Determine the element of lowest atomic number that contains a complete d subshell in the ground state. element symbol:

The element that contains a complete d subshell in the ground state : Cu

In an atom, there are energy levels in the shell and sub shell.

This energy level is expressed in terms of electron configurations.

Writing the electron configuration starts from the lowest to the highest subshell's energy level (following Aufbau rules)

Electron filling in subshells using the following sequence

1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁶, 5s², 4d¹⁰, 5p⁶, 6s², etc.

And the element whose electron configuration occupies the sub shell d lies in the transition group or group B in the periodic system

Electrons can move the shell up or down by releasing energy or absorbing energy

So it can be concluded that there are 2 conditions:

Ground state is the state of electrons filling shell with the lowest energy levels.

Excited state is the state of electrons which occupies a higher energy level

In filling electrons in subshell d there are half-full and full rules because of the tendency of electrons to get stability (not following the Aufbau rule)

Example

Cr atomic number 24: [Ar] 4s² 3d⁴ [Ar] 4s¹ 3d⁵

Cu atomic number 29: [Ar] 4s² 3d⁹ [Ar] 4s¹ 3d¹⁰

Mo atomic number 42: [Cr] 5s² 4d⁴ [Cr] 5s¹ 4d⁵

So the element of the lowest atomic number that contains a complete subshell in the ground state has an electron configuration

1s², 2s², 2p⁶, 3s², 3p⁶, 4s¹, 3d¹⁰

which is owned by the element Cu

electron transitions  

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the locations and properties of two electrons  

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It's sublevel  

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a possible full set of quantum numbers  

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it's filtration, but this is an assumption so my apologies if i got this