We talked about quantum numbers and atomic orbitals. In this section, we will focus our attention mainly on writing the electronic configuration of atoms and the rules associated with it. First, let's talk about the ground state electronic configuration. The ground state configuration means the electronic configuration at the lowest energy state.
There are certain rules that should be applied to the filling of orbitals. In order to do that properly, we need to know the Aufbau principle. According to the Aufbau principle, the filling order of electrons obeys a general pattern in which the electrons try to occupy the orbitals in such a way as to minimize the total energy; that is, they occupy the lowest energy orbitals first and then step-by-step go to the next available higher energy levels successively. Of course, there are some exceptions to these generalizations. Some students find it hard to remember the order of filling. The diagram given below may help you. So take a close look. Filling order can be depicted as follows:
This is an easy way to remember the general order of electron-filling in the subshells. The order of filling is 1s, 2s, 2p, 3s, 3p, 4s, 4p, 5s, . . ., and so on.
An s orbital is spherical in shape. All p orbitals have dumbbell shape with two lobes aligned along an axis (see page 47). All d orbitals have slightly complex shapes (see page 48) and are beyond the scope of our discussion. Each orbital can accommodate a maximum of two electrons. Hence, an s subshell, (only one orbital) can accommodate a maximum of two electrons. Similarly, p (three orbitals), d (five orbitals), and f (seven orbitals) subshells can have maximum of six, ten, and fourteen electrons, respectively.
Some possible combinations of quantum numbers for atomic orbitals
Write the electronic configuration of lithium.
From the periodic table, we can get the atomic number of lithium. The lithium atom has 3 electrons. As we know, 1s subshell is the first one to be filled. The s orbital can hold a maximum of 2 electrons. We have one electron remaining. It will occupy the 2s subshell which is the next energy level. So the third electron will occupy the 2s subshell. Hence, the configuration of lithium is 1s22s1.
Write the electronic configuration of oxygen in its ground state form.
The oxygen atom contains 8 electrons. The first 2 electrons will go to the 1s level. The next 2 electrons will occupy the 2s level. We have 4 electrons remaining. What is the next subshell according to the filling order? It is 2p. The p subshell can hold a maximum of 6 electrons in its orbitals. So the remaining 4 electrons will occupy the 2p level. Hence, the configuration of oxygen is 1s2 2s2 2p4.