# What element has the ground state electron configuration [Ar]3d^10 4s^1?

$Z$ $=$ $18 + 11$ $=$ $29$
$\left[A r\right]$ denotes an inert gas core, an argon core, i.e. there are 18 electrons that the valence electrons builds on. There are 11 additional valence electrons. If there are 29 electrons, there are 29 protons in the nucleus; $Z$ $=$ $29$. How do I know this? Because, the element must be assumed to be neutral.
So, you just look up your handy Periodic Table, and see what element has $Z$ $=$ $29$.