Video Transcript
Which diagram shows the correct placement of the first six electrons in the following graphical representation of the electronic configuration of an element?
An electronic configuration is the representation of the arrangement of electrons distributed among the orbitals and subshells. The graphical representations, or diagrams, (A) through (E) shown here display the orbitals within a subshell as a box in order from lowest to highest energy and uses arrows to show how many electrons there are in each orbital, with the up or down direction of the arrow indicating the spin of the electron. In order to determine which answer choice correctly depicts the first six electrons in the orbitals provided, we need to remember a couple of rules.
The first is that the order in which electrons fill orbitals is called the aufbau principle and states that electrons will occupy the lowest energy orbitals before filling the higher energy orbitals. That being said, the order of filling energy orbitals can be remembered using this condensed version of the aufbau energy diagram. We can demonstrate this by drawing diagonal arrows, starting with electrons filling the 1s orbital first, followed by the 2s orbital with the filling of the 2p orbitals next in line and the 3s orbital after that. This pattern is continued down the line until all electrons have been accounted for and repeated each time by starting from the top.
Since this question focuses on the first six electrons, the correct electronic configuration will demonstrate a full 1s and 2s orbital, each with two electrons, followed by two electrons in the 2p subshell. Upon looking at the answer choices, we can eliminate diagrams (B), (D), and (E) because they do not demonstrate one or both of the 1s and 2s orbitals with two electrons or as completely filled orbitals. Both diagrams (A) and (C) are possible answers to this question because they each have a total of six electrons and both show two electrons each in the 1s, 2s, and 2p orbitals.
To determine which of the two is the correct answer, there is a second rule we must keep in mind. This is called Hund’s rule, which states that electron subshells are singly occupied with spin-up state electrons, an up arrow, before they are doubly occupied with spin-down state electrons, or a down arrow. For this question, this applies to the 2p subshell because it contains three orbitals. And all three orbitals reside at the same energy level.
As a result, electrons will spread out amongst the three orbitals such that each 2p orbital is half filled with spin-up state electrons before pairing up a spin-up state and a spin-down state electron within the same orbital. This is because there are repulsion forces between the pairing electrons within any one atomic orbital. Based on this information, diagram (A) can be eliminated because the two electrons in the 2p subshell should be spread out into different 2p orbitals. And they should both be in the spin-up state.
Diagram (C), on the other hand, depicts the correct orbital filling and spin state for the first six electrons. Therefore, which diagram shows the correct placement of the first six electrons in an electronic configuration? The answer is diagram (C).
