3-2: Molecular Models

This lesson is a little tough to do without a molecular modelling kit, which looks like this:

Here's how to use a kit like this:

• The differently-coloured spheres represent different common atoms. Depending on who makes the kit the colours might change, but typically white is hydrogen, black is carbon, red is oxygen and blue is nitrogen.

• The little sticks (grey and white) can show you how the atoms are connected together. These represent chemical bonds or connections between the atoms.

• Each atom will have different numbers of holes in the sphere: hydrogen will have one, carbon will have four, oxygen will have two, and nitrogen will have three.

• These holes correspond to the number of connections each atom can make with others.

• When building a molecule out of atoms, there can't be any empty holes. In the real world, that will be an unstable molecule that will immediately react with something else to make all those connections complete.

Let's take a closer look at the picture, and a few of the simple molecules you can build with a kit like this.

Water

Here's a water molecule built with a molecular modelling kit:

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Water has a formula of H₂O, which means there are two atoms of hydrogen and one atom of oxygen. As you can see, the oxygen atom in the middle is bonded to each of the hydrogen atoms. The oxygen can make two bonds, and the hydrogen can only make one. We can simplify this a bit with a structural diagram, also called a skeletal diagram (among other things).

Instead of using the colour, we just write the symbol of the element. The single lines show that each bond between the oxygen and the hydrogen is a single bond, which means that only one pair of electrons is shared here. (More on that in Grade 10 Science.)

Hydrogen

There are seven elements that only exist as a diatomic molecule: they don't exist as single atoms, only as pairs of atoms in a molecule. Conveniently, six of these seven elements make a shape like a number 7:

...and then there's hydrogen, up in the top-left corner. Here's a molecule of hydrogen H₂:

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The structural diagram of hydrogen gas is similarly simple:

Two atoms, a single bond between them, and that's it. As far as molecules go, it will never get any simpler!

Carbon dioxide

This one gets a little strange. Carbon has four connections to make with other atoms, and each oxygen atom can form two connections. We know the formula is CO₂, so it has one carbon atom and two oxygen atoms. It's tempting just to put one stick between the carbon and each of the oxygens, but that would leave lots of empty spaces. To fix that, we make two connections between the carbon atom in the centre and each of the oxygen atoms:

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If that looks complicated, count the bonds between the carbon and each of the oxygen atoms. The carbon has two bonds with the left oxygen, and two more with the right one. Since each carbon has two bonds with each of the oxygen atoms, we say that the bond between the carbon and each oxygen is a double bond. The structural diagram shows this with a double line:

Nitrogen

Like hydrogen, nitrogen is a diatomic element, so its formula is N₂. Since nitrogen can make three bonds with another atom, and each nitrogen atom in the N₂ molecule can only bond with the other atom, that means there's a triple bond between the atoms.

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That triple bond takes so much energy to break that your body can't do it. You need nitrogen or you will die -- it's a vital element that's used in many molecules in your body, including proteins. Almost 80% of the air you breathe is nitrogen, and it circulates through your bloodstream... but, since your body has no way to break it apart, you breathe it right back out. Here's the structural diagram:

As you can imagine, the triple bond is shown with three lines.