CBSE Class 11-science Answered

• Orbitals with lower value of (n + l) are filled first as they have lower energy.

• If two orbitals have the same value of (n + l), then orbital with lower value of n will have lower energy.

• Energies of the orbitals in the same subshell decrease with increase in atomic number.

• Filling of electrons: The filling of electrons into the orbitals of different atoms takes place according to Aufbau principle, Pauli’s exclusion principle and Hund’s rule of maximum multiplicity.

• Aufbau principle: In the ground state of the atoms, the orbitals are filled in order of their increasing energies. The order in which the orbitals are filled is as follows

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 4f, 5d, 6p, 7s...

It is based on the (n + l) rule. It states that the orbital with lower value of (n + l) has lower energy.

• Pauli exclusion principle: No two electrons in an atom can have the same set of four quantum numbers. Only two electrons may exist in the same orbital and these electrons must have opposite spin.

• Hund’s rule of maximum multiplicity: Pairing of electrons in the orbitals belonging to the same subshell (p, d or f) does not take place until each orbital belonging to that subshell has got one electron each, i.e. it is singly occupied.

• Stability of completely filled and half filled subshells:
  • Symmetrical distribution of electrons.
  • Exchange energy.