Atomic and Molecular Masses

This is so wrong.

Bohr model? More like I’m bored of seeing this incorrect model.

Let’s Start With 1 Mole of 12C12C chosen semi-arbitrarily

Each 12C atom has 6 protons, 6 neutrons, and 6 electrons.

Therefore, 1 mole of 12C has: 6 moles of protons, 6 moles of neutrons, and 6 moles of electrons.

Let’s Start With 1 Mole of 12C

But a mole is chosen so that 1 mole of protons weighs 1 g (electrons are too small to make any difference).

So our 1 mole of 12C has:6 grams of protons, 6 grams of neutrons, and ~0 grams of electrons.

Therefore, 1 mole of 12C weighs….12 grams!(Keanu moment)

This is the Point of Moles!Avogadro’s number is actually chosen so that this will be true.

1 mole of any element has a mass, in grams, equal to its atomic mass.

1 mole of 63Cu weighs 63 g. 1 mole of 79Br weighs 79 g

1 mole of 14C weighs 14 g etc.

These all have the same NUMBER OF ATOMS—one mole. Because some are heavier than others, the masses are different.

This is the Point of Moles!

Each of these beakers is 1 mole (6.022*1023) atoms.

Because the masses and densities are different, there are very different masses and volumes in each beaker.

If we had the same mass of each element, the number of atoms would be different instead.

Real SamplesActual samples of elements come as a mix of isotopes.

For example, any natural sample has the following mix:

12C = 98.9% 13C = 1.1%14C = essentially zero, but you can see it’s there because it’s radioactive

Therefore, the weighted average of carbon’s mass is0.989*12 + 0.011*13 = 12.011(which is the atomic mass on the periodic table)

1 mole of 12C weighs exactly 12 grams, 1 mole of any naturally-occurring mix of carbon will weigh 12.011 grams

This makes sense—a few of those carbon atoms are a little bit heavier.

(if we had a really small sample, we might get all 12C by chance, but our sample sizes will always be in the gajillions)

Using Atomic Masses

Two Different Conversion Factors

Number of Atoms Moles Grams

Avogadro’s Number. Same no matter what the element is

Atomic Mass. Different value for each element.

Putting Them Together

You Can Do It All Together

Molecular/Formula WeightsTake a simple compound: H2O

1 mole of water has 2 moles of hydrogen atoms, and 1 mole of oxygen atoms

Each mole of hydrogen atoms weighs 1.008 g, and each mole of oxygen atoms weighs 15.999 g.

Therefore, 1 mole of water weighs 2*1.008 + 15.999 = 18.015 g (the sum of the atomic masses)

This is the molecular weight (or molecular mass, or molar mass).

For an ionic compound, this would be the formula weight (same thing, but not molecular since there are no molecules).

You use it just like atomic mass: to convert between grams and moles

Summary 1 mole of an element weighs its atomic mass in grams

Because real samples have mixed isotopes, you use the average mass from the periodic table.

This is a conversion factor for going from moles to grams, or grams to moles.

For compounds, the molecular/formula weight is just the sum of the elements' atomic masses.