MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY
Molar quantities for chemical elements. Clockwise from upper left, these are: carbon, sulphur, iron, copper, magnesium. The different amount of each element represents a quantity known as the mole. One mole of any sample contains the same number of molecules or atoms, but the weight and volume are different. The mole, one of the base international (SI) units, was defined in 1971 in relation to the number of atoms in 12 grams of the most abundant isotope of carbon. Known as Avogadro's number, this immensely large quantity is over 600 billion trillion atoms, or a six followed by 23 zeros.
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