IF YOU HAVE TROUBLE UNDERSTANDING ELECTRON CONFIGURATION –
Let’s look at vanadium (V). (Chromium is a little more complex so I’ll explain it further). First, we need to check the atomic numbers of V (which is 23). This is because V is an atom (and not an ion) in this instance.
Next, follow the order of the table below (which is very simple in case you get lost in the future (link down)).
You might have noticed that the order of elements does not follow simple logic. 4d is after 4s and 4d after 5s. It is therefore important to understand how to build the table above when it comes to electron configuration.
It is important to know how many electrons a subshell can hold (s,p,d,f).
Start with the ssubshell, which holds 2 electrons.
The p-subshell can contain 4 more electrons than the ie. 2+4=6 electrons,
The d-subshell can contain 4 more electrons than the ie. 6+4=10 electrons
The f-subshell can store 4 more electrons than the ie. 10+4=14 electrons.
You will have noticed that all subshells now have four more electrons than their predecessors.
Vandium is the case
First, we have 2 electrons in the 1s subshell. Now we have 23-2=21 electrons
The 2 electrons then fill the 2s subshell. Now, we have 21-2=19 electrons
Six electrons then fill the 2p subshell. Now, we have 19-6=13 electrons
The 3s subshell is then filled with 2 electrons. Now we have 13-2=11 electrons
The 3p subshell is then filled with 6 electrons. Now, we have 11-6=5 electrons
The 4s subshell is then filled with 2 electrons. Now, we have 5-2=3 electrons
The 3d subshell is now partially filled by the final 3 electrons.
The way we write this is : 1st subshell 2nd subshell…
For V, the electron configuration logically would be :
This can be expressed in a simpler way by substituting a portion of the electron configuration for a noble gas with [symbol noble gas]. As in the above example, the electron configuration for Argon (Ar) can be replaced by [symbol of noble gas]. The electron structure of V can then be written as follows:
[Ar]
This Bozeman Science video will help clarify the concept of electron configuration.
This article explains how electron configurations are written, shows us how to create the table above, and may clear up any doubts you might have about this topic.
A ANSWER TO YOUR PROBLEM
The electron configuration of Cr according to the above method should be: or [Ar] but instead it is: or[Ar]. This violates the Aufbau Principle which states that electrons orbiting one atom or more fill the lowest energy levels (subshells), before filling higher levels.
It is important to understand that a subshell that is exactly half-full is more stable than one that is only half-full. The electron is able to move from the 4s subshell into the 3d subshell, which provides greater stability and makes the change more favorable.
This is an exception to the Aufbau principle. Tungsten (W), which has the same valency of Chromium as Chromium, but W does not follow the Aufbau principle. Copper (Cu) is another metal that doesn’t follow this principle. It has an electron configuration of [Ar] instead of [Ar] (to fulfill the d orbital). Subshells that are fully filled are more stable than sublevels that are only partially filled.