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When the $3d$ orbital is complete, the new electrons will enter in:
A. $4p$ orbital
B. $4f$ orbital
C. $4s$ orbital
D. $4d$ orbital

Last updated date: 13th Jun 2024
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Hint: To know the next energy level in which the electron will enter, apply Aufbau’s principle and use the formula $(n+l)$, where $n$ is the principal quantum number and $l$ is the azimuthal quantum number.

Complete step by step solution:
The energy of the orbital majorly depends upon the value of $n$and $l$, where $n$ refers to the principal quantum number and $l$ refers to the azimuthal quantum number. This was given by Aufbau. Aufbau’s principle decides the filling up of electrons in the orbitals.
So, according to Aufbau’s principle, the electrons in an atom would fill principal energy levels in order of increasing energy by $(n+l)$ rule. According to him, the rule says that the subshell with a lower value will be having the lowest energy and it is to be needed to fill up first.
So, here in the given question, given that the orbital is $3d$.
So, here the value of the principal quantum number will be $3$ and the value of azimuthal quantum number will be $2$. So, according to the rule mentioned above its value will be $5$. Now, the next orbital to be filled will either have the same value with lower $n$ or it will have a value equal to $6$.
So, from the option given let us find which one has these above criteria.
First option shows, $4p$-orbital, where by applying the $(n+l)$, we get $(4+1)=5$. Here, the principal quantum number is four and the azimuthal quantum number of p-orbital is one.
Likewise, the next option will have $4+3=7$, as for f-orbital the azimuthal quantum number is three.
Similarly, the third option will have $4+0=4$, as for s-orbital the azimuthal quantum number is zero.
And the last option will have, $4+2=6$, as azimuthal quantum number for d-orbital is two.
So, the next electron will either fill in $4p$-orbital or $4d$orbital. But the electron will fill in the $4p$-orbital first and, as it has the lowest principal quantum number compared to the $4d$ orbital.

Hence, the correct option is A.

Note: If two subshells have the same value of $(n+l)$ then the subshell with the lower value of $n$ has lower energy and it should be filled first. The $n$ describes the average size and energy of the orbital to a greater extent while $l$ describes the shape of the orbital and energy of the orbital to greater extent.