# What does ''population'' mean in regards to the excited and ground states of an atom?

I have a problem I'm working on. It seems simple enough, but there is a term I'm not familiar with. It asks for the ''relative populations of the first excited and the ground states for helium gas in thermal equilibrium at 10,000K.'' We are learning about the partition function in my statistical physics class.

• If $P_0$ is the population of the ground state and $P_1$ is the population of the first excited state it's asking for the ratio $P_1/P_0$. – John Rennie Feb 10 '16 at 18:19
• @JohnRennie right but what is the definition of a population? – whatwhatwhat Feb 10 '16 at 18:23
• The population is just the number of helium atoms in the state – John Rennie Feb 10 '16 at 18:56