Multiple choice

The lightest substance known is

  1. air

  2. hydrogen

  3. helium

  4. oxygen
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Hydrogen is the lightest known substance, with an atomic mass of approximately 1 atomic mass unit (amu). It's the first element in the periodic table and the most abundant element in the universe. Under standard conditions, hydrogen exists as a diatomic gas (H₂) and is about 14 times lighter than air. Helium is the second lightest element, while air is a mixture of gases and oxygen is much heavier.

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