Structure of Atom Part - 9 | Class 11th

·       d-orbitals:

For d-orbitals, Ɩ = 2 and m_Ɩ = -2, -1, 0, +1 and +2. i.e., there are five possible orientations for d orbitals. So there are 5 types of d-orbitals. They are d_xy, d_yz, d_zx, d_x²_-y² and d_z². The shapes of the first four d-orbitals are double dumb-bell and that of the fifth one, d_z², is dumb-bell having a circular collar in the xy-plane. The five d- orbitals have equivalent energies. For d-orbitals the number of radial nodes is 2 and the total number of nodes is n-2. Boundary surface diagrams for d-orbitals are as follows:

·       f-orbitals:

For f-orbitals, Ɩ = 3 and m_Ɩ = -3, -2, -1, 0, +1, +2 and +3. i.e., there are seven possible orientations for f orbitals. So there are 7 types of f-orbitals. They are fx³, fy³, fz³, fx(y²-z²), fy(z²-x²), fz(x²-y²) and fxyz. They have diffused shapes.

ü  Electronic Configuration of Atoms: The arrangement of electrons in various orbitals is called the electronic configuration. This arrangement is obtained on the basis of following rules,

Ø Aufbau principle:  Electrons are filled in the various orbitals in the increasing order of their energies, i.e., orbital having lowest energy will be filled first and the orbital having highest energy will be filled last.

                 This rule has two sub rules:

a)     The various orbitals are filled in the increasing order of their (n+Ɩ) value.

b)     If two orbitals have the same (n+Ɩ) values, the orbital with the lower n value is filled first this rule is called Bohr-Bury rule. The increasing order of orbitals is as follows:

1s < 2s < 2p < 3s <3p < 4s <3d < 4p <5s < 4d <5p < 6s < 4f < 5d < 6p < 7s

  Ø  Pauli’s Exclusion Principle: No two electrons in an atom can have all the four quantum numbers same.  It can also be stated as – An orbital can have maximum two electrons and they must be of opposite spin.
                        

Ø  Hund’s Rule of Maximum Multiplicity: No electron pairing takes place until each orbital is first singlifilled, e.g., N (7) has electronic configuration 1s² 2s², 2p_x¹ 2p_y¹ 2p_z¹ according to Hund’s rule.

Degenerate orbitals: Orbitals having the same energy are called degenerate orbitals.

Shielding Effect or Screening Effect:  Due to the presence of electrons in the inner shells, the electron in the outer shell will not experience the full positive charge on the nucleus. So due to the screening effect, the net positive charge experienced by the electron from the nucleus is lowered and is known as effective nuclear charge.

ü  Stability of Completely Filled and Half Filled Subshells:

·       For atoms having half filled or completely filled electronic configurations have extra stability compared to other atoms. This is due to their symmetrical distribution of electrons and greater exchange energy. For example, the electronic configuration of Cr is [Ar] 3d⁵4s¹ and not 3d⁴4s². This is because d⁵ represents a half filled configuration and has extra stability. Similarly for Cu the electronic configuration is [Ar] 3d¹⁰4s¹ and not 3d⁹4s².

Another factor that explain stability of half-filled orbitals is exchange energy. More the number of exchanges more is the stability.