7 2.2 Electronic Configuration
Figures
Figure 2.2.4: Boron Electronic Diagram.
An electronic energy diagram for a boron atom. A vertical arrow on the left denotes increasing energy levels. Subshells 1s, 2s, 2p, 3s and 3p are written from bottom to top, with horizontal lines representing electron orbitals: s subshells have a single horizontal line, while p subshells have 3 lines in each.
An up and a down arrow are found on both the 1s and 2s horizontal line, representing 4 electrons in total. The 2p subshell has only 1 up arrow on the left horizontal line, representing an unpaired electron. There are 5 arrows in total, representing the 5 electrons needed to be allocated. The higher energy levels have no arrows and remain unfilled.
Figure 2.2.5: Carbon, Nitrogen and Oxygen Electronic Energy Diagrams
Electronic energy diagrams for Carbon, Nitrogen and Oxygen atoms, respectively. A vertical arrow on the left denotes increasing energy levels. Subshells 1s, 2s, 2p, 3s and 3p are written from bottom to top, with horizontal lines representing electron orbitals: s subshells have a single line, while p subshells have 3 lines in each.
On the left is Carbon. Both the 1s and 2s subshells have two arrows on each line, one pointing up and the other down, representing four electrons in total. The 2p subshell has two unpaired electrons, each represented by an up arrow on the first two underscores. The carbon atom has a total of six electrons, with the 3s and 3p energy levels remaining unfilled
Next, in the middle, is Nitrogen. The arrangement of electrons in the subshells is similar to Carbon, except there is an extra unpaired electron in the 2p subshell on the third horizontal line.
On the right is Oxygen. The arrangement of the electrons is nearly the same as Nitrogen, except there is now a paired electron in the first line of the 2p subshell.
Practice questions
Multiple choice questions
Question 1

 Write the electron configuration of Na.
 [latex]1s^{2}2s^{2}2p^{5}3s^{2}[/latex]
 [latex]1s^{1}2s^{2}2p^{3}3s^{4}[/latex]
 [latex]1s^{2}2s^{2}2p^{6}3s^{1}[/latex]
 [latex]1s^{2}2s^{2}2p^{4}3s^{2}[/latex]
 Write the electron configuration of Na.
Question 2
 Write the electron configuration of Ca using noble gas configuration.
 [latex][Ar]3d^{2}[/latex]
 [latex][Ca]3d^{2}[/latex]
 [latex][Ar]4s^{2}[/latex]
 [latex][Ca]4s^{2}[/latex]
Solutions
 The first two electrons go to the 1s orbital as a pair. The next two electrons are placed in the orbital 2s. The next six electrons are placed in 2p orbitals as three pairs. The remaining electron is placed in the 3s orbital.
 For Ca, Ar is the last noble gas that comes before Ca. The electron configuration of Ca is written as [Ar], which denotes Ar’s electron configuration followed by the configuration of remaining electrons.
[latex]Z = 18: 1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}[/latex] (This is written as [latex][Ar] ) Z = 20: 1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}[/latex] (After 3p orbital, electrons fill into the 4s orbital, not the 3d orbital). The noble gas configuration of Ca is [latex][Ar]4s^{2}[/latex].