<P> There are around 94 naturally occurring elements on earth . The atoms of each element have a nucleus containing a specific number of protons (always the same number for a given element), and some number of neutrons, which is often roughly a similar number . Two atoms of the same element having different numbers of neutrons are known as isotopes of the element . Different isotopes may have different properties - for example one might be stable and another might be unstable, and gradually undergo radioactive decay to become another element . </P> <P> The hydrogen nucleus contain just one proton, Its isotope deuterium, or heavy hydrogen, contains a proton and a neutron . Helium contains two protons and two neutrons, and carbon, nitrogen and oxygen - six, seven and eight of each particle, respectively . However, a helium nucleus weighs less than the sum of the weights of the two hydrogen nuclei which combine to make it . The same is true for carbon, nitrogen and oxygen . For example, the carbon nucleus is slightly lighter than three helium nuclei, which can combine to make a carbon nucleus . This difference is known as the mass defect . </P> <Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (October 2014) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (October 2014) (Learn how and when to remove this template message) </Td> </Tr>

The tremendous amount of energy released by nuclear reactions is required to bond what together