Heisenberg principle

Question 2.9: State Heisenberg's uncertainty principle.

Answer.

It states that it is impossible to determine the exact position and momentum of electron simultaneously with absolute precision.

If we determine one value with more accuracy, then second value is determined with less accuracy and vice versa.

Question 2.10: Why is 4s orbital lower in energy than 3d orbital?

Answer

Energy of an orbital can be determined using n + ℓ rule or Wiswesser's rule. According to this rule, orbital with lower n + ℓ value has lower energy than orbital with higher n + ℓ value. For 4s orbital, n =4 and ℓ = 0, so its n + ℓ (i.e., n + ℓ = 4 + 0 = 4) value is 4 while for 3d orbital, n =3 and ℓ = 2, so its n + ℓ value (i.e., n + ℓ = 3 + 2 = 5) is 5 . Therefore, 4s orbital has lower energy than 3d orbital.

Question 2.11: What is n + ℓ rule?

Answer.

According to this rule, energy of the orbitals can be determined by n + ℓ rule or Wiswesser's rule. This rules states that the energy of orbital is determined from n + ℓ value.

The orbital with lower n + ℓ value has lower energy than orbital with higher n + ℓ value. If two orbitals have same n + ℓ values, than orbital with lower n value will have lower energy than orbital with higher n value.

For example, for 4s orbital, n =4 and ℓ = 0, so its n + ℓ (i.e. n + ℓ = 4 + 0 = 4) values is 4 while for 3p orbital, n =3 and ℓ = 1, so its n + ℓ (i.e. n + ℓ = 3 + 1 = 4) value is also 4. So, 3p orbital has lower energy than 4s orbital because of lower n value.

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