The mass of copper (Cu) is 63.546 atomic mass units (u), which translates to a molar mass of 63.55 grams per mole. This is the standard atomic weight recognized by the International Union of Pure and Applied Chemistry (IUPAC), and it’s the number you’ll find on virtually every periodic table.
What the Number 63.546 Represents
Copper’s atomic weight isn’t the mass of a single copper atom. It’s a weighted average of the masses of copper’s two naturally occurring stable isotopes, adjusted for how common each one is in nature. Every copper atom has 29 protons in its nucleus, but the number of neutrons varies.
Cu-63 has 29 protons and 34 neutrons, with an isotopic mass of 62.9296 u. It makes up about 69.17% of all natural copper. Cu-65 has 29 protons and 36 neutrons, with an isotopic mass of 64.9278 u, and accounts for the remaining 30.83%. When you average those two masses according to their abundance, you land at 63.546 u.
Why the Value Has a Small Uncertainty
You may see copper’s atomic weight written as 63.546(3), where the “(3)” indicates an uncertainty of plus or minus 0.003. This uncertainty exists not because of measurement limitations but because the ratio of Cu-63 to Cu-65 actually varies slightly depending on where in the world the copper comes from. A copper-chloride mineral from Chile, for instance, yields an atomic weight closer to 63.542, while a copper-carbonate mineral from Arizona pushes it up to 63.549. IUPAC expanded the uncertainty to 0.003 back in 1969 specifically to account for this natural variability.
For most chemistry coursework and lab calculations, rounding to 63.55 g/mol is perfectly acceptable and is the value most textbooks use.
Using Copper’s Molar Mass in Calculations
The molar mass of 63.55 g/mol means that one mole of copper atoms, roughly 6.022 × 10²³ individual atoms, weighs 63.55 grams. This is the conversion factor you need for stoichiometry problems. If a reaction calls for 0.5 moles of copper, you’d weigh out about 31.8 grams. If you have 127.1 grams of copper wire, you’re working with approximately 2 moles.
Copper in the Human Body
Your body contains a surprisingly small amount of copper. A healthy adult carries between 50 and 120 milligrams total, most of it concentrated in the liver, brain, and muscles. The recommended daily intake for adults is 900 micrograms (less than 1 milligram). Pregnant women need slightly more, around 1,000 micrograms per day. Despite these tiny quantities, copper plays essential roles in energy production, iron metabolism, and building connective tissue.