Which element was originally used as the standard for determining atomic weights?

• A. Hydrogen
• B. Oxygen
• C. Carbon-12
• D. Nitrogen

Answer: (A)

Verified step by step guidance

1. Understand that atomic weights (now called relative atomic masses) are determined by comparing the mass of atoms to a standard reference.
2. Historically, the first element used as the standard for atomic weights was hydrogen because it is the lightest element and simplest atom.
3. Later, oxygen was used as a standard because it was easier to work with experimentally and was abundant in compounds.
4. In modern times, carbon-12 isotope is used as the standard for atomic masses, defined exactly as 12 atomic mass units.
5. Therefore, the element originally used as the standard for determining atomic weights was hydrogen.