Weighing Atoms

Why it matters

Imagine trying to weigh a single grain of sand on a kitchen scale. Now imagine weighing an atom — the scale reads zero, but the atom isn't weightless.

Predict first

Why do chemists use Carbon-12 as the standard for atomic mass instead of Hydrogen-1?

Carbon-12 provides a stable, reproducible benchmark. Using 1/12th of its mass as the unit (amu) ensures that the atomic masses of most elements are close to whole numbers, simplifying calculations.

• Carbon is the most abundant element in the universe
• Carbon-12 has a stable nucleus and allows integer masses for most common elements — correct
• Hydrogen is too light to measure accurately
• Carbon's physical state is the defining reason for the standard

What you do

Add up the atomic masses (H=1, C=12, N=14, O=16, Na=23, Cl=35.5) and match each compound to its molecular mass.

Check yourself

What is the relative atomic mass of Carbon-12?

By definition, the atomic mass of Carbon-12 is exactly 12 amu. The value 12.011 is the *average* atomic mass of natural carbon, which includes Carbon-13.

If an atom has 6 protons and 8 neutrons, what is its relative atomic mass?

Relative atomic mass ≈ protons + neutrons. 6 + 8 = 14 amu. This is the isotope Carbon-14.

Why is the atomic mass of Chlorine listed as 35.5 amu in the periodic table?

Natural chlorine is a mix of isotopes. The periodic table shows the *average* atomic mass, weighted by abundance, not the mass of a single isotope.

• 12.011 amu
• 12.000 amu — correct
• 6.000 amu
• 1.000 amu
• 6 amu
• 8 amu
• 14 amu — correct
• 12 amu
• Chlorine atoms lose half their mass during reactions
• It is the weighted average of its naturally occurring isotopes (Cl-35 and Cl-37) — correct
• Chlorine has 35.5 protons
• The measurement instrument is inaccurate