Nickel ( /ˈnɪkəl/ ni-kəl) is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal. Nickel belongs to the transition metals and is hard and ductile. Pure nickel shows a significant chemical activity that can be observed when nickel is powdered to maximize the exposed surface area on which reactions can occur, but larger pieces of the metal are slow to react with air at ambient conditions due to the formation of a protective oxide surface. Even then, however, nickel is reactive enough with oxygen that native nickel is rare on Earth's surface, being mostly confined to the interiors of larger nickel–iron meteorites that were protected from oxidation during their time in space. On Earth, such native nickel is always found in combination with iron, in keeping with those elements' origin as major end products of the nucleosynthesis process in supernovas. An iron–nickel mixture is thought to compose Earth's inner core.^{[citation needed]}