Video Transcript
Which of the following diagrams
represents the electronic configuration of a chromium atom?
First of all, chromium is a
transition element, which is an element that has atoms with incomplete d subshells
or can form cations with incomplete d subshells. The atomic number of chromium is
24, so we know that a neutral chromium atom has 24 protons and 24 electrons. Chromium is found in period four
and group six on the periodic table, and it is in the d block. The electronic configuration of
chromium is 1s2 2s2 2p6 3s2 3p6 4s1 3d5. The first five subshells contain
the core electrons of chromium and can be condensed to the symbol for argon. The valence electrons of chromium
are found in the 4s and 3d subshells, so we can see that chromium has six valence
electrons.
In the provided diagrams, the blue
box represents the 4s subshell and the five green boxes represent the 3d
subshell. Each arrow represents an
electron. And each box represents an orbital,
which can hold a maximum of two electrons. We notice that all the diagrams
contain six valence electrons, so our job is to determine how to correctly arrange
these six electrons in the orbitals. According to the aufbau principle,
electrons fill lower-energy subshells before they fill higher-energy ones. Answer choices (A) and (D) appear
to violate this principle because the 4s subshell should be filled with two
electrons before the 3d subshell is filled.
In addition, according to Hund’s
rule, every orbital in a subshell is singly occupied before any orbital is doubly
occupied. It looks as though answer choices
(B) and (C) violate this rule because one electron should be placed in each orbital
in the 3d subshell before pairing them up.
Using this reasoning, we might
conclude that the correct answer is answer choice (E), but we’d be wrong. Chromium is an example of a period
four transition metal with an irregular electronic configuration. The correct electronic
configuration of chromium contains one valence electron in the 4s subshell and five
valence electrons in the 3d subshell. Therefore, the diagram that
represents the electronic configuration of a chromium atom is the diagram in answer
choice (A).
