From Halopedia, the Halo wiki
Lead, symbol Pb (Latin plumbum, a lead weight) is a dense, bluish-gray metallic element that was one of the first known metals. Thousands of years since its discovery, humanity still uses the material in construction. The atomic number of lead is 82; the element is in group 14 (or IVa) of the periodic table.
Metallic lead is a soft, malleable, ductile metal, often used to make bullets. Lead is toxic when taken internally, although this is usually not a problem with ammunition.
Because of its high density and nuclear properties, lead is used extensively as protective shielding for radioactive material.
Lead is used by the UNSC in the construction of SOEIV drop capsules, radiation lining to protect reactor engineers from lethal exposure, and was used by the surviving Spartan-II's in Operation: FIRST STRIKE to reinforce a captured T-25 TC "Spirit" from radiation and stress during exit from slipspace on its own.
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