The reason moles are important can be illustrated this way: what if you had some carbon and some hydrogen? You might want each carbon atom to buddy up with one hydrogen molecule.

How do you make sure each carbon atom and each hydrogen molecule is paired and no one is left out? You can’t just put together equal mass of each one, because they weigh different amounts. What you want to do is add equal moles of each. A mole is 6.022 × 10^23 of the basic chemical entities that make up each.

Add 12 grams of carbon and 2 grams of hydrogen and you will have equal numbers of each: 1 mole.

A mole is more or less just a number for counting something. The Clear Science staff like to compare it to a dozen. You know that a dozen of something means there are 12 of that thing—for example a dozen doughnuts.

A mole of something is 6.022 × 10^23 of that thing. This is called Avogadro’s number, and if you write it out not in scientific notation, it is:

• 602,214,179,000,000,000,000,000

The standard used to define a mole is the number of carbon atoms in 12 grams of carbon-12This excellent website has pictures showing moles of various substances, including carbon, which we have borrowed for the graphic above.

You calculate how many moles you have as shown, with the number of grams per mole you read off the periodic table. (Atomic mass, which is the number with decimal points in it.)

Since electron clouds keep atoms from touching because they repel, you might be tempted to think the universe should fly apart. Actually there is a push and pull, modeled by scientists as something called the Lennard-Jones potential.

When neutral atoms get very close their electron clouds repel each other. But they are also attracted to each other by something called van der Waals forces. This means there is a happy medium, a place of minimal energy, where they like to sit in relation to each other.

Atoms can’t fall through each other because their electron clouds repel each other. Push them together as hard as you want, and they will resist. They have force fields around them. Atoms don’t actually ever touch—their force fields do. Those 2 tiny electrons smeared around these atoms do that, even though the empty space inside the atoms is huge.

(BS alert: It’s not actually a force, but this is the easiest way to describe it. Fair warning.)

To understand why atoms can’t fall through each other—despite the huge amount of empty space in them—we first need to think about electrons. Remember, the nucleus has protons, which are positive. The electrons are negative and attracted to the protons, so they orbit the nucleus.

A hydrogen atom has one proton and one electron. However, remember we also said the physics for electrons were strange. Namely, they are quantum mechanical. What this means is that they are kind of everywhere around the nucleus, without being anywhere exactly. Weird, right?

What this means is they are kind of a blur, or a smear. Think of the space around the nucleus as being a cloud, all kind of negative.

So what’s an atom?

1. a part at the center (the nucleus)
2. parts going around the center (orbitals, which “orbit”)

It is not wildly incorrect to think of this as being like the solar system, with the sun at the center, and the planets orbiting. So: there is a huge solar system that we are part of, riding on the Earth. And: if you shrink down to see the atoms that make up elements, they are tiny solar systems.

[BS ALERT: It is incorrect to think of atoms as exactly like like solar systems, for reasons we might go into later. But in general, the metaphor is a good one.]

It’s the nucleus that gives an atom its identity. We’ll talk about that next.