Question

How did we know that copper and chromium have different electronic configuration??

Answer 1

Due to the different number of electrons between the two elements.

Explanation:

The atomic number of Chromium is 24 and that of Copper is 29.

Therefore, the atomic number equals the number of protons as well as the number of electrons if the element is in its neutral state of no charge, which in this case it is.

So the number of electrons for Chromium will be 24 and Copper will have 29 electrons.

So hence the electronic configuration of the two will be:

Chromium: `1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 3d^5, 4s^1`

Copper: `1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 3d^10, 4s^1`

The "to the power" values indicate the number of electrons that are in the specific orbital, so in `1s^2` there are 2 electrons in the specific s-orbital.

So to clarify the difference between the two electronic configurations lie in the `3d` orbital, Chromium having 5 electrons and Copper having 10 in that specific orbital.

s-orbitals can have a maximum of 2 electrons
p-orbital can have a maximum of 6 electrons
d-orbital can have a maximum of 10 electrons and a
f-orbital can have a maximum of 14 electrons

To understand why Chromium and Copper don't completely fill the 4s orbital first and instead add the electrons into the 3d orbital you can look up @mrpauller's answer in: "Why do electron configurations of chromium and copper contradict the Aufbau principle?"

Hope I helped :)

Answer 2

HOW we know is by conducting experiments with other known compounds. The way that they combine tells us about the electronic structure necessary to form the resulting compounds.

Explanation:

The study of how the elements were discovered, and the development of the Periodic Table can be extremely interesting. The types of experiments devised, and the deductions required to determine the basic properties of the elements is amazing.
Fundamentally, science is all about OBSERVING natural phenomena. Our theories try to make sense of it all, and then propose new experiments to confirm or refute our hypotheses.

One way of doing that is to react both chromium and copper with similar compounds, and then measure what comes out of it. The differences in mass tell us how the different elements react (combine) with other things. From those measurements we can calculate how many oxygen atoms attach to chromium or copper, for example, and from that we can deduce the electronic structures necessary for the observed differences.