Atoms

by Chris Woodford. Last updated: January 24, 2024.

Suppose you had to build yourself a world exactly like the one we live in. Where would you start? You'd need people... cars... houses... animals... trees... and billions of other things. But if you had a few dozen different types of atom, you could build all these things and more: you'd just join the atoms together in different ways. Atoms are the tiny building blocks from which everything around us is constructed. It's amazing to think you can make anything out of atoms, from a snake to an ocean liner—but it's absolutely true! Let's take a closer look.

Artwork: From the hair on your head to the t-shirt on your back, everything in the world is made of atoms. I've greatly exaggerated their size in this illustration. On my screen, each of the atomic red dots is about 10 million times bigger than a typical atom. (Your screen may be bigger or smaller than mine, or scaled differently, so take that as a very rough approximation.)

What is an atom?

Take anything apart and you'll find something smaller inside. There are engines inside cars, pips inside apples, hearts and lungs inside people, and stuffing inside teddy bears. But what happens if you keep going? If you keep taking things apart, you'll eventually, find that all matter (all the "stuff" that surrounds us) is made from different types of atoms. Living things, for example, are mostly made from the atoms carbon, hydrogen, and oxygen. These are just three of over 100 chemical elements that scientists have discovered. Other elements include metals such as copper, tin, iron and gold, and gases like hydrogen and helium. You can make virtually anything you can think of by joining atoms of different elements together like tiny LEGO® blocks.

Photo: What does an atom look like? You can see one if you have the right kind of microscope or camera! This photo shows strontium atoms "flying" in a cube while being stimulated with precision laser light. By courtesy of National Institute of Standards and Technology (NIST).

An atom is the smallest possible amount of a chemical element—so an atom of gold is the smallest amount of gold you can possibly have. By small, I really do mean absolutely, nanoscopically tiny: a single atom is hundreds of thousands of times thinner than a human hair, so you have absolutely no chance of ever seeing one unless you have an incredibly powerful electron microscope. In ancient times, people thought atoms were the smallest possible things in the world. In fact, the word atom comes from a Greek word meaning something that cannot be split up any further. Today, we know this isn't true. In theory, if you had a knife small and sharp enough, you could chop an atom of gold into bits and you'd find smaller things inside. But then you'd no longer have the gold: you'd just have the bits. All atoms are made from the same bits, which are called subatomic particles ("sub" means smaller than and these are particles smaller than atoms). So if you chopped up an atom of iron, and put the bits into a pile, and then chopped up an atom of gold, and put those bits into a second pile, you'd have two piles of very similar bits—but there'd be no iron or gold left.

What are the parts of an atom?

Most atoms have three different subatomic particles inside them: protons, neutrons, and electrons. The protons and neutrons are packed together into the center of the atom (which is called the nucleus) and the electrons, which are very much smaller, whizz around the outside. When people draw pictures of atoms, they show the electrons like satellites spinning round the Earth in orbits. In fact, electrons move so quickly that we never know exactly where they are from one moment to the next. Imagine them as super-fast racing cars moving so incredibly quickly that they turn into blurry clouds—they almost seem to be everywhere at once. That's why you'll see some books drawing electrons inside fuzzy areas called orbitals.

Artwork: Atoms contain protons and neutrons packed into the central area called the nucleus, while electrons occupy the space around it. In simple descriptions of the atom, we often talk about electrons "orbiting" the nucleus like planets going around the Sun or satellites whizzing around Earth, although that's a huge oversimplification. Note also that this picture isn't drawn to scale! Most of an atom is empty space. If an atom were about as big as a baseball stadium, the nucleus would be the size of a pea in the very center and the electrons would be somewhere on the outside edge.

What makes an atom of gold different from an atom of iron is the number of protons, neutrons, and electrons inside it. Cut apart a single atom of iron and you will find 26 protons and 30 neutrons clumped together in the nucleus and 26 electrons whizzing around the outside. An atom of gold is bigger and heavier. Split it open and you'll find 79 protons and 118 neutrons in the nucleus and 79 electrons spinning round the edge. The protons, neutrons, and electrons in the atoms of iron and gold are identical—there are just different numbers of them. In theory, you could turn iron into gold by taking iron atoms and adding 53 protons, 88 neutrons, and 53 electrons to each one. But if that were as easy as it sounds, you can bet all the world's chemists would be very rich indeed!

But let's suppose you could turn atoms into other atoms very simply. How would you make the first few chemical elements? You'd start with the simplest atom of all, hydrogen (symbol H), which has one proton and one electron, but no neutrons. If you add another proton, another electron, and two neutrons, you get an atom of helium (symbol He). Add a further proton, another electron, and two more neutrons, and you'll have an atom of the metal lithium (symbol Li). Add one proton, one neutron, and one electron and you get an atom of beryllium (symbol Be).

See how it works? In all atoms, the number of protons and the number of electrons is always the same. So nitrogen has 7 protons and 7 electrons, calcium has 20 protons and 20 electrons, and tin has 50 protons and 50 electrons. The number of neutrons is very roughly the same as the number of protons, but sometimes it's rather more. So bromine has 35 protons and 35 electrons, but 45 neutrons. Platinum has 78 protons, 78 electrons, and 117 neutrons. The number of protons in an atom is called the atomic number and it tells you what type of atom you have. An atomic number of 1 means the atom is hydrogen, atomic number 2 means helium, 3 means lithium, 4 is beryllium, and so on. The total number of protons and neutrons added together is called the relative atomic mass. Hydrogen has a relative atomic mass of 1, while helium's relative atomic mass is 4 (because there are two protons and two neutrons inside). In other words, an atom of helium is four times heavier than an atom of hydrogen, while an atom of beryllium is nine times heavier.

How do atoms make molecules and compounds?

Atoms are a bit like people: they usually prefer company to being alone. A lot of atoms prefer to join up with other atoms because they're more stable that way. So hydrogen atoms don't exist by themselves: instead, they pair up to make what is called a molecule of hydrogen. A molecule is the smallest amount of a compound: a substance made from two or more atoms.

Some people find molecules and compounds confusing. Here's how to remember the difference. If you join two different chemical elements together, you can often make a completely new substance. Glue two atoms of hydrogen to an atom of oxygen and you'll make a single molecule of water. Water is a compound (because it's two different chemical elements joined together), but it's also a molecule because it's made by combining atoms. The way to remember it is like this: compounds are elements joined together and molecules are atoms joined together.

Not all molecules are as small and simple as water. Molecules of plastics, for example, can be made of hundreds or even thousands of individual atoms joined together in incredibly long chains called polymers. Polythene (also called polyethene or polyethylene) is a very simple example of this. It's a polymer made by repeating a basic unit called a monomer over and over again—just like a coal train made by coupling together any number of identical trucks, one after another:

What are isotopes?

To complicate things a bit more, we sometimes find atoms of a chemical element that are a bit different to what we expect. Take carbon, for example. The ordinary carbon we find in the world around us is sometimes called carbon-12. It has six protons, six electrons, and six neutrons, so its atomic number is 6 and its relative atomic mass is 12. But there's also another form of carbon called carbon-14, with six protons, six electrons, and eight neutrons. It still has an atomic number of six, but its relative atomic mass is 14. Carbon-14 is more unstable than carbon-12, so it's radioactive: it naturally disintegrates, giving off subatomic particles in the process, to turn itself into nitrogen. Carbon-12 and carbon-14 are called isotopes of carbon. An isotope is simply an atom with a different number of neutrons that we'd normally expect to find.

Artwork: Carbon-12 and carbon-14 are both isotopes of carbon: different variations that have different numbers of neutrons (blue). Carbon-14 has two more neutrons (yellow) than carbon-12, but both have six protons (red) and six electrons (green).

How do atoms make ions?

Atoms aren't just packets of matter: they contain electrical energy too. Each proton in the nucleus of an atom has a tiny positive charge (electricity that stays in one place). We say it has a charge of +1 to make everything simple (in reality, a proton's charge is a long and complex number: +0.00000000000000000016021892 C, to be exact!). Neutrons have no charge at all. That means the nucleus of an atom is effectively a big clump of positive charge. An electron is tiny compared to a proton, but it has exactly the same amount of charge. In fact, electrons have an opposite charge to protons (a charge of −1 or −0.00000000000000000016021892 C, to be absolutely exact). So protons and electrons are a bit like the two different ends of a battery: they have equal and opposite electric charges. Since an atom contains equal number of protons and electrons, it has no overall charge: the positive charges on all the protons are exactly balanced by the negative charges on all the electrons. But sometimes an atom can gain or lose an electron to become what's called an ion. If it gains an electron, it has slightly too much negative charge and we call it a negative ion; it it loses an electron, it becomes a positive ion.

Artwork: A lithium atom (Li) forms a positive ion (Li +) by "losing" an electron. A fluorine atom (F) forms a negative ion (F −) by gaining an electron.

What's so good about ions? They're very important in many chemical reactions. For example, ordinary table salt (which has the chemical name sodium chloride) is made when ions of sodium join together with ions made from chlorine (which are called chloride ions). A sodium ion is made when a sodium atom loses an electron and becomes positively charged. A chloride ion forms in the opposite way when a chlorine atom gains an electron to become negatively charged. Just like two opposite magnet poles, positive and negative charges attract one another. So each positively charged sodium ion snaps onto a negatively charged chloride ion to form a single molecule of sodium chloride. When compounds form through two or more ions joining together, we call it ionic bonding. Most metals form their compounds in this way.

The electrical charge that ions have can be useful in all sorts of ways. Ions (as well as electrons) help to carry the electricity through batteries when you connect them into a circuit.

How many atoms are there in something?

If atoms are so tiny, there must be zillions and zillions of them in all the things around us... but how many are there, exactly?

Chemists have a handy way of talking amount these vast numbers of atoms—by using the rather unusual word mole. A mole of something (anything) has exactly 6.022 × 10²³ particles in it, which is a short way of saying 602,200,000,000,000,000,000,000 or 602 billion trillion. This strange amount is called Avogadro's number (or Avogadro's constant) after Italian chemist Amedeo Avogadro (1776–1856), who thought up the idea. Avogadro's original hypothesis was that a certain volume of any gas will contain the same number of molecules as the same volume of any other gas providing both gases are at the same temperature and pressure.

So how much is a mole? When we're talking about atoms, a mole is the relative atomic mass in grams. So a mole of carbon is 12g, because carbon's relative atomic mass is 12, and it contains 620 billion trillion atoms. A mole of aluminum is 27g, because aluminum's relative atomic mass is 27. A mole of aluminum also contains 620 billion trillion atoms.

We can also use moles to talk about molecules. A mole of a compound contains 602 billion trillion molecules. A molecule of water has a relative molecular mass of 18 (that's 16 for the oxygen atom, plus two hydrogens, making 18). A mole of water weighs 18g and contains 620 billion trillion molecules.

Photo: A mole of any substance contains the same number of elementary particles (atoms, molecules, ions, electrons, or anything else). Here you can see 18g of water, 12g of carbon, 63g of copper, and 27g of aluminum. Each of these is a mole and contains 602 billion trillion atoms (or molecules, in the case of water). Photo courtesy of National Institute of Standards and Technology Digital Collections, Gaithersburg, MD 20899.

A brief history of atoms

Who discovered the atom, how, and when? Let's quickly nip back through history...

• 450 BCE: Ancient Greek philosophers Leucippus and Democritus became the first people to propose that matter is made of atoms.
• 1661: Anglo-Irish chemist Robert Boyle (1627–1691) suggested that chemical elements were the simplest forms of matter.
• 1789: Frenchman Antoine Lavoisier (1743–1794), widely known as the "father of modern chemistry," set out a list of chemical elements (which he defined as substances that can't be broken down through a chemical reaction). This was an important stepping stone on the way to the full Periodic Table.
• 1803: English scientist John Dalton (1766–1844) published the atomic theory of matter. He realized each chemical element was made up of atoms.
• 1815: English physician William Prout (1785–1850) suggested the weights of different elements are simple multiples of the weight of a hydrogen atom—not quite true, but another important clue to understanding how atoms are made.
• 1869: Building on the insights of Lavoisier, Dalton, Prout and others, a Russian chemist called Dmitri Mendeleyev (1834–1907) found a logical way of organizing the chemical elements with a neat structure called the Periodic Table.
• 1896: French physicist Henri Becquerel (1852–1908) accidentally discovered radioactivity.
• 1917: New Zealand-born English physicist Ernest Rutherford (1871–1937) "split" the atom: he proved that atoms are made of smaller particles, eventually concluding they had a heavy, positively charged nucleus and a largely empty area around them.
• 1919: British physicist Francis Aston (1852–1908) discovered a large number of atomic isotopes using mass spectrometry.
• 1938: German physicists Otto Hahn (1879–1978) and Fritz Strassmann (1902–1980) achieved the first nuclear fission (splitting up of heavy atoms to make lighter ones).
• 1945: The United States dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki.
• 1960s–1970s: Particle physicists figured out how several fundamental forces hold small, "subatomic" particles together to make atoms. Their ideas gradually became known as the Standard Model.
• 2013: Scientists used a quantum microscope to take the first pictures inside a hydrogen atom.

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On other sites

• The Particle Adventure: One of the best simple websites explaining atoms and the world inside them.
• Structure of Matter: This very good interactive slideshow from the Nobel Prize website explains, in 22 slides, all about atoms and the other particles inside them. [Archived via the Wayback Machine.]
• Dark matter and dark energy: Most of the "stuff" in the universe isn't conventional matter or energy, as we've always conceived it: it's actually "dark matter" and "dark energy." This NASA website explains what these things are and how they relate to conventional matter and energy.

Articles

• Ultrasensitive Microscope Reveals How Charging Changes Molecular Structures by Dexter Johnson, IEEE Spectrum, July 11, 2019. What happens to the structure of molecules when they gain or lose electric charges?
• Scientists Took an M.R.I. Scan of an Atom by Knvul Sheikh, The New York Times, July 1, 2019. Scientists from IBM Almaden and the Institute for Basic Sciences in Seoul discover what happens when you look at atoms in a body scanner.
• Do Not Adjust Focus. Those Blobs Are Atoms by Kenneth Chang, The New York Times, October 5, 2004. The quest to take pictures of an atom continues!

Books for younger readers

• Can you feel the force? by Richard Hammond. Dorling Kindersley, 2015. Includes a short introduction to atoms, quantum theory, and particle physics.
• The Periodic Table by Adrian Dingle. Oxford University Press, 2014.
• Atoms and Molecules by Chris Woodford and Martin Clowes. Rosen, 2012. (Previously published by Blackbirch.) One of my own books, this charts the history of atomic theory from ancient times to modern.
• Splitting the Atom by Alan Morton. Evans, 2005.A brief history of atom theory.
• How to split the atom by Hazel Richardson. Oxford University Press, 1999. A zany, fun guide that will appeal to lovers of the Horrible Science series.
• Eyewitness: Matter by Christopher Cooper. Dorling Kindersley, 1992. A drier, but solid introduction; good for school projects.

Books for older readers

• Atom by Piers Bizony. Icon, 2017. The story of how scientists came to understand atoms.
• The Elements: A Visual Exploration of Every Known Atom in the Universe by Theodore Gray. Black Dog & Leventhal, 2012. An entertaining trip through the Periodic Table.
• Mr. Tompkins in Paperback by George Gamov. Cambridge, 2012. A very vivid introduction to the world inside atoms from one of the 20th-century's most creative physicists. Suitable for older teens and above.
• The Fly in the Cathedral by Brian Cathcart. Farrar, Straus and Giroux, 2005. Excellent, easy-to-understand account of how Ernest Rutherford and his team figured out the structure of atoms. Also published in paperback by Penguin.
• Six Easy Pieces by Richard Feynman. Penguin, 1998. This book is by no means as "easy" as its title suggests, but the final chapter does contain a pithy explanation of quantum theory and its various puzzles that people with a basic grounding in physics can hope to understand.

Videos

• What is a Higgs boson?: Don Lincoln, a scientist at Fermilab, explains the hottest question in subatomic science—in terms most of us can understand!
• What is antimatter?: Another good simple explanation from Don Lincoln.
• How J.J. Thomson discovered the electron: This is a great little video that explains how scientists such as Thomson came to the conclusion that electrons must be charged particles inside atoms.