When there are pi bonds as well as the sigma bonds, and therefore a double or triple bond. When it can change where that double bond is you can draw a resonance structure. Ozone, O3, has two resonance structures because you can rearrange the bonds in it.
A homogeneous mixture is uniform throughout, like water that has dissolved gases. You cannot easily distinguish the individual parts of the mixture. Many other drinks are considered homogeneous too. Laundry detergent and colognes are other examples of homogeneous mixtures. Homogeneous mixtures can also be a solid (ie. steel), liquid or gas form. It does not always have to be a liquid.
Radium, that is the element that he discovered.
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1.14912614 x 10^-25 moles would be your answer
