Hydrogen has three naturally occurring isotopes which figure into the average atomic mass found on the periodic table (1.00974): hydrogen-1, hydrogen-2, and hydrogen-3. which would you guess is most abundant? explain your answer

The number following the name of the element is the number of subatomic particles inside the nucleus of the atom. This means that it is the mass number of the isotope. The average atomic mass of the element is the sum of the products of the percentage abundance and mass number of the naturally occurring isotopes. 

Since, the average atomic mass of the hydrogen is nearest to 1 then, the most abundant isotope should be hydrogen-1. 

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1. o2(g)as a product