What should I consider when building electron configurations?

1 Answer
Jun 12, 2016


Everything is based on energy. Nature always is most stable in the lowest energy configurations.


There are just a few rules/definitions to keep in mind in order to 'build up' atomic configurations properly. The first is the definition of the four quantum numbers (n, l , #m_l, m_s#).

(a) The Principle Quantum number, n defines the energy of an electron in an atom and the size of an orbital.
(b) The Angular Momentum Quantum number, l defines the shape of an orbital.
(c) The Magnetic Quantum number, #m_l# defines the orientation of an orbital in space.
(d) The Electron Spin Quantum Number #m_s# defines the spin direction of an electron in its orbital.

The four quantum numbers n, l , #m_l, and m_s# enable us to label completely an electron in any orbital in any atom. In a sense, we can regard the set of four quantum numbers as the “address” of an electron in an atom, somewhat in the same way that a street
address, city, state, and postal code specify the address of an individual.

For example, the four quantum numbers for a 2s orbital electron are n = 2, l = 0, #m_l# = 0, and #m_s# = +1/2 or -1/2 . It is inconvenient to write out all the individual quantum numbers, and so we use the simplified notation (n, l , #m_l, m_s#). For the preceding example, the quantum numbers are either (2, 0, 0, +1/2 ) or (2, 0, 0, -1/2 ). The value of #m_s# has no effect on the energy, size, shape, or orientation of an orbital, but it determines how electrons are arranged in an orbital.

There are four key principles or rules to keep in mind while constructing an atomic configuration.

  1. The Heisenberg Uncertainty Principle says that it is impossible to simultaneously know both the momentum and the position of an electron with certainty.
  2. The Aufbau Principle states that electrons fill subshells within an energy level from lowest level to highest.
  3. The Pauli Exclusion Principle says that “No two electrons in an atom can have the same four quantum numbers.”
  4. Hund's Rule states “the most stable arrangement of electrons in subshells is the one with the greatest number of parallel spins.” [electrons filling orbitals in a subshell with equal energy first spread out (parallel spins) before pairing up (opposite spins)].

With these basic rules you should be able to write out the electronic configuration of any atom or ion.