[DannyBoi]

Hi, in my teacher's lecture notes and in Atkins Physical Chemistry 9th edition on page 752, the equation for mean free path which a molecule travels in between collisions is,

λ = kT/σp

but in the solution manual for Atkins, on wikipedia, and on various google search results, I get the equation:

λ = kT/2^1/2σp

I e-mailed my teacher and he said it was a typo in the manual. But I see this sqrt(2) in every mean free path equation outside the book and his notes. Which is the right equation?