A
chemical element or
element is a species of
atoms having the same number of
protons in their
atomic nuclei (i.e. the same
atomic number, or
Z). There are 118 elements that have been identified, of which the first 94 occur naturally on
Earth with the remaining 24 being
synthetic elements. There are 80 elements that have at least one stable
isotope and 38 that have exclusively
radioactive isotopes, which
decay over time into other elements.
Iron is the
most abundant element (by
mass) making up Earth, while
oxygen is the
most common element in the Earth's crust.