whats the noble has configuration for chromium?
is it [Ar] 4s^2 3d^4
Nope, nope, nope.
[Ar] 3d5 4s1
you need some practice on looking at energy levels, and memorizing them.
how is it 4s1?
There are two exeptions to the filling order: Copper and Cr.
https://www.brightstorm.com/science/chemistry/the-atom/exceptions-to-electron-configuration/
To determine the electron configuration of an element, you need to identify its atomic number and understand the filling order of the electron orbitals.
The atomic number of chromium (Cr) is 24. To find its electron configuration, you start by filling the orbitals in order of increasing energy level and follow the Aufbau principle, Hund's rule, and the Pauli exclusion principle.
The filling order for the electron orbitals is as follows:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, etc.
Now let's fill the orbitals for chromium:
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^4
As you can see, the electron configuration of chromium is [Ar] 4s^2 3d^4. The [Ar] represents the preceding noble gas configuration of argon (atomic number 18), which includes the filled orbitals: 1s^2 2s^2 2p^6 3s^2 3p^6.