The electron configuration of an atom gives the distribution of electrons among atomic orbitals in the atom. Two general methods are used to show electron configurations. The subshell notation uses numbers to designate the principal shells and the letters s, p, d, and f to identify the subshells. A superscript following the letter indicates the number of electrons in the designated subshell. The ground state electron configuration for nitrogen would be 1s22s22p3. A drawback to this method of showing the electron configuration is that it does not tell us how the three 2p electrons are distributed among the three 2p orbitals. We can show this by using an orbital diagram in which boxes are used to indicate orbitals within subshells and arrows to represent electrons in these orbitals. The direction of the arrows represent the directions of the electron spins. The orbital diagram for nitrogen is
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The way we arrive at electron configurations such as the one for nitrogen above is to use a set of rules collectively called the aufbau principle.
- Electrons occupy orbitals of the lowest energy available
- No two electrons in the same atom may have all four quantum numbers alike
- When entering orbitals of the same energy, electrons initially occupy them singly ant with the same spin
- Electrons fill orbitals in order of the quantum number sum (n + l). For equal (n + l) sums, fill levels in order of increasing n.
A mnemonic diagram for the aufbau principle known as the diagonal rule is shown here
The aufbau principle is really a thought process in which we think about building up an atom from the one that preceeds it in atomic number, by adding a proton and neutrons to the nucleus and one electron to the appropriate atomic orbital.
There are some exceptions to the to the aufbau principle. The first is chromium (Z = 24), the aufbau principle predicts the an electron configuration of [Ar]3d44s2 but experimentally we find it to be [Ar]3d54s1. The next exception found is that of copper (Z = 29), the predicted electron configuration is [Ar]3d94s2 but experimentally we find it to be [Ar]3d104s1. The reason for these and other exceptions are not completely understood, but it seems that a half-filled 3d subshell in the case of chromium or a completely-filled 3d subshell in the case of copper lend a special stabilty to the observed electron configurations. There is no need to dwell on these exceptions, the point to remember is that the aufbau principle predicts the correct electron configuration most of the time and that the energy of the predicted electron configuration is very close to the ground state energy.
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