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Oddo—Harkins rule Rule stating that the cosmic abundance of elements with an even atomic number is greater than that of adjacent elements with an odd atomic number. Consequently, a graph plotting relative atomic abundance against increasing atomic number (Z) displays a ‘toothed’ curve, rather than a smooth line. The reason for this is connected with processes such as helium burning (see nucleosynthesis). 42He is a basic building block, and so additions produce even numbers, e.g. 42He + 42He → 84Be;84Be + 42He → 126C.