There must be 7 electrons present in the 3d energy level.
The atomic number is 27.
Thus, for 1st energy level, there is only 1 subshell present which is 1s² containing 2 electrons.
For the 2nd energy level, there are 2 subshells which are
2s² = 2 e⁻s
2p⁶ = 6 e⁻s
The number of electrons at the third energy level, x, can be assumed.
Then, 1s²2s²2p⁶3s²3p⁶4s²3dˣ (valence = 2 requires 2 electrons in 4s).
In the third level, there must be 7 electrons
since 27 - (2+2+6+2+6+2) = 7 = x.
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