Final Chemistry Notebook

Other atomic weights are listed on the periodic table (see our Periodic Table module). For each
element listed, measuring out a quantity of the element equal to its atomic weight in grams will yield
6.02 x 10 23 atoms of that element.
The atomic weight of an element identifies both the mass of one mole of that element and the total
number of protons and neutrons in an atom of that element. How can that be? Let's look at hydrogen.
One mole of hydrogen atoms will weigh 1.01 grams.
A hydrogen atom
with its single electron
Each hydrogen atom consists of one proton surrounded by one electron. But remember, the electron
weighs so little that it does not contribute much to an atom's weight. Ignoring the weight of hydrogen's
electrons, we can say that one mole of protons (H nuclei) weighs approximately one gram. Since
protons and neutrons have about the same mass, a mole of either of these particles will weigh about
one gram. For example, in one mole of helium, there are two moles of protons and two moles of
neutrons - four grams of particles.
Molecular weight
If you stand on a scale with a friend, the scale will register the combined weight of both you and your
friend. When atoms form molecules, the atoms bond together, and the molecule's weight is the
combined weight of all of its parts.
For example, every water molecule (H2O) has two atoms of hydrogen and one atom of oxygen. One
mole of water molecules will contain two moles of hydrogen and one mole of oxygen.
2 moles
hydrogen
Mole/weight relationships
of water and its parts
1 mole
oxygen
1 mole
water
+ =
A bottle filled with exactly 18.02 g water will contain 6.02 x 10 23 water molecules. The concept of
fractions and multiples described above also applies to molecules: 9.01 g of water would contain 1/2
mole, or 3.01 x 10 23 molecules. You can calculate the molecular weight of any compound simply by
summing the weights of atoms that make up that compound.
93