Introduction to the Mole

You probably first heard this trick question in primary school. 

Q What weighs more, a pound of feathers or a pound of lead  

A Silly question They both weigh the same, of course. 

These days we talk of kilogrammes not pounds, but the same is true of a kilogramme of charcoal, which is just atoms of pure carbon, and a kilogramme of atoms of pure lead. But even though they have the same mass, you don t seem to be getting much lead for your money, when compared with the amount of carbon. 

What is the difference The mass of an atom comes from the number of protons and neutrons present in the nucleus very little contribution comes from the electrons, as they are so small. Lead has 82 protons and 125 neutrons, making a mass number of 207, whereas carbon has only 6 protons and 6 neutrons, (atomic mass 12) and hydrogen, with only 1 proton has an a mass number of 1. It follows that 207 kilogrammes of lead will contain the same number of atoms of lead as there are carbon atoms in 12 kilogrammes of carbon, and the same number of atoms as there are hydrogen atoms in one kilogramme of hydrogen. 

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