Principles in Chemistry

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Principles in Chemistry

Pauli’s Exclusion Principle

It states that “No two electrons in an atom can have the same set of four quantum numbers”.

In other words “An orbital can have a maximum two electrons and these must have opposite signs”.

Aufbau Principle

It states that “Orbital of lowest energy is filled first, before the filling of orbitals having higher energy starts”.

So, the order of relative energies is : 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d and so on.

(n + l) Rule

Lower the value of (n + l) for an orbital, the lower is its energy.

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If two orbitals have same (n + l) value, the orbital with lower value of n has lower energy and hence is filled first.

Exceptions of Aufbau Principle

The electronic configurations having half – filled or completely filled orbitals are more stable.

In light of this fact the electronic configurations of Cu (29) and Cr (24) can be studied as follows :

The electronic configuration of Cr24 according to Aufbau’s principle :

1s², 2s², 2p6, 3s², 3p6, 4s², 3d4

The actual configuration of Cr24 : 1s², 2s², 2p6, 3s², 3p6, 4s¹, 3d5

Since 3d subshell is more stable when it has 5 electrons in it i.e., it is half filled so the latter configuration is actual and more stable of Cr.

The elements not obeying Aufbau’s principle

Cr24, Cu29 Nb41, Mo42, Tc43, Ru44, Rh45, Pb46, Ag47, La57, Gd64, Pt78, Au79 etc.

Hund’s Rule of Maximum Multiplicity

It states that “Electron pairing in any orbital (s, p, d, f) cannot take place until each orbital of the same sublevel contains one electron.”

e.g., O8=>2, 6
1s², 2s², 2p4
1s², 2s², 2p²x, 2p¹y, 2p¹z

Heisenberg’s Uncertainity Principle (1926)

It states that “It is impossible to specify at any given moment both the position and momentum of an electron.”

If δx and δp be the uncertainties with respect to the position and the momentum, then uncertainty principle can be expressed as:

Δx . Δp ≥ h/4 π

Bohr’s Principle

It states thatx “An electron can resolve only in those orbits whose angular momentum (mvr) is an integral multiple of the factor (h/2π)”

Bohr – Bury Scheme

  • The maximum no. of electrons in each shell is 2n², where n is the principal quantum number.
  • The maximum no. of electrons in outermost orbit can be 8 and in its penultimate orbit can be 18.
  • The outermost shell can contain not more than 2 electrons, if the next to the outermost has not reached its maximum requirement (e.g.),
  • Ca20 ⇒ 2, 8, 8, 2;
  • Sc21 ⇒ 2, 8, 9, 2;
  • Zn30 ⇒ 2, 8, 18, 2;
  • Kr36 ⇒ 2, 8, 18, 8;
  • Rb37 ⇒ 2, 8, 18, 8, 1.