# [Ar]4s^2 3d^10 4p^4 is the noble gas configuration of which element?

Dec 31, 2016

That's not a noble gas configuration. That is, however, a noble gas shorthand for the electron configuration of selenium.

Notice how the number of electrons corresponds to the number of protons in a neutral element. Thus, if you start from the $\left[A r\right]$ portion of the electron configuration, you should have accounted for $18$ electrons.

In fact, the noble gas portion of the configuration represents the core electrons of the unidentified element. Therefore, everything after it is non-core (the $4 s$ and $4 p$ electrons are your valence electrons here, for the post-transition elements on the fourth row).

Again, since the number of electrons equals the number of protons in a neutral element, and there are two $4 s$, ten $3 d$, and four $4 p$ electrons, there must be

$18 + 2 + 10 + 4 = 34$

total electrons in the element, which correspond to $34$ total protons in the element. When you look it up, you should locate selenium and find that it has atomic number $34$.