg1.674*10⁻²⁷ kg

Rutherford’s Atomic Model

In his famous alpha particle scattering experiment, Rutherford tried to study the arrangement of the subatomic particle inside the atom.
This model suggest that :-

  • The Atom is nearly empty.
  • Positive mass (neutron was not discovered till then) is concentrated in a very small space called nucleus inside the atom.
  • Electrons revolves around the nucleus at high speed in circular path called orbits.

Niels Bohr’s  Atomic Model

According to Bohr each orbit is associated with a particular amount of energy and hence named them as energy levels.
Electrons can revolve only in these fixed paths and not in any path between two energy levels.

Radius of first Bohr orbit is 5.291*10⁻¹¹m.

Energy of the first Bohr orbit is -13.6 eV.

Atomic number (Z)

Atomic number can be defined as the number of proton present in the atom. It is also equal to the number if electrons present in a neutral atom.

     Atomic number of sodium (Na) is 11 that means that it has 11 proton in its nucleus. Atomic number of sodium can be written as 11Na.

Mass Number (A)

Mass number of an element is the sum of the number of protons and number of neutrons that present in the nucleus. eg- Mass number of carbon is 14 and it can be written as C14.

      Mass number (A) = e + p
  Where , e = numbers of electrons , p = numbers of protons

How to find numbers of Electron, protons and neutrons in any given element ?

Suppose, Chlorine is the given element whose atomic number (Z) is 17 and mass number (A) is 40. We have to find number of electron, protons and numbers of neutrons present in ₁₇Cl⁴⁰
We know, Atomic number (Z) of an element = number of protons or numbers of electrons present in the atom.
Therefore, in Cl  atom Z=17 that’s why number of electron (e) =17
Number of proton (p) = 17.
And we also know,
     Mass Number (A) = No. of protons (p) + no. of neutrons (n)
Therefore numbers of neutrons = (A – e) = 40 – 17 = 23.

Some more concept on Atomic Structure Chemistry Notes


Isotopes are the Atoms of same elements having same atomic number but different in mass number. eg – protium(₁H¹), Deuterium(₁H²), Tritium (₁H³) these are the isotopes of hydrogen. Isotopes have similar chemical properties but different physical properties.

Hydrogen(₁H¹) is the only atom in which neutrons are not present.

Protium(₁H¹) is the lightest isotope whereas Lead(Pb²⁰⁸) is the heaviest isotopes.


Atoms of different element having same mass number are called isobars. Physical properties are same in case of isobars, as physical properties depends on mass number. eg- ₁₄S⁴⁰ and ₁₇Cl⁴⁰.


Atoms of different elements having same number of neutrons but different mass number. eg – ₁₁Na²³, ₁₂Mg²⁴ both have 12 neutrons.


The three-dimensional space, present around the nucleus of an atom, where probability of finding an electron is maximum, is called orbital. Each orbital is associated with a fixed number of electron.

Shell and subshell of an atom

The Orbitals or path of different energies in which the electrons revolve, are called Shell. The energy of innermost shell is lowest and of outermost shell is largest.

These shells are represented by K, L, M, N, etc symbols. Where energy of shell K < L < M < N.

Each orbit or shell consist of one or more subshell depending upon the three dimensional graphic plot of electronic wave function and the internal atomic composition.

Each subshell is associated with one or more orbitals that are represented by the small letters s, p, d, f respectively.

The maximum number of electron present in s, p, d, f subshell are 2, 6, 10, and 14 respectively.

Quantum numbers

It is the combination of four numbers which shows position or energy, subshell, orientation and spin of an electron.

Principal quantum number

It tell us about the energy of the orbital in which the electron is present. It also tell us about the distance of the orbital from the nucleus. It is represented by a positive integer (n). The value of of n increases, distance of the orbital from the nucleus as well as the energy increases.

Azimuthal quantum number (l)

It describe the subshell or angular momentum of the electrons ( shape of the orbital in three dimension). Its value ranges from 0 to (n-1).

Magnetic Quantum number (m)

It determines the direction of an orbital in the space. Its value ranges from -1 to +1 including 0.

Spin quantum numbers

It determine the spin of the electron. There are two possible spins – clockwise and anticlockwise.

Bohr Bury Scheme for Electronic Configuration

According to this rule, the electrons enter in the orbit in accordance with the following rules :-

  • Maximum Number of electron in a shell is given by the formula 2n². Where n = 1, 2, 3,…… For K, L, M, … Shells.

For K shell maximum number of electrons is 2

For L shell maximum number of electrons is 8 (2n²=2*2²=8)

For M shell maximum number of electrons is 18 (2n²=2*3³=18)

  • The outermost shell can have maximum of 8 electrons. This rule is also known as octet rule.

Geometric representation(Atomic Structure) of electronic configuration

Atomic structure and Electronic configuration of sodium atom

Eg:- sodium(₁₁Na) has atomic number 11 and we know atomic number = number of proton or electron present in an atom.
We can say that sodium(Na) has 11 electron in its atom.
When we arrange these electrons in shells we will find K, L and M shells with 2, 8 ,1 electrons respectively.

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