The lightest chemical elements is/are:

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Hydrogen

Helium

Both 1 and 2

Oxygen

answer is C.

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Detailed Solution

Concept- A species of atoms with the same number of protons in their atomic nuclei are referred to as chemical elements. Among all chemical elements, hydrogen and helium are the lightest. Lightest pertains to lower molecular weight. The atomic number of an element in the periodic table can be used to determine its weight. Chemical elements having an atomic number of 10 are heavier than those with an atomic number of 5. Therefore, the periodic table clearly illustrates the lightest and heaviest elements. The weight of an element is determined by the atomic number. The atomic number of hydrogen and helium is respectively so they are the lightest elements.
Hence, the correct answer is option 3.

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