Multiple choiceQuestion 1 · Recall
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By the end you can define the unified atomic mass unit, define relative isotopic mass, and convert between the mass of one atom in kg and its relative isotopic mass.
One atom of carbon-12 has a mass of about . Written out, that's 0.000 000 000 000 000 000 000 000 019 9 kg. Nobody wants to do arithmetic with that.
So chemists don't weigh atoms in kilograms. They pick one atom as the standard and compare every other atom with it.
The standard is the carbon-12 atom, .
Unified atomic mass unit, u
One twelfth of the mass of one atom of carbon-12.
Picture a carbon-12 atom as a chocolate bar with exactly 12 squares. Snap off one square: that's 1 u. Now weigh every other atom in squares. Hydrogen-1 is about 1 square; oxygen-16 is about 16.
Where the analogy breaks: atoms aren't made of twelve u-sized chunks, and most atoms are not an exact whole number of squares. Oxygen-16 is 15.995 u, not 16 u.