Answer
Please see the work below.
Work Step by Step
In an electronic configuration, the noble gas core is shown in square brackets. For writing the electronic configuration of an element, the recently completed noble gas core is used.
It simplifies the writing of electronic configuration since by writing the noble gas core we need not mention its configuration.
For example, the electronic configuration of Calcium with atomic number 20 can be represented using an Argon core.
Cacium confiuration = $[Ar]4s^{2}$
