Suppose you have a dozen carbon atoms, a dozen gold atoms, and a dozen iron atoms. Even though you have the same number of each, would you expect them all to have the same mass? Explain.

NO!

Explanation:

Not considering isotopes for the moment.  All the atoms of any one element have the same mass.   From the periodic table,  and using whole numbers, you may note that the mass of one atom of hydrogen is one atomic mass unit (amu) and the mass of one carbon atom is 12 amu

So, the one carbon (C) atom is 12 times more massive than one hydrogen (H) atom

If you have the same number or each  the carbon atoms will always be 12 times more massive.

The fundamental, concept, is that (again neglecting isotopes) since all the atoms of any one element are the same, we count out atoms by massing the atoms.   We choose to measure masses in grams, and the counting units is the mole--a number.   Like we count out eggs by the number 12, which we call a dozen,   we count out atoms by the number mole which is a huge, huge number 6.02  x 10 to the 23rd power.  

To comprehend how large the number mole is, I had various students count out a mole of atoms or a mole of molecules by massing them, and then each brought the mole of what they had into a gym which measured 100ft by 100ft.  

The question then became, if you had a mole of raindrops, average diameter 0,04cm, how high would the gym walls have to be to hold the mole of raindrops, assuming no compressing of the water molecules.

The answer is nearly 1,000 times the distance between the earth and the moon.   The reaction is WOW, they must be so small,   YEP!

They got extra credit if they could come up with another accurate and fun way of describing the number mole.

it forms an anion that has a larger radius. anions have a negative charge and are larger than the original atom.