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
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.)
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
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
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
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
What are the parts of an atom?
Most atoms have three
different subatomic particles inside them: protons,
and electrons. The protons and neutrons are
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
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
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
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
you add another proton, another electron, and two neutrons, you get
an atom of helium (symbol He). Add a further proton, another electron,
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).
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
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.
What is the Periodic Table?
Suppose you make a list of the chemical elements in order of their atomic number (how many protons they have), starting with hydrogen (H). You'll find that elements with similar chemical properties (how they react with things) and physical properties (whether they're metals or non-metals, how they conduct heat and electricity, and so on) occur at regular intervals—periodically, in other words. If you rearrange your list into a table so similar atoms fall underneath one another, you get a diagram like this, which is called the Periodic Table. The columns are called groups and the rows are called periods.
Artwork: The Periodic Table of the elements.
So what? Atoms in a certain group (column) tend to have similar properties. So, for example, the red column on the right contains the Noble Gases (helium, neon, argon, krypton, and so on), which are relatively unreactive. The pink column on the left contains the alkali metals (lithium, sodium, potassium, and so on), which are relatively reactive metals (you probably know that some of them react violently with water, for example, to produce explosive hydrogen gas). If you know where a certain element sits in the table, and you know a little bit about the properties of the elements above, below, and either side, you can often figure out what the properties of that element will be.
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:
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.
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
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
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.
How do we know atoms exist?
Artwork: Molecules are built from atoms: In the early 19th century, English chemist John Dalton (1766–1844) realized that atoms join together in simple
ratios. Water forms when two hydrogens snap onto one oxygen. Chemical reactions like this
make sense if the elements exist as simple building blocks: atoms, in other words.
If we can't see atoms, how do we know they're there? That's a very good question!
Science is all about evidence, so what evidence do we have that atoms really exist?
It comes in a variety of different forms:
Chemists have long known that when we combine different elements in chemical reactions,
the ingredients join in simple ratios. So, for example, in water we know that there
are twice as many hydrogen atoms as oxygen atoms (H2O), making
a ratio of 2:1. In salt (sodium chloride) there are equal numbers of sodium and chlorine atoms (NaCl),
so the ratio is 1:1. We can easily explain this if chemical elements really exist as
simple particles (atoms, in other words), which snap together like building blocks.
Some substances are radioactive: they naturally split into simpler substances and give off tiny
particles or energy in the process. Again, this makes sense if atoms exist and they're built
from smaller particles (protons, neutrons, and electrons).
Scientists can split big atoms into smaller ones. In one very
famous series of experiments in the early 20th century, a team led by
Ernest Rutherford (a New-Zealand-born physicist) fired particles at atoms and watched what happened. This showed how the bits were arranged inside a typical atom (with the nucleus at the center).
We have plenty of evidence for tiny particles called electrons: they
power things like electricity and magnetism.
An English physicist named J.J. Thomson
discovered electrons in 1897. This
discovery helped scientists to realize that atoms are made of even smaller bits.
Unlike these earlier scientists, we can actually see atoms; just look at the photo of sulfur atoms up above!
Seeing that picture would have delighted Rutherford, Thomson, and the other pioneers of atomic science.
Now, scientists are even starting to see inside atoms. Thanks to the development of really powerful
electron microscopes, we can peer deep inside things at
their internal atomic structure. In 2013, for example, scientists used a quantum
microscope to produce the first picture of the inside of a hydrogen atom. Amazing!
There's plenty more evidence where that came from, but this will do for starters. It shows us that our theory of what atoms
are and how they are built is a very good one: the theory agrees with the things we see around us in the world and it's
confirmed by many different kinds of evidence. It's not a complete theory, however: we still have an enormous amount to learn
about atoms and the bits and pieces lurking inside them!
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.
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.
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.
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
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.
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!
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