<P> Heavier heavy metals are not usually formed this way since fusion reactions involving such nuclei would consume rather than release energy . Rather, they are largely synthesised (from elements with a lower atomic number) by neutron capture, with the two main modes of this repetitive capture being the s - process and the r - process . In the s - process ("s" stands for "slow"), singular captures are separated by years or decades, allowing the less stable nuclei to beta decay, while in the r - process ("rapid"), captures happen faster than nuclei can decay . Therefore, the s - process takes a more or less clear path: for example, stable cadmium - 110 nuclei are successively bombarded by free neutrons inside a star until they form cadmium - 115 nuclei which are unstable and decay to form indium - 115 (which is nearly stable, with a half - life 7004300000000000000 ♠ 30 000 times the age of the universe). These nuclei capture neutrons and form indium - 116, which is unstable, and decays to form tin - 116, and so on . In contrast, there is no such path in the r - process . The s - process stops at bismuth due to the short half - lives of the next two elements, polonium, and astatine, which decay to bismuth or lead . The r - process is so fast it can skip this zone of instability and go on to create heavier elements such as thorium and uranium . </P> <P> Heavy metals condense in planets as a result of stellar evolution and destruction processes . Stars lose much of their mass when it is ejected late in their lifetimes, and sometimes thereafter as a result of a neutron star merger, thereby increasing the abundance of elements heavier than helium in the interstellar medium . When gravitational attraction causes this matter to coalesce and collapse new stars and planets are formed . </P> <P> The Earth's crust is made of approximately 5% of heavy metals by weight, with iron comprising 95% of this quantity . Light metals (~ 20%) and nonmetals (~ 75%) make up the other 95% of the crust . Despite their overall scarcity, heavy metals can become concentrated in economically extractable quantities as a result of mountain building, erosion, or other geological processes . </P> <P> Heavy metals are primarily found as lithophiles (rock - loving) or chalcophiles (ore - loving). Lithophile heavy metals are mainly f - block elements and the more reactive of the d - block elements . They have a strong affinity for oxygen and mostly exist as relatively low density silicate minerals . Chalcophile heavy metals are mainly the less reactive d - block elements, and period 4--6 p - block metals and metalloids . They are usually found in (insoluble) sulfide minerals . Being denser than the lithophiles, hence sinking lower into the crust at the time of its solidification, the chalcophiles tend to be less abundant than the lithophiles . </P>

Which heavy metal has highest concentration in earth's crust