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What is the shape of a molecule?

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Well, a molecule is mostly empty space.

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Almost all of its mass is concentrated

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in the extremely dense nuclei of its atoms.

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And its electrons,

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which determine how the atoms

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are bonded to each other,

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are more like clouds of negative charge

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than individual, discrete particles.

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So, a molecule doesn't have a shape

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in the same way that, for example,

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a statue has a shape.

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But for every molecule,

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there's at least one way

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to arrange the nuclei and electrons

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so as to maximize the attraction

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of opposite charges

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and minimize the repulsion

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of like charges.

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Now, let's assume that the only electrons

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that matter to a molecule's shape

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are the outermost ones from each participating atom.

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And let's also assume

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that the electron clouds in between atoms,

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in other words, a molecule's bonds,

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are shaped kind of like sausages.

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Remember that nuclei are positively charged

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and electrons are negatively charged,

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and if all of a molecule's nuclei

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were bunched up together

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or all of its electrons were bunched up together,

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they would just repel each other and fly apart,

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and that doesn't help anyone.

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In 1776, Alessandro Volta,

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decades before he would eventually invent batteries,

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discovered methane.

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Now, the chemical formula of methane is CH4.

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And this formula tells us

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that every molecule of methane

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is made up of one carbon and four hydrogen atoms,

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but it doesn't tell us what's bonded to what

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or how they atoms are arranged in 3D space.

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From their electron configurations,

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we know that carbon can bond

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with up to four other atoms

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and that each hydrogen can only bond

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with one other atom.

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So, we can guess

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that the carbon should be the central atom

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bonded to all the hydrogens.

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Now, each bond represents

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the sharing of two electrons

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and we draw each shared pair of electrons as a line.

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So, now we have a flat representation

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of this molecule,

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but how would it look in three dimensions?

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We can reasonably say

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that because each of these bonds

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is a region of negative electric charge

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and like charges repel each other,

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the most favorable configuration of atoms

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would maximize the distance between bonds.

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And to get all the bonds

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as far away from each other as possible,

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the optimal shape is this.

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This is called a tetrahedron.

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Now, depending on the different atoms involved,

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you can actually get lots of different shapes.

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Ammonia, or NH3, is shaped like a pyramid.

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Carbon dioxide, or CO2, is a straight line.

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Water, H2O, is bent like your elbow would be bent.

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And chlorine trifluoride, or ClF3,

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is shaped like the letter T.

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Remember that what we've been doing here

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is expanding on our model of atoms and electrons

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to build up to 3D shapes.

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We'd have to do experiments

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to figure out if these molecules

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actually do have the shapes we predict.

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Spoiler alert:

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most of the do, but some of them don't.

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Now, shapes get more complicated

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as you increase the number of atoms.

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All the examples we just talked about

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had one obviously central atom,

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but most molecules,

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from relatively small pharmaceuticals

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all the way up to long polymers

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like DNA or proteins, don't.

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The key thing to remember

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is that bonded atoms will arrange themselves

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to maximize the attraction between opposite charges

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and minimize the repulsion between like charges.

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Some molecules even have two or more

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stable arrangements of atoms,

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and we can actually get really cool chemistry

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from the switches between those configurations,

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even when the composition of that molecule,

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that's to say the number and identity of its atoms,

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has not changed at all.