Here's why:
* [He] represents the electron configuration of helium, which is 1s². This is the core configuration of fluorine.
* 2s² indicates that there are two electrons in the 2s subshell.
* 2p⁵ indicates that there are five electrons in the 2p subshell.
Fluorine is in the second period of the periodic table, meaning it has two electron shells. The noble gas helium precedes fluorine, so its electron configuration is used as a shortcut for the core electrons.
