Atomic Orbital Diagram Principle: Hund՚S Rule and Energy State of Electron

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Learning Outcomes

After studying this lesson, you shall be able to:

  • Aufbau principle
  • Pauli Exclusion Principle
  • Hund՚s rule

Energy State of Electron

  • When the electrons are in the available orbitals with the lowest energy, we call it as the ground-state electronic configuration of the atom.
  • One or more electrons can jump into a higher energy orbital, if energy is applied to the atom in the ground state. We call it as an excited-state electronic configuration.
Energy State of Electron

The following principles are used to determine which orbitals occupy the electrons:

According to the Aufbau principle, an electron always goes into the available lower energy orbital.

The relative energies of the atomic orbitals follow:

  • 1s atomic orbital is closer to the nucleus and lower in energy than atomic orbital, which is lower in energy and closer to the nucleus than atomic orbital.
  • While comparing atomic orbitals in the same shell, s atomic orbital is lower in energy than p atomic orbital, and p atomic orbital is lower in energy than d atomic orbital.
Atomic Orbitals in Same Shell

According to the Pauli Exclusion Principle,

only two electrons can occupy each atomic orbital, and the two electrons must be of opposite spin.

  • The single electron of a hydrogen atom occupies atomic orbital,
  • The second electron of a helium atom fills the atomic orbital,
  • The third electron of a lithium atom occupies atomic orbital,
  • The fourth electron of a beryllium atom fills the atomic orbital.
  • The fifth electron of a boron atom occupies any one of the three degenerate atomic orbitals
Energy State of Electron

According to the Hund՚s rule,

when there are degenerate orbitals, an electron will occupy an empty orbital before it starts to pair up.

Degenerate Orbitals


1. Which of the following tells that the two electrons in atomic orbital must be of opposite spin.

a) Aufbau principle

b) Pauli Principle

c) Hund՚s rule

d) None of the above

Answer: B

2. Which of the following principles are used to determine how orbitals occupy the electrons.

a) Pauli Principle

b) Hund՚s rule

c) Aufbau principle

d) All of the above

Answer: D

# Aufbau principle

#Pauli Exclusion Principle

#Hund՚s rule

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