An element with a high relative atomic mass but low atomic number is an element that is relatively light in terms of the number of protons in its nucleus but has a high mass due to the presence of additional neutrons. This situation arises in isotopes of elements where the number of neutrons is significantly higher than the number of protons.
One example of such an element is hydrogen-2, also known as deuterium. Deuterium has an atomic number of 1, which means it has only one proton in its nucleus, just like regular hydrogen. However, deuterium has an additional neutron in its nucleus, giving it a relative atomic mass of approximately 2 atomic mass units (amu), which is double the mass of ordinary hydrogen.
Another example is helium-3. Helium-3 has an atomic number of 2, indicating that it has two protons in its nucleus, just like regular helium. However, helium-3 has one additional neutron, which gives it a relative atomic mass of approximately 3 amu.
Both deuterium and helium-3 are isotopes of their respective elements, meaning they have the same atomic number but different atomic masses due to varying numbers of neutrons.