Express the product of 2.2 mm and 6.00 mm

Ionic compounds are made of ions, and yet the overall charge of an ionic compound is neutral. why?

Joan writes four numbers on the board in standard form, and then she writes their scientific notation

Consider hydrogen in an excited state n = 5n=5 that emits photons to reach the ground state. there are various possible transitions other than straight to the ground state that can occur; for example, it can drop to the n = 3n=3 state followed by the n = 3n=3 to the ground state transition. which of the possible transitions will result in the emission of a photon in the visible region?