Hello, I am reading the volume 2 of the Feynman's Lectures on Physics, and something is bothering me when he calculates the dipole moment of a single atom induced by an extern field : https://books.google.co.uk/books?id...=onepage&q=feynman dipole single atom&f=false Indeed, he states that : " p = qex " But why ? I would use in general : " p = Zqex " where Z is the number of electrons in the atom. x is the displacement of the center of charges of the electrons, and thus x is also the displacement of each electron. Could you explain his reasoning ? It is not the first time he uses a single electron charge instead of Z in his calculations, and I do not understand. Thanks. PS : And qe is obviously equal to the charge of one electron and note equal to Z*(charge), because he uses then e² = q²e/4piε0 (we can also see it in the mechanical equation of motion). Moreover, when he the does his calculations for the Helium atom, he keeps using qe = charge of one electron and does not use a new value for Z. And that is exactly what I find weird: he gets the good results event without having taken Z = 2.