Video Transcript
Which of the following elements in
the first series of transition elements have irregular electronic
configurations? (A) Chromium and scandium, (B)
scandium and manganese, (C) chromium and copper, (D) scandium and copper, (E)
chromium and manganese.
To begin, transition elements are
elements that have atoms with incomplete d subshells or can give rise to cations
with incomplete d subshells.
The first series of transition
elements is found in period four in the d block in groups three through 11 on the
periodic table. Zinc is not considered a transition
element because its atoms and ions do not have incomplete d subshells.
When looking at the answer choices,
we can see that all of these elements are in fact transition elements in period
four. In this question, we need to
identify which of the elements in the answer choices have irregular electronic
configurations. Let’s clear some space to examine
the electronic configurations of the period four transition elements.
All period four transition elements
have 18 core electrons, which are represented by the electronic configuration 1s2
2s2 2p6 3s2 3p6. This part of the electronic
configuration can be condensed to the noble gas argon, which contains 18
electrons.
The valence electrons of all period
four transition elements are found in the 4s and 3d subshells. We can see different combinations
of electrons in the 4s and 3d subshells, but there is an important similarity. All period four transition elements
have two electrons in the 4s subshell, except chromium and copper. These two electronic configurations
appear to violate the aufbau principle, which states that electrons fill the
lowest-energy subshells before they fill the higher-energy ones. The 4s subshell is lower in energy
than the 3d subshell and should be filled with two electrons first before filling
the 3d subshell.
In the orbital diagrams, we can see
that the 3d subshell of chromium is exactly half full. And in copper, it is completely
full. Explaining why copper and chromium
have these irregular electronic configurations is not straightforward and is beyond
the scope of this video.
The answer choice that correctly
identifies two period four transition elements with irregular electronic
configurations is answer choice (C) chromium and copper.
