Question

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

Answer 1

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

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Notice how the number of electrons corresponds to the number of protons in a neutral element. Thus, if you start from the `[Ar]` 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 `4s` and `4p` 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 `4s`, ten `3d`, and four `4p` 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`.