Pick a unit
Each one takes about two minutes and ends with a quick check.
What everything is made of
Everything you can touch is made of atoms, tiny balls about a ten-millionth of a millimetre across. One grain of salt holds more atoms than there are stars in our galaxy.
Each atom has a tiny centre, the nucleus, made of two kinds of particle: protons, which carry a plus charge, and neutrons, which carry no charge. Around the centre, much lighter electrons, each with a minus charge, move about in a fuzzy cloud.
The centre is tiny compared with the whole atom. If an atom were a football stadium, its nucleus would be a pea on the centre spot, with the electrons buzzing round the stands. Atoms are mostly empty space.
The number of protons decides what kind of atom it is, its element. 1 proton is always hydrogen, 6 is carbon, 7 nitrogen, 8 oxygen, 30 zinc. Normally an atom has as many electrons as protons, so the plus and minus cancel out.
What each particle does. Protons decide the element, and their plus charge holds the electrons in place. Neutrons help glue the centre together: protons all repel each other, and neutrons add a powerful short-range pull (the strong force) that stops the nucleus flying apart. Electrons do all of chemistry: every bond, every reaction, is electrons moving or being shared.
Protons and neutrons are made of even smaller particles called quarks. Electrons, as far as anyone can tell, have no smaller parts at all.
The atoms of life. Out of 118 elements, just four make up about 96% of your body by weight: oxygen, carbon, hydrogen and nitrogen. Then come calcium (bones and teeth) and phosphorus (DNA and bones).
Count atoms instead of weighing them and hydrogen wins: about 6 of every 10 atoms in you are hydrogen, because it’s so light. Most of it is in water, which makes up more than half your weight.
Curious? Go deeperIsotopes, radioactivity and carbon dating
Atoms of the same element can have different numbers of neutrons. These versions are called isotopes. Most carbon has 6 neutrons (carbon-12); about 1 in 100 has 7 (carbon-13); and a tiny trace has 8 (carbon-14).
Too many or too few neutrons make a nucleus unstable. It eventually breaks down and gives off radiation: that’s radioactivity. Carbon-14 breaks down at a steady rate (half of it is gone after about 5,730 years), so measuring how much is left in old bones or wood tells archaeologists their age. That’s carbon dating.
Curious? Go deeperHow small, and how empty?
An atom is roughly 0.1 to 0.5 millionths of a millimetre across. Line up ten million of them and you’d have about a millimetre.
Almost all the mass (over 99.9%) is in the nucleus, which is about 100,000 times smaller than the atom. So why don’t you fall through the floor? Because the electron clouds of your shoes and the floor push each other away: minus repels minus.
Curious? Go deeperWhere did all these atoms come from?
Right after the Big Bang, the universe made almost only hydrogen and helium. Even today they make up about 98% of all the normal matter in the universe.
Every heavier atom was cooked later, inside stars. Stars fuse hydrogen into helium, then helium into carbon and oxygen, and the biggest stars go on up to iron. Elements heavier than iron come from dying stars: ageing giant stars, exploding stars, and collisions between neutron stars (the main source of gold). So the carbon in your body and the oxygen you breathe were made inside stars long before the Sun existed.
An atom is a tiny plus centre (protons and neutrons) with minus electrons around it. The number of protons decides the element.
What makes a carbon atom a carbon atom?
The periodic table, made friendly
There are 118 known elements. The periodic table lists them in order of protons: hydrogen (1), helium (2), lithium (3) and so on, in rows.
Each new row starts a new electron layer (Unit 3). And elements in the same column have the same number of outer electrons, so they behave alike. Lithium and sodium, in the first column, both fizz in water. The last column, helium, neon and argon, hardly reacts with anything.
The four atoms that build most of living things, hydrogen, carbon, nitrogen and oxygen (outlined above), all sit near the top. Metals like zinc and iron sit further down, in a middle block we meet in Unit 11.
Curious? Go deeperFamilies, and who made the table
In 1869 the Russian chemist Dmitri Mendeleev arranged the 63 elements known then by weight and behaviour. He left gaps for elements nobody had found yet and predicted what they’d be like. When gallium and germanium were discovered, they matched his predictions closely.
Some columns are famous families:
- Alkali metals (first column): lithium, sodium, potassium. Soft, and so eager to give away their one outer electron that they fizz or even explode in water.
- Halogens (second-to-last column): fluorine, chlorine. One electron short of full, so they grab electrons hard.
- Noble gases (last column): helium, neon, argon. Full outer layer, so they almost never react. That’s why helium is safe in balloons.
- Transition metals (the wide middle block): iron, copper, zinc, gold. This is where zinc lives.
Across a row, atoms get smaller, because more protons pull the electrons in tighter. Down a column, they get bigger, because each row adds a layer. About three quarters of all elements are metals. Some symbols come from Latin: Na is natrium (sodium), Fe is ferrum (iron), Au is aurum (gold).
The periodic table orders elements by protons. Same column means the same number of outer electrons, so similar behaviour.
Lithium and sodium sit in the same column. What do they share?
Electrons live in layers
Electrons don’t crowd into one cloud. They fill layers around the centre, inside first. The first layer holds 2 electrons. The second holds 8. For the atoms we meet here, the third holds 8 as well.
It helps to picture each layer after the first as 4 seats, each seat holding a pair of electrons. Only the outer layer matters for chemistry: it’s the part other atoms touch.
Atoms are most settled when their outer layer is full. Helium, neon and argon already are, so they hardly react. Every other atom gets there by sharing, giving or taking electrons. That’s all of chemistry in one sentence. Slide below to build atoms one proton at a time.
Carbon: 6 electrons. 2 fill the first layer and 4 sit in the outer layer, so it needs 4 more to be full.
Curious? Go deeperWhat the "seats" really are
Layers are really energy levels: electrons further out have more energy. Within a layer, electrons sit in regions with particular shapes, called orbitals. These are the seats.
The first layer has one ball-shaped seat (called s), so it holds 2. The second layer has one ball-shaped seat and three dumbbell-shaped seats (called p) pointing at right angles: that’s exactly the 4 seats, holding 8.
Each seat holds at most 2 electrons, and the two must have opposite “spin”, a tiny built-in magnetism. Bigger atoms add more seat shapes: the third layer can actually hold 18, but its 5 extra seats (10 electrons) fill later, which is why the transition metals sit in the middle of the periodic table.
Curious? Go deeperWhy fireworks have colours
When an electron gets a kick of energy (from heat or electricity), it can jump to a higher layer. When it falls back, it gives the energy out as light. Each element’s layers are spaced differently, so each gives out its own colours.
That’s why sodium street lamps glow orange-yellow, strontium makes red fireworks and copper makes blue-green ones. Astronomers use the same colours to tell what distant stars are made of.
Electrons fill layers: 2, then 8, then 8. Atoms want a full outer layer, and chemistry is how they get one.
Oxygen has 8 electrons. How many are in its outer layer?
Giving and taking: ions and salt
Some atoms don’t share. They hand electrons over completely. Sodium has 1 lonely electron in its outer layer; chlorine is 1 short of full. So sodium gives its electron to chlorine, and both end up with full outer layers.
Now sodium has one more proton than electrons, so it’s plus (Na⁺). Chlorine has one extra electron, so it’s minus (Cl⁻). Charged atoms like these are called ions. Plus and minus attract, so they stick, and billions of them stack into a neat grid: a grain of table salt, a crystal.
Sharing (Unit 4) and giving (this unit) are the two big ways atoms join. Most molecules in living things, and in the Mirror Molecules lesson, are held together by sharing.
Curious? Go deeperIons keep you alive
Your nerves work by moving sodium ions (Na⁺) and potassium ions (K⁺) in and out of nerve cells, which makes a tiny electric pulse. Calcium ions (Ca²⁺) make muscles contract and build bones. Chloride ions (Cl⁻) help make the acid in your stomach.
These charged “electrolytes” are why sports drinks contain salts. Ions can carry more than one charge, too: magnesium gives away 2 electrons to become Mg²⁺, and oxygen can take 2 to become O²⁻.
Curious? Go deeperWhy salty water carries electricity
Pure water barely conducts electricity. Dissolve salt in it and the Na⁺ and Cl⁻ ions float free, so they can drift towards the plus and minus ends of a battery, carrying current. Solid salt doesn’t conduct, because its ions are locked in the crystal.
Sharing and giving are really two ends of one scale: the bigger the difference in pulling strength between two atoms (next unit), the more one-sided the sharing, until it becomes giving.
Some atoms give or take electrons instead of sharing. They become charged ions, and opposite charges stick, as in salt.
Why does sodium become plus?
Tug-of-war: slightly plus, slightly minus
Sharing isn’t always fair. Some atoms pull shared electrons harder: oxygen and nitrogen pull hard, carbon is in the middle, and hydrogen and metals pull weakly.
When the pull is uneven, the electrons sit closer to the stronger atom. It ends up slightly minus, and its partner slightly plus. Chemists write these as δ− and δ+ (δ is the Greek letter d, for “a little”).
Water is the famous example. Its oxygen pulls electrons from both hydrogens, so water has a slightly minus end and two slightly plus ends. Water molecules cling to each other, plus to minus. That’s why water forms drops, and why it pulls salt apart: water’s minus ends tug the Na⁺ ions, its plus ends tug the Cl⁻.
Oil is mostly carbon and hydrogen, which share almost fairly, so oil has no charged ends. Water molecules would rather cling to each other than to oil, so the two don’t mix.
Curious? Go deeperThe pulling-strength scale
Chemists measure pulling strength with a number called electronegativity. On the usual scale: fluorine 4.0 (the strongest), oxygen 3.4, nitrogen 3.0, carbon 2.6, hydrogen 2.2, zinc 1.7, sodium 0.9.
The bigger the difference between two bonded atoms, the more lopsided the sharing. Carbon and hydrogen (2.6 vs 2.2) share almost fairly; oxygen and hydrogen (3.4 vs 2.2) don’t; sodium and chlorine (0.9 vs 3.2) are so different that sodium simply hands its electron over.
Curious? Go deeperHydrogen bonds: why ice floats and DNA zips up
A slightly plus hydrogen on one molecule is pulled toward a spare pair on an oxygen or nitrogen of the next. This pull is called a hydrogen bond. It’s much weaker than a real bond, but there are billions of them.
Hydrogen bonds are why water boils at 100°C, when similar-sized molecules boil far below freezing. They make ice an open lattice that is lighter than liquid water, so ice floats. And they hold the two strands of your DNA together like a zip, weak enough to unzip when a cell copies it.
Uneven sharing makes slightly plus and slightly minus ends. Those tiny charges make water sticky, dissolve salt, and let molecules grab each other.
In a carbon–oxygen bond, which atom ends up slightly minus?
Molecules are 3D
Molecules aren’t flat drawings. They’re 3D, and their shape comes from one rule: electron pairs push each other as far apart as they can. Count the groups around a central atom (each bond or spare pair is one group; a double bond counts once):
- 4 groups point to the corners of a little pyramid (chemists say tetrahedron), like methane.
- Water has 4 groups too, but 2 are spare pairs, so its atoms make a bent shape. Ammonia, with 1 spare pair, is a low pyramid.
- 3 groups spread into a flat triangle, like the carbon in C=O.
- 2 groups make a straight line, like carbon dioxide.
- Rings that share electrons all the way round, like benzene, lie completely flat.
Spin the models below. Shape decides which molecules fit together, how a medicine fits its pocket in your body, and whether a molecule has a mirror twin (Unit 12).
Methane
Drag to spin · pinch to zoom
Methane, CH₄: 4 bonds push apart to the corners of a pyramid.
Curious? Go deeperThe angles, exactly
The pyramid (tetrahedron) puts bonds 109.5° apart, as in methane. Spare pairs take up a bit more room than bonds, so they squeeze the bonds closer: 107° in ammonia, 104.5° in water. A flat triangle has 120° angles, and a straight line 180°.
Chemists call this rule VSEPR, short for “valence shell electron pair repulsion”: outer electron pairs repel each other.
Curious? Go deeperShape is how molecules recognise each other
Smell and taste sensors, enzymes and medicines all work by shape: a molecule has to fit a pocket, like a key in a lock. Change the shape slightly and it may no longer fit.
Benzene and similar rings are flat because their shared electrons spread right round the ring, in clouds above and below it. Rings like this appear in DNA, in many medicines and in the nitrogen ring of the Mirror Molecules lesson.
Electron pairs push apart, so molecules have set 3D shapes: pyramid, bent, flat triangle, straight line, flat ring.
Why is water bent instead of straight?
Reading chemists’ drawings
Chemists draw the same molecule in several ways. Here is ethanol, the alcohol in drinks. The formula just counts atoms. Ball and stick shows every atom and bond.
The line drawing is the shorthand you’ll see most: every corner or line end is a carbon, hydrogens on carbons are left out (add them yourself until each carbon has 4 bonds), and every other atom is written as a letter.
For 3D, chemists use wedges. A solid wedge points toward you, out of the page. A dashed wedge points away, into the page. Plain lines lie flat in the page.
That’s how a flat drawing shows which mirror twin it means (Unit 12): swap one solid and one dashed wedge and you’ve drawn the other twin.
Curious? Go deeperOther ways chemists write molecules
- Condensed formula: CH₃CH₂OH for ethanol, listing atoms group by group.
- Text codes for computers: ethanol is “CCO” in a system called SMILES. Databases use codes like this to search millions of molecules.
- Space-filling models: atoms drawn as overlapping balls at their true sizes, showing how bulky a molecule really is.
Line drawings save time: a sugar molecule with 24 atoms becomes a simple ring with a few letters. Once you know “corners are carbons, add hydrogens until each carbon has 4 bonds”, you can count every atom yourself.
In line drawings, corners are carbons and hydrogens on carbons are hidden. Wedges show what points toward you or away.
In a line drawing, an unlabelled corner is…
Reactions: atoms swap partners
A chemical reaction is atoms swapping partners: some bonds break, new bonds form, and new molecules come out. Atoms are never created or destroyed, only rearranged.
Burning natural gas (methane) in air: CH₄ + 2 O₂ → CO₂ + 2 H₂O. Count them: 1 carbon, 4 hydrogens and 4 oxygens on each side.
So why does gas need a spark? Breaking the old bonds needs a push first, like rolling a ball over a hill before it can roll down the other side. That hill is the energy barrier.
Once over the top, the new bonds release more energy than the push cost: that’s the heat of the flame. A high hill means a slow reaction, because few molecules get over. A low hill means a fast one. Heating things up helps more molecules over the top.
Curious? Go deeperEnergy in, energy out
Breaking a bond always costs energy; making one always releases it. If the new bonds release more than the old ones cost, the reaction gives out heat, like burning. If they release less, the reaction soaks up heat: instant cold packs work this way.
Curious? Go deeperWhat makes reactions faster
- Heat: hotter molecules move faster and hit harder, so more get over the hill. For many reactions, 10°C warmer roughly doubles the speed. That’s why fridges slow food going off.
- Crowding: more molecules packed together meet more often.
- Surface: powders react far faster than lumps. Fine flour dust in the air can even explode.
- Helpers: catalysts lower the hill (next unit).
Chemists count molecules in huge batches called moles: one mole is about 602 thousand billion billion (6.02 × 10²³) particles. A mole of water is just 18 grams, about a tablespoon.
Reactions rearrange atoms, and none are lost. They need a push over an energy hill: the lower the hill, the faster the reaction.
Methane burns into CO₂ and water. Where do the oxygen atoms in the water come from?
Helpers: catalysts
A catalyst, or helper, makes a reaction faster by giving it a lower hill. It does this by holding the ingredients in just the right position, so much less push is needed.
The helper isn’t used up: it lets go of the product and grabs the next ingredients. That’s why a pinch is enough.
Catalysts are everywhere. Your body runs on them: enzymes are protein catalysts that digest food and build your cells. A car’s catalytic converter uses metals to clean exhaust gas.
A handed catalyst can do even more: by holding ingredients one way round, it decides which mirror twin gets made. That’s the heart of the Mirror Molecules lesson.
Curious? Go deeperThe catalysts that run the world
- Enzymes can speed up reactions in your body by millions of times or more, at body temperature. Without them, digesting a meal would take years.
- Fertiliser: an iron catalyst helps turn nitrogen from the air into ammonia, the base of most fertiliser. Food grown with it feeds about half the people on Earth.
- Car exhaust: platinum, palladium and rhodium in a catalytic converter turn poisonous carbon monoxide into carbon dioxide, and nitrogen oxides back into nitrogen.
A catalyst changes how fast a reaction reaches its end, not where it ends up. Nobel Prizes in Chemistry for catalysts include 2001, 2021, and Kagan and Soai in 2026.
Catalysts lower the energy hill by holding ingredients in position. They aren’t used up, so a little goes a long way.
Why is a pinch of catalyst enough?
Metals and borrowed bonds
Metals like sodium, iron and zinc pull electrons weakly, so they give them up easily. Sodium becomes Na⁺ in salt, and iron loses electrons to oxygen when it rusts.
Many metal atoms also have empty seats: room in their outer layer that their own electrons don’t fill. An empty seat can’t grab like a hand, but it can accept an atom that brings its own spare pair. Chemists call this a coordinate bond; we’ll call it a borrowed bond.
Zinc has 2 electrons to share, so it has 2 firm hands, plus about 2 empty seats. In the Mirror Molecules lesson, a nitrogen’s spare pair fills one of zinc’s empty seats, linking molecules into teams.
Your blood does the same trick. Haemoglobin, the protein in red blood cells, holds iron atoms, and oxygen sits in an iron’s empty seat on its trip from your lungs to your muscles.
Curious? Go deeperWhy metals shine and conduct
In a lump of metal, each atom lets its outer electrons wander. The atoms sit in a grid surrounded by a shared “sea” of electrons. That sea carries electricity, reflects light (so metals shine) and lets layers of atoms slide without breaking (so metals bend instead of shattering).
Curious? Go deeperEmpty seats in your blood, and why carbon monoxide is deadly
Each haemoglobin protein in your red blood cells holds four iron atoms, and each iron has an empty seat where an oxygen molecule can sit. In the lungs oxygen hops on; in your muscles it hops off.
Carbon monoxide (CO) fits the same seat, but grabs it about 200 times more strongly than oxygen and doesn’t let go easily. So even a little CO in the air stops blood carrying oxygen. That’s why CO alarms matter.
Zinc is everywhere in your body too: about 2 to 3 grams in total, working inside hundreds of enzymes. Some proteins use zinc to hold a “zinc finger” shape that grips DNA.
Metals give up electrons easily, and many have empty seats that accept another atom’s spare pair: a borrowed bond.
What can fill a metal’s empty seat?
Handedness: mirror molecules
When a carbon holds four different things, they can be arranged two ways that are mirror images, like your hands. Same atoms, same bonds, but no amount of turning makes one into the other.
Chemists call such molecules chiral and the two versions enantiomers; we call them mirror twins.
Mirror twins behave the same in most ways, but anything handed can tell them apart. Your nose is handed, so left carvone smells of spearmint and right carvone of caraway. Medicines work by fitting handed pockets in your body, so often only one twin works.
Ordinary reactions make both twins, half and half. Making just one is hard, and that’s what the 2026 Nobel Prize in Chemistry was about: read the Mirror Molecules lesson.
Curious? Go deeperMore mirror twins around you
- Limonene: one twin is the main smell of orange peel; its mirror twin smells more like pine or lemon.
- Life’s choice: proteins are built almost entirely from “left-handed” amino acids, and DNA uses “right-handed” sugar.
- Medicines: in ibuprofen only one twin does most of the work; your body slowly converts the other twin into it.
Chemists name twins in two ways: L/D (an older system, used for amino acids and sugars) and R/S (a rule based on ranking the four partners). Neither letter tells you which way the molecule twists light: that has to be measured.
A carbon with four different partners has a mirror twin. Twins share atoms but not shape, and handed things like your nose can tell them apart.
What does a carbon need to have a mirror twin?
Remember these 6 things
- Atoms are a plus centre with minus electrons around it. The number of protons decides the element.
- Full outer layer. Atoms share, give or take electrons until their outer layer is full.
- A bond is a shared pair. Hydrogen makes 1, oxygen 2, nitrogen 3, carbon 4; leftover electrons are spare pairs.
- Uneven sharing makes slightly plus and slightly minus ends, which let molecules grab each other.
- Shapes. Electron pairs push apart, so molecules are 3D, and a carbon with four different partners has a mirror twin.
- Reactions rearrange atoms over an energy hill; catalysts lower the hill and aren’t used up.
Atom Quiz
Twelve quick questions, one for each unit. Every answer comes with the reason why.
Questions people ask about basic chemistry
What is an atom made of?
An atom has a tiny centre, the nucleus, made of protons (plus charge) and neutrons (no charge), with much lighter electrons (minus charge) moving around it. The number of protons decides which element it is: 1 for hydrogen, 6 for carbon, 8 for oxygen.
What is a chemical bond, in simple words?
A chemical bond is what holds two atoms together. Usually it is a pair of electrons shared between them: both atoms’ plus centres are pulled toward the shared minus pair. Some atoms bond by handing electrons over instead, becoming charged ions that stick together, as in salt.
Why do atoms bond at all?
Atoms are most settled when their outer layer of electrons is full. Most atoms’ outer layers are only part full, so they share, give or take electrons with other atoms until the layer is full. Helium and neon already have full outer layers, which is why they hardly react.
Why is water bent and why does it matter?
Water’s oxygen has two bonds and two spare pairs of electrons, and all four push apart, so the molecule is bent. Because oxygen also pulls shared electrons harder, water has a slightly minus end and slightly plus ends, so water molecules cling together and dissolve salt.
What does a catalyst do?
A catalyst speeds up a reaction by lowering its energy barrier, usually by holding the ingredients in the right position. It is not used up, so a small amount can help again and again. Enzymes in your body and the metals in a car’s catalytic converter are catalysts.
What makes a molecule chiral (handed)?
A molecule is chiral when it cannot be turned to match its mirror image, like a hand. The commonest cause is a carbon atom holding four different groups. The two mirror versions, called enantiomers, have the same atoms but different 3D shapes, so handed things like smell sensors can tell them apart.