Atomic Theory Introduction

Atomic Theory Introduction

Rutherford model of the atom:

  • The nucleus is the center of the atom,
  • It is the heaviest part
  • The nucleus consist of the protons and the neutrons
  • The electrons revolve round the nucleus in circular or elliptical path

Neils Bohr model of the atom

  • Electrons revolve round the nucleus in fixed circular paths known as the orbits
  • The electron can travel indefinitely within an energy level without loosing energy
  • Every orbit (or Shell) has a definite energy
  • The orbits or shells further away from the nucleus has higher energy
  • Electrons in a lower orbit can absorb energy and move to a higher orbit, these electrons are known as excited electrons
  • Excited state electrons can come to the ground state by emitting the additional energy it possesses (Emission Spectra)


Draw back of Bohr model

  • It cannot explain fine spectra of atoms
  • It cannot explain the Geometry of molecules

Wave Mechanical model of atom

  • Proposed by Werner Heisenberg
  • Based on Uncertainty principle or probability of locating an electron (Position and momentum are not certain)
  • Leading to concept of orbitals or “3-Dimensional space around the nucleus where there is a very high probability of locating the electrons”
  • The different orbitals are s, p, d, and f
  • Every electron has a quantum address by which it can be identified.
  • Also known as Quantum Numbers ( There are 4 quantum numbers n, l, m & s )

What did the quantum duck say? Quark!! Quark!!

  • What are the advantages of Quantum Mechanical model
  •  It is able to distinguish every electron in an atom with a set of 4 quantum numbers
  • Can explain the fine spectra of an atom
  • Can explain Zeeman effect, stark effect, Compton effect etc
  • Can be used to predict the geometry of molecules
  • Explains chemical bonding (Hybridization) in a more realistic way

The ‘Aufbau Order’ based on (n + l) rule




You may verify the sequence of arrangement using the
(n + l) rule The orbital having a lower n+l value has a lower energy. If two orbitals have the same n+l value the one having lower value for n has lower energy and therefore gets filled first





Electronic configuration for carbon would be 1s2 2s2 2p2 (2 + 2+2 = 6 = Z)

Electronic configuration for nitrogen would be 1s2 2s2 2p3 (2 + 2+3 = 7 = Z)

What are quantum numbers used for?
A set of 4 quantum numbers is used to distinguish every electron present in an atom. It may be compared to the Postal Code of your address.

Sometimes referred to as quantum address.

e.g. 2px








Which among the following values are possible for quantum numbers?








Electron Energy Level Diagram based on Aufbau Principle (EELD)

Atomic Theory














Draw EELD diagrams for the following elements.

Atomic number = 9, 14, 23, 24, 28, 29, 42
Draw the shapes of the p, and d orbital’s with the appropriate axis