The outer electronic configuration of copper is

Atomic number of Cu is 29.

Thus, its electronic configuration can be written as, ${\mathrm{Cu}}_{29}=$​$1{\mathrm{s}}^2,2{\mathrm{s}}^2,2{\mathrm{p}}^6,3{\mathrm{s}}^2,3{\mathrm{p}}^6,3{\mathrm{d}}^{10},4{\mathrm{s}}^1$

(Since, fully filled orbitals are more stable, so one electron from 4s orbital transfers into 3d orbital.)

The electronic configuration of neon is,
${\mathrm{Ne}}_{10}=1{\mathrm{s}}^2,2{\mathrm{s}}^2,2{\mathrm{p}}^6$
Thus, using inert gas, the configuration of Cu can also be represented as,
${\mathrm{Cu}}_{29}=\left[\mathrm{Ne}\right]3{\mathrm{s}}^2,3{\mathrm{p}}^6,3{\mathrm{d}}^{10},4{\mathrm{s}}^1$