What does the 3 indicate in 1s22s22p63s1?

The value 3, indicates the principal energy level or the main energy level for where the respective subshells will be found in. It is used to denote the electrons residing in their subshells of that energy.

Atomicity occurs to prevent updates of the database occurring only partially, which can cause greater problems than rejecting the whole series outright.some are monatomic, diatomic, triatomic etc because it shows the number of atoms. for example, monatomic means it contains only one atom.