Lead

Lead, symbol Pb (Latin plumbum, a lead weight), dense, bluish-gray metallic element that was one of the first known metals. 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 in ODST HEV pods, and was used by the surviving Spartan II's in Operation: FIRST STRIKE to reinforce a captured Spirit for exiting slipspace.