A chemical element<span> or </span>element<span> is a species of </span>atoms<span> having the same number of </span>protons<span> in their </span>atomic nuclei<span> (i.e. the same </span>atomic number<span>, or </span>Z).[1]<span> There are 118 elements that have been identified, of which the first 94 occur naturally on </span>Earth<span> with the remaining 24 being </span>synthetic elements<span>. There are 80 elements that have at least one stable </span>isotope<span> and 38 that have exclusively </span>radioactive isotopes<span>, which decay over time into other elements. </span>Iron<span> is the </span>most abundant element<span> (by </span>mass<span>) making up Earth, while </span>oxygen<span> is the </span>most common element in the Earth's crust.<span>[2]</span>