How we can write electronic configuration of subshell and need eg
The order for filling subshells is (sometimes called the Aufbau filling order):
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p
Each subshell has a maximum number of electrons that it can hold. The s-type subshells can hold a maximum of two electrons, the p-type subshells can hold a maximum of six electrons, the d-type subshells can hold a maximum of ten electrons, and the f-type subshells can hold a maximum of fourteen electrons.
To write an electron configuration using the subshell notation, a combination of the subshell followed by a superscript indicating the number of electrons in that subshell is used. Thus, for the first two elements, we would write their electron configurations as:
H: 1s1 and He: 1s2
For the next element, Li, we can't put a third electron into the 1s subshell because it is full. Thus, we would need to got to the next available subshell - the 2s.
The filling of subshells would continue to build upon the previous element and fill subshells completely before going on to the next subshell.
You can see that, however, this would get to be quite a chore when we reach larger elements like lead (Pb). Thus, it is preferential to use a shorthand method that utilizes the configuration of the noble gases (group 8A). To do a shorthand configuration for any element, count backwards from that element until you reach a noble gas. Write that element in brackets. Then, continue forward with next subshell(s) - see the attached version of the periodic chart that shows the subshell order with respect to the elements.
For example, if we wanted to do the shorthand configuration for sodium (Na), you would count back one element to neon (Ne). Put this element symbol in brackets and then, noting that the next correct subshell is 3s, include the rest of the electrons as we did with the smaller elements.
Na: [Ne] 3s1
Copper is in the ninth column of the transition metals in the d block of the fourth energy level of the periodic table. This would make the electron configuration for copper, 1s2 2s2 2p6 3s2 3p6 4s2 3d9 or in noble gas configuration [Ar] 4s^2 3d^9.
However, because the 3d orbital is so much larger then the 4s orbital and the 3d orbital only needs one more electron to be filled, the 3d orbital pulls an electron from the 4s orbital to fill this empty space.
This makes the actual electron configuration for copper
[Ar] 4s^1 3d^10.