Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile. Pure nickel shows a significant chemical activity, though larger colletions of the metal are slow to react with air at ambient conditions due to the formation of a protective oxide surface. However, nickel is reactive with oxygen to the extent causes that native nickel is rare on Earth's surface, and is mostly confined to the interiors of larger nickel iron meteorites which were protected from oxidation in space. Such native nickel is always found on Earth alloyed with iron, in keeping with the element's origin as a major end-product of the nucleosynthesis process, along with iron, in supernovas. An iron-nickel alloy is thought to compose the Earth's core.