____ is the lightest element known.

Explanation

**Step-by-Step Text Solution:**

1. **Understanding the Question**: The question asks for the lightest element known. This requires us to identify the element with the lowest atomic mass.

2. **Identifying Elements**: In the periodic table, elements are organized by their atomic number and mass. The atomic mass of an element is a key factor in determining its weight.

3. **Analyzing Hydrogen**: Among all the elements, hydrogen is known to have the lowest atomic mass. It consists of only one proton and one electron, which contributes to its status as the lightest element.

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