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

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

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Answer 1 · 2018-03-14T12:25:02

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:

Aufbau Principle - Wikimedia Commons

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

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

$1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{2} 3 d^{3}$ $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^3$ , or $[Ar] 4 s^{2} 3 d^{3}$ $["Ar"]4s^2 3d^3$ .

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

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

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

$[Ar] 4 s^{2} 3 d^{8}$ $["Ar"]4s^2 3d^8$

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

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

$[Ar] 4 s^{2} 3 d^{10}$ $["Ar"]4s^2 3d^10$

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

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