# What element in the fourth period is an exception to the Aufbau principle?

Mar 14, 2018

Copper and chromium

#### Explanation:

The aufbau principle states that electrons are placed in orbitals of lower energy levels before placing themselves in higher energy levels. It goes like this:

So first, we have $1 s$, then $2 s$, then $2 p$, then $3 s$, followed by $3 p , 4 s , 3 d$, and so on and so forth.

Writing down the electron configuration of vanadium, the element just before chromium, we have:

$1 {s}^{2} 2 {s}^{2} 2 {p}^{6} 3 {s}^{2} 3 {p}^{6} 4 {s}^{2} 3 {d}^{3}$, or $\left[\text{Ar}\right] 4 {s}^{2} 3 {d}^{3}$.

Obviously, chromium should be same, except that we have $3 {d}^{4}$, right? Well, no. Since $3 {d}^{4}$ is very unstable, as one orbital is left unfilled while all others are (partly) filled. So one electron is taken from the $4 s$ orbital, and since the orbital still has one electron, it's okay to do so. So chromium's electron configuration will be:

$\left[\text{Ar}\right] 4 {s}^{1} 3 {d}^{5}$, not $\left[\text{Ar}\right] 4 {s}^{2} 3 {d}^{4}$. So in manganese, the next element, the $4 s$ orbital is completely filled.

Similarly, for nickel, the element just before copper, the config is:

$\left[\text{Ar}\right] 4 {s}^{2} 3 {d}^{8}$

For the same reasons as before, copper's config is:

$\left[\text{Ar}\right] 4 {s}^{1} 3 {d}^{10}$, instead of $\left[\text{Ar}\right] 4 {s}^{2} 3 {d}^{9}$. Zinc, the element after copper, has this config:

$\left[\text{Ar}\right] 4 {s}^{2} 3 {d}^{10}$

It all ties up.