by Chris Woodford. Last updated: March 10, 2015.

S uppose 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.

Photo: Methane molecules flying down a tiny carbon nanotube. Courtesy of US Department of Energy.

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.

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 about 100,000 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.

Photo (above): What does an atom look like? You can see one if you have the right kind of microscope. This photo shows sulfur atoms arranged on a layer of copper deposited onto a crystal of ruthenium. By courtesy of US Department of Energy/Brookhaven National Laboratory.

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. as chemistry teacher Jim Clarke points out very clearly. 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. The number of neutrons is very roughly the same as the number of protons, but sometimes it's rather more. 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.

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.

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 do we know atoms exist?

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:

  1. 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 (H 2 O), 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.
  2. 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).
  3. 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).
  4. 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.
  5. 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 theme!

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.

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.
  • 1803: English scientist John Dalton (1766–1844) published the atomic theory of matter. He realized each chemical element was made up of atoms.
  • 1869: 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.
  • 1911: 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.


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