Relative masses.

Step 1 of 5

Too light to weigh

A carbon atom weighs about 0.00000000000000000000002 g, that is $2 \times 10^{-23}$⁠ grams. No balance can weigh a single atom. But to know how much of one substance reacts with how much of another, it is enough to know how heavy each atom is relative to the others.

So chemists use relative atomic mass, $A_r$⁠: an atom’s mass compared with an atom of carbon-12, whose mass is set at exactly $12$⁠. In the Unit on atoms this was the mean mass of the isotopes. Every periodic table prints it:

Element	H	C	O	Na	Al	Cl	Ca	Fe	Cu	Au
$A_r$⁠	$1.008$⁠	$12.01$⁠	$16.00$⁠	$22.99$⁠	$26.98$⁠	$35.45$⁠	$40.08$⁠	$55.85$⁠	$63.55$⁠	$196.97$⁠

The Scene shows a laboratory balance with carbon powder on it, and Look inside open at Particles: the sheets of carbon atoms in graphite, magnified about a hundred million times. Beside it are the balance’s reading and what follows from it.

Step 2 of 5

Formula mass

A molecule or formula unit weighs as much as its atoms together. The relative formula mass, $M_r$⁠, is the sum of the relative atomic masses of every atom in the formula. The small number below a symbol says how many atoms of that kind there are: CO₂ has one carbon and two oxygens,

$M_r(\mathrm{CO_2}) = 12.01 + 2 \times 16.00 = 44.01$$M_r(\mathrm{CO_2}) =$$12.01 + 2 \times 16.00 =$$44.01$⁠

The Scene shows water, H₂O, with the readings hiding everything but the mass on the balance. Work out the formula mass of water.

Step 3 of 5

How many atoms of each kind

In the formula of calcium carbonate, chalk, CaCO₃, the $3$⁠ belongs to the oxygen alone: one calcium, one carbon and three oxygens. When a formula is long, it helps to write a small table: each element, how many atoms of it, and their mass together.

Step 4 of 5

Counting atoms

Glucose, the sugar the body burns, has the formula C₆H₁₂O₆. In the Scene Look inside is open at Particles: a crystal of glucose molecules, each a ring of carbons and an oxygen with OH groups around it. How many atoms are in one molecule?

Step 5 of 5

Find it

The more atoms in a formula, and the heavier they are, the bigger the formula mass. The readings hide the formula mass again. Work it out yourself, and find in Substance the substance whose formula mass is about $100$⁠.