Here's how to break it down:
* [Ar] represents the electron configuration of the noble gas Argon, which has the same electron configuration as the first 18 electrons of selenium.
* 3d10 represents the 10 electrons in the 3d subshell.
* 4s2 represents the 2 electrons in the 4s subshell.
* 4p4 represents the 4 electrons in the 4p subshell.
This notation is a shorthand way of writing the full electron configuration, which is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4.
