<Tr> <Td> 81 </Td> <Td> thallium </Td> <Td> Tl </Td> <Td> 0.01 </Td> <Td> </Td> </Tr> <Tr> <Td> 83 </Td> <Td> bismuth </Td> <Td> Bi </Td> <Td> 0.01 </Td> <Td> </Td> </Tr> <P> The mass - abundance of the nine most abundant elements in the Earth's crust is approximately: oxygen 46%, silicon 28%, aluminum 8.2%, iron 5.6%, calcium 4.2%, sodium 2.5%, magnesium 2.4%, potassium 2.0%, and titanium 0.61% . Other elements occur at less than 0.15% . For a complete list, see abundance of elements in Earth's crust . </P> <P> The graph at right illustrates the relative atomic - abundance of the chemical elements in Earth's upper continental crust--the part that is relatively accessible for measurements and estimation . </P>

What element is the most abundant on the earths crust