The nucleus contains both protons and neutrons. An element has a fixed number of protons but may exist with various numbers of neutrons. The sum of the protons and neutrons is the mass number. For example, helium exists as 3He(2 protons and one neutron) or as 4He (2 protons and 2 neutrons). The two forms of helium are called isotopes of helium. Isotopes of an element have the same chemical properties but different weights. Some elements have many isomers. Tin (Sn) has about 38 known isotopes.
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