Final answer:
In the electron configuration notation 1s², the number 1 represents the first energy level, which is the closest to the nucleus and contains the lowest energy electrons.
Step-by-step explanation:
In the notation 1s², the number 1 represents the energy level of an electron in an atom. This is noted as the principal quantum number, which defines the location of the energy level and is indicative of the electron orbiting closer to the nucleus and having a lower energy state. The number before the subshell letter (in this case, 's') tells us which energy level or shell the electron occupies, with 1 being the first and lowest energy level.
The electron configuration notation describes how many electrons are within a particular sublevel or orbital by using superscript numbers. For example, in the electron configuration of helium, 1s², the superscript '2' indicates that there are two electrons in the 1s sublevel, which is the only sublevel in the first energy shell.