Final answer:
The rounded atomic mass of helium is 4.003 amu. The mass of an He atom is slightly less than the sum of its constituent particles, resulting in a mass defect. This lost mass is converted into energy during fusion reactions.
Step-by-step explanation:
The rounded atomic mass of helium is 4.003 amu. The mass of an He atom is slightly less than the combined masses of its constituent subatomic particles, which is known as the mass defect. The mass lost in the transformation from hydrogen atoms to helium atoms is converted into energy during the fusion reaction.
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