💧 Molecules
Atoms holding hands
Most people draw a water molecule as a straight line, but it’s bent, and the bend explains a lot about how water behaves.
Try drawing a water molecule in your head.
Most people draw three beads in a row.
A real water molecule is bent, and the bend changes how water behaves.
The puzzle
How do three atoms fit together?
A water molecule has three atoms: one big oxygen and two small hydrogens. The question is what shape they make when they join.
Your turn: tap your guess first. You’ll remember the answer better if you’ve guessed before you find out. When those three atoms join up, what shape do they make?
A water molecule is shaped like…
Every water molecule has this V shape. If it were straight, it wouldn’t behave like water, and you’ll test that further down.
The two arms make a wide V, about 104.5° apart.
More than half of your body is water, so you’re full of that angle.
Build it
Atoms join by sharing electrons
To make a molecule, atoms share electrons. Each atom holds one end of the same pair, like two hands gripping the same rope. Your turn: drag the two hydrogens onto the oxygen and watch them lock into the V.
You’ll see it hasn’t lined up straight. The oxygen has two pairs of spare electrons on top, and they push the hydrogens down into a V. The two dots on each arm are the shared electrons that hold the atoms together.
The bend matters more than you might think.
It gives water a hidden property.
The why
The bend gives water a + end and a − end
Oxygen pulls the shared electrons closer to itself. Press the button and you’ll see the molecule get a − end and a + end.
A molecule with a + end and a − end is called polar. Its two ends pull on other molecules a bit like the ends of a tiny magnet.
The why
Opposite ends pull together
Because every water molecule has a + end and a − end, the + end of one pulls on the − end of its neighbour. Your turn: drag the loose molecule near the other and see which ends pull together.
That pull is called a hydrogen bond. One on its own is weak, but in a drop of water every molecule is held to its neighbours this way, billions of them at once.
Because water is bent and sticky…
it does things that shape the world around you.
The why
What the bend does for water
Pick one of these to see it happen.
The molecules at the top are pulled down and sideways by their neighbours, so the surface acts like a stretched skin. It’s strong enough for a pond-skater to walk on, and it’s why spilt water forms round beads.
Water sticks to the walls of the tube and creeps up, and it goes higher in the thinner tube. The same stickiness helps a tree lift water through tiny tubes from its roots to its top leaves, with no pump.
This is the strangest one. Almost every substance shrinks when it freezes, so the solid sinks in its own liquid. Water expands when it freezes, so ice floats.
Nearly every solid sinks in its own liquid.
When water freezes, its molecules lock into an open cage with gaps in it…
so ice floats, and fish can live under it until spring.
Explore
The bend makes the difference
You can check that the bend matters by taking it away. Your turn: straighten the molecule into the line most people draw, and you’ll see the + and − ends disappear.
The same three atoms in a different shape would make a different substance. In chemistry, a molecule’s shape decides what it does.
Diamond and the graphite in pencil lead are made of the same atoms.
The only difference is how the atoms are arranged.
Everything around you is atoms joined together in a particular shape.
Pass it on
A diamond and the graphite in your pencil are made of the same carbon atoms. The atoms are arranged differently, which is why diamond is the hardest thing you own and graphite rubs off on your fingers. Soap works because one end of its molecule is attracted to water and the other end to grease. The smell of an orange and the way a protein folds into exactly the right knot both depend on how atoms are joined and what shape they make.
So if someone draws water as a straight H–O–H, or says atoms just stick together, you can put them right on both. Water is a bent V of about 104.5°, held together by shared electrons, and the bend gives it a + end and a − end. You can also show them the open cage and explain why ice floats.