Note the average atomic mass of copper on the nano-balance. Add atoms to the
larger balance until it registers the same number in g) as the reading on the nano-balance
(in u). Use the Exponent slider to help get the correct amount. Stop adding atoms when the
readings on both balances matchi exactly to the nearest 0.001 y).
How many atoms did you need to add?

One atom of copper has a mass of  1.08    1  g .

One gram of copper contains   9.47   1  atoms.

Explanation:

Copper's molar mass expresses the mass of 1 mole of copper atoms. One mole of any substance contains exactly  6.022   1 atoms or molecules of that substance - this is known as Avogadro's number.

In your case, 1 mole of copper will contain  6.022   1 atoms of copper.

Copper haa a molar mass of 63.456 g. The mass of an individual copper atom is calculated by dividing the mass by copper atoms of 1 mole.

                       = 1  Cu  (63.456 g  / 6.022    1)

                       =  1.08    1  g  

To get how many copper atoms are in one gram of copper, you can write  = 1 g copper   ( 6.022   1 atoms of copper  / 63.546  g )

                        =  9.47   1  atoms.

The mass has escaped into a gas which you can not see so the law stays true.