What Is an Atom?

Introduction

Take a piece of pure copper wire and cut it in half. Both halves are still copper. Cut one half again, and again, and again. Could you go on for ever? Chemists answer no. After an enormous number of cuts you would reach a single particle that is still copper, but which cannot be split further without it ceasing to be copper. That particle is an atom . Everything you can touch, breathe or eat is built from atoms, so understanding what an atom is marks the real start of chemistry.

Core explanation

Definition. An atom is the smallest particle of an element that still has the chemical identity of that element. A single copper atom is still copper: it would react in the ways copper reacts. If you could somehow break that atom apart, the pieces would no longer be copper at all.

Atoms and elements. An element is a pure substance made of only one kind of atom. Copper contains only copper atoms; oxygen gas contains only oxygen atoms; carbon in the form of diamond contains only carbon atoms. About 118 elements are known, so there are about 118 different kinds of atom. Every material in the universe is made from these kinds of atom, joined together in different combinations.

Atoms are extremely small. A typical atom is roughly one ten-millionth of a millimetre across. You cannot see an individual atom with your eyes or with an ordinary light microscope. Even a speck of dust contains many billions of billions of atoms.

One atom is not the same as a lump. Some properties we observe belong to large collections of atoms, not to one atom alone. A single copper atom is not "orange-brown" and does not "conduct electricity" the way a wire does; those properties appear when huge numbers of copper atoms are packed together. What a single atom does keep is its chemical identity — the kind of reactions it can take part in and the kinds of particle it can bond with.

Atoms have an inner structure. Although the atom is the smallest piece of an element , it is not the smallest thing that exists. Atoms contain even smaller particles (protons, neutrons and electrons), which you will study later. The important point for now is that splitting an atom destroys its identity as that element, which is why the atom is the basic unit of chemistry.

Atoms join together. Atoms rarely stay alone. They usually bond to other atoms to form molecules or large regular arrangements called lattices. Water, for example, is made of molecules in which two hydrogen atoms are joined to one oxygen atom.

Step-by-step reasoning

To decide whether a particle counts as an atom of an element:

1. Ask whether the substance is an element (only one kind of atom). 2. Imagine dividing a sample of it into smaller and smaller pieces. 3. The last piece that still behaves chemically like that element is one atom. 4. Anything smaller would be a sub-atomic particle and would no longer be that element.

Visual explanation

Picture a long wall built from identical bricks. You can remove half the wall, then half again, and each piece is still "brick wall". Eventually you hold one brick. Break that brick and you have only dust and fragments, not a brick. In this picture the wall is a sample of an element and the single brick is one atom. The simulation lets you zoom in on a sample until individual atoms appear as separate spheres.

Real-world analogy

Think of the letters of the alphabet. A small set of letters can be combined to build every word in a dictionary. In the same way, a small set of about 118 kinds of atom combines to build millions of different substances. A single letter is the smallest unit that still means something in writing, just as an atom is the smallest unit that still means something chemically.

Real-world example

Gold leaf used to decorate picture frames and buildings can be beaten until it is only about 0.1 micrometres thick. That sounds astonishingly thin, yet it is still several hundred gold atoms thick. However thin the leaf, it keeps the colour and resistance to tarnish of gold, because it is still made of gold atoms.

Why?

Why does cutting a sample of an element eventually have to stop? Because matter is not continuous like an endlessly divisible jelly; it is made of separate particles. Once you reach one atom, there is nothing smaller that is still the same element. The identity of the element belongs to the atom, not to any smaller part of it.

Common misconception

"An atom is the smallest particle that exists." Atoms are made of smaller particles — protons, neutrons and electrons. The correct statement is that an atom is the smallest particle of an element that keeps that element's chemical identity.

Worked example

Question: A student says: "A single atom of iron is grey and magnetic, just like an iron nail." Is the student right?

Reasoning: Colour and magnetism of a nail are properties of a very large number of iron atoms packed together in a solid. A single atom does not have a colour in the everyday sense and is not a magnet like a nail. What the atom keeps is its chemical identity as iron.

Answer: No. A single iron atom is still iron chemically, but bulk properties such as colour and magnetism belong to large collections of atoms.

Quick check

1. What is an atom? Answer: The smallest particle of an element that still has the chemical identity of that element.

Exam focus

Learn the definition word for word: "the smallest particle of an element that can take part in a chemical reaction" or "that keeps the identity of the element" are both accepted. Do not write "the smallest particle of matter", which is wrong.

Advanced insight

Scientists can now image and even move single atoms using scanning tunnelling microscopes. Experiments like this show that some properties change when very few atoms are involved: a cluster of just a few dozen gold atoms can behave very differently from a gold bar, which is the basis of much of nanoscience.

Summary

An atom is the smallest particle of an element that keeps the chemical identity of that element. Each element is made of one kind of atom, and about 118 kinds are known. Atoms are far too small to see directly, contain smaller sub-atomic particles, and usually join to other atoms. Bulk properties such as colour belong to large collections of atoms, not to one atom.

Practice questions

1. Define an element in terms of atoms. Answer: An element is a pure substance made of only one kind of atom. 2. Why is it wrong to say an atom is the smallest particle of matter? Answer: Atoms contain smaller particles (protons, neutrons and electrons), so they are not the smallest particles of matter; they are the smallest particles of an element. 3. A sample of pure sulfur is ground into a very fine powder. Is each grain of the powder an atom? Explain. Answer: No. Each grain still contains an enormous number of sulfur atoms; grinding only makes smaller collections of atoms. 4. Explain why roughly 118 kinds of atom can build millions of different substances. Answer: Atoms can join together in many different combinations and arrangements, just as a few letters can build a huge number of words.