Care must be exercised in the use of the abbreviated notation 1s¹(­)2s¹(¯) to indicate the configuration and spin of a many-electron atom. In the present example, all we mean to imply is that the total component of the spin is zero. We do not imply that the electron in the 1s orbital necessarily has a spin "up" and that in the 2s orbital a spin "down." The situation could equally well be described by the notation 1s¹(¯)2s¹(­). There is no experimental method by which we can distinguish between electrons in an atom, or, for that matter, determine any property of an individual electron in a many-electron system. Only the total magnetic moment, or total angular momentum, may be determined experimentally.