> How many valence electrons does a helium atom have?
The electron configuration of Helium is simply 1s2. We see that its outermost shell is the s shell and it contains 2 electrons, therefore the number of valence electrons is also 2.
> What is the formula of the ion formed when potassium achieves noble-gas electron configuration?
Potassium has a electron configuration of [Ar] 4s1. To have an electron configuration of only [Ar] which is a noble gas, the one electron from 4s1 should be removed, hence:
K+