In the ground state, an atom of which element has two valence electrons?
se
cu
ni
cr

In the event that you investigate where Chromium is on the intermittent table, it is one electron far from having 5 electrons in its 3d-subshell and a more prominent general strength. Consequently, with a specific end goal to accomplish this more attractive state, one of the 4s electrons is elevated to the 3d sub shell, accomplishing the half-full orbital and bringing about just a single electron staying in the 4s (valence) shell. Copper does this too.

Because a model would make the investigation easier to understand

Both models are effective because they show the valence electrons, and these valence electrons indicate the reactivity of an element.