Terminology - the "upper state" Hi there! I'm reading a paper at the moment that describes a formula for an absorption coefficient with regard to radiative transfer. One of the variables in it is referred to as "N2". It describes N2 as "The number density of atoms in the upper state". With oxygen as an example, would N2 simply refer to the number of atoms in the outer shell? ie: 6? My apologies if this is a bit obvious. I'm afraid I'm new to all this, and this terminology doesn't immediately appear to be standard.