Electronic Structure of Atoms

Electron configurations

Electron Configurations

The way in which electrons are distributed among the various orbitals is called the electron configuration

Orbitals are filled in order of increasing energy, with no more than two electrons per orbital


This element has 3 electrons. We would thus begin by placing two electrons in the 1s ground state, or lowest energy, orbital. These two electrons would have opposite magnetic spin quantum numbers. We would then place the third electron in the next highest energy level orbital - the 2s orbital:


or, "1s two, 2s one".

Writing electronic configurations

What do we do now with the next element, Carbon (6 electrons)? Do we pair it with the single 2p electron (but with opposite spin)? Or, do we place it in another 2p orbital?

The second 2p electron in Carbon is placed in another 2p orbital, but with the same spin as the first 2p electron:

Hund's rule: for degenerate orbitals, the lowest energy is attained when the number of electrons with the same spin is maximized

Electrons repel each other, by occupying different orbitals the electrons remain as far as possible from one another

Electronic configurations can also be written in a short hand which references the last completed orbital shell (i.e. all orbitals with the same principle quantum number 'n' have been filled)

The electrons in the stable (Noble gas) configuration are termed the core electrons

The electrons in the outer shell (beyond the stable core) are called the valence electrons

Something curious

The noble gas Argon (18 electrons) marks the end of the row started by Sodium

Will the next element (K with 19 electrons) put the next electron one of the 3d orbitals?

The 4s orbital would be filled when we have an element with 20 electrons (Calcium).

Then we go back and fill up the 3d orbitals, which can hold a maximum of 10 electrons

Thus, the 4th row of the periodic table is 10 elements wider than the previous row - we have available five 'd' orbitals we can fill (with 10 electrons). These 10 elements are the Transition Elements, or Transition Metals.

With Cerium (element 58) the 'f' orbitals enter the picture. These orbitals can hold 14 electrons.

1996 Michael Blaber