Electron configuration



The electron configuration of an atom is a representation of the distribution of its electrons throughout the various energy shells. It is useful for rationalizing chemical properties and describing the chemical bonds that hold atoms together.

Neils Bohr proposed the idea of using electron configurations of atoms, based on his own model, to better understand them.

Electron configurations show how many electrons of an atom are in an energy shell, and how many are in each sub-shell of that energy shell. An atom's nth electron shell can contain 2 n2 electrons. For example, the first energy shell can contain 2 electrons and the second energy shell can contain 8.

There are sub-shells of energy shells which are labelled s, p, d, and f. These sub-shells can contain 2, 6, 10, and 14 electrons respectively. The broken down electron capacity of each energy shell can be seen as follows: Like energy shells, sub-shells are filled from lowest energy (s) to highest energy (f). It can also be seen that not every energy shell contains every sub-shell (n = 1 is missing p, d, and f, n = 2 is missing d and f, and n = 3 is missing f).

While the Bohr model of the atom displayed each energy shell and the number of electrons that were able to be on that shell, it did not display the number of electrons on each separate sub-shell.

Finding the electron configuration
The notation for the electron configuration of an atom is xyz, where x is the energy shell (or period/row on the periodic table), y is the sub-shell, and z is the number of electrons in that sub-shell. Once that sub-shell is filled, another xyz will appear beside it to indicate the next sub-shell.

Since the energy shells go upwards in terms of energy, it's easy to see the pattern that electron configurations follow.

Start at the first energy shell. There is only one sub-shell, s, which can contain 2 electrons. This means that for the first energy shell, the highest electron configuration (the electron configuration for the last element on that row, helium) is 1s2. On the second energy shell, there are two sub-shells (s and p). We know that s can contain 2 electrons (2s2 ) and that p  can contain 6 (2p6). To express the highest electron configuration of that energy shell (now being neon as it is the last element on that period), we place all the sub-shells beside each other. This means the electron configuration of neon is 1s2 2s2 2p6.

Hydrogen, for example, is 1s1. This means that hydrogen has one energy shell (first period/row), one sub-shell in that energy shell (s), and one electron in that sub-shell.

There are different ways of finding out the atom from the electron configuration. For 1s2 2s2, you can see that there are 2 electrons in the s sub shell of the first energy shell and 2 electrons in the s sub shell of the second energy shell. With the total number of electrons (4 in this case) you can see that this atom is Beryllium, since it has 4 electrons. Basically the sum of the superscripts of every sub-shell is the atomic number of the element that it represents.

When facing an electron configuration of an atom that has a longer electron configuration, finding the sum of every single superscript becomes an extremely slow method of finding the atom. Another way of finding what atom is being expressed by the electron configuration is to look at the last sub-shells to see which energy shell it is on, and then how many electrons into that energy shell it is. For example, 1s2, 2s2, 2p6, 3s2, 3p6, 4s2. By looking at the last sub-shell we can see that it is on the 4th period of the periodic table of elements. It is also the second element on the 4th period of the periodic table of elements, as it has 2 electrons in the 4s sub-shell. By looking at a periodic table, you can see that the 2nd element of the 4th row is calcium, which happens to be the atom expressed by that electron configuration.

'''The d-block and f-block have their energy levels reduced by 1 and 2, respectively. The d-block thus begins at 3d, not 4d as one would expect. The f-block begins at 4f, not 6f. '''

e.g. Gadolinium: [Xe]6s24f75d1 because the f-block is 2 energy levels lower than its period suggests. The 5d1 is due to a half-filled orbital being inherently more stable. The extra electron is knocked into the nearest energy level, which happens to be 5d.