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Is Deuterium On The Periodic Table . Hydrogen is the first element in the periodic table and has the atomic number one. Each atom has an integer number of neutrons, but the periodic table gives a decimal value because it is a weighted average of the number of neutrons in the isotopes of each element. Elements, Atoms, and the Periodic Table from saylordotorg.github.io There are three isotopes of hydrogen namely, protium 1h1,. Hydrogen is the first element in the periodic table and has the atomic number one. The periodic table lists the atomic weight for each element, which can be used to find mass number, for hydrogen, for example, the atomic weight is 1.008.