Atoms and Elements

Introduction

Oxygen in the air, carbon in a pencil and gold in a ring are all elements. What makes each one a separate element, different from all the others? The answer lies in their atoms. Every element is built from its own kind of atom and no other. Linking the everyday idea of an element with the particle idea of an atom lets you classify any substance as an element, a compound or a mixture.

Core explanation

One element, one kind of atom. An element is a pure substance that contains only one kind of atom. A sample of pure copper contains copper atoms only; a sample of pure sulfur contains sulfur atoms only. Because it contains just one kind of atom, an element cannot be broken down into simpler substances by chemical reactions.

What makes atoms "the same kind"? Atoms contain a central nucleus with positively charged protons. The number of protons is called the atomic number , and it decides which element an atom belongs to:

- every atom with 1 proton is hydrogen; - every atom with 6 protons is carbon; - every atom with 8 protons is oxygen; - every atom with 79 protons is gold.

Change the number of protons and you change the element. This is why atomic number, not appearance or mass, is the true identity card of an atom.

Same element, slightly different masses. Atoms of one element can differ slightly in mass because they can have different numbers of neutrons. These varieties are called isotopes . Most carbon atoms have 6 neutrons, but about 1% have 7. They are all still carbon atoms, because they all have 6 protons, and they react in the same way.

Elements may contain molecules. "One kind of atom" does not mean the atoms are always separate. In oxygen gas, pairs of oxygen atoms join to form O₂ molecules. In diamond, carbon atoms join into a giant network. Both are still elements, because all the atoms are of the same kind.

Elements, compounds and mixtures.

Substance type Particle picture --- --- Element Only one kind of atom (alone or joined together) Compound Two or more kinds of atom chemically joined in a fixed ratio Mixture Two or more substances mixed but not chemically joined

Water (H₂O) is a compound because each particle contains hydrogen and oxygen atoms joined together. Air is a mixture of elements such as nitrogen and oxygen and compounds such as carbon dioxide.

How many elements? There are 118 known elements. About 90 occur naturally on Earth; the rest have been made in laboratories, often only a few atoms at a time.

Step-by-step reasoning

To decide whether a particle diagram shows an element:

1. Look at all the atoms in the diagram. 2. Check whether they are all the same kind (same colour or symbol). 3. If yes, it is an element, even if atoms are joined in pairs or groups. 4. If different kinds of atom are joined, it is a compound; if different particles are just mixed, it is a mixture.

Visual explanation

Draw three boxes. In the first, all circles are blue, some in pairs: an element. In the second, every particle is a red circle joined to two white circles: a compound. In the third, blue pairs and red-white particles are scattered together: a mixture. Colour tells you the kind of atom.

Real-world analogy

Think of a box of LEGO bricks all of one colour and size. However you build with them — towers, walls or separate bricks — you are using only one kind of brick. That is like an element. Mix bricks of several colours locked together in a set pattern, and you have something like a compound.

Real-world example

Pure gold used in electronic connectors is an element: every atom has 79 protons. Jewellers' 18-carat gold, by contrast, is a mixture (an alloy) of gold with metals such as silver and copper, added to make it harder.

Why?

Why can't an element be broken down chemically? Chemical reactions only rearrange atoms and share or transfer electrons; they never change the number of protons in a nucleus. Since an element contains only one kind of atom, there is nothing simpler for a chemical reaction to separate it into.

Common misconception

"If atoms are joined together, the substance must be a compound." Atoms of the same element can join together, as in O₂ or diamond. A compound needs atoms of two or more different elements chemically joined.

Worked example

Question: A diagram shows particles, each made of two identical green atoms joined together, and nothing else. Is the substance an element, a compound or a mixture?

Reasoning: Every atom in the diagram is the same kind (green). Atoms being joined in pairs does not make it a compound, because no different elements are joined. There is only one kind of particle, so it is not a mixture.

Answer: An element, made of diatomic molecules (like Cl₂).

Quick check

1. What decides which element an atom belongs to? Answer: Its atomic number, the number of protons in its nucleus.

Exam focus

Particle-diagram questions are very common. Remember: one kind of atom means an element; different atoms chemically joined means a compound; different particles not joined means a mixture. Justify your answer by referring to the kinds of atom shown.

Advanced insight

In nuclear reactions, such as radioactive decay or reactions inside stars, the number of protons in a nucleus can change, so one element really can turn into another. This is a nuclear change, not a chemical one, and it involves energies millions of times larger than those in chemical reactions.

Summary

Each element is made of only one kind of atom, identified by its atomic number (number of protons). Atoms of the same element may have different masses (isotopes) and may be joined into molecules or networks. Compounds contain different kinds of atom chemically joined in a fixed ratio; mixtures contain different substances not chemically joined.

Practice questions

1. Define an element in terms of atoms. Answer: A substance made of only one kind of atom, all with the same atomic number. 2. An atom has 8 protons. Which element is it? Answer: Oxygen. 3. Explain why O₂ is an element but CO₂ is a compound. Answer: O₂ contains only oxygen atoms, whereas CO₂ contains carbon and oxygen atoms chemically joined together. 4. Two atoms both have 6 protons but different masses. Are they the same element? Explain. Answer: Yes. They are both carbon because they have the same atomic number; they are isotopes with different numbers of neutrons.