Elements, Compounds & Mixtures
Learn to classify matter as elements, compounds, or mixtures by examining what atoms are bonded together and whether substances combine to form new properties.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Elements, Compounds & Mixtures, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Everything around you is made of matter, and all matter is built from atoms. But atoms don't always stay by themselves — sometimes they join together in specific ways, and sometimes they just sit next to each other. Understanding whether something is an element, a compound, or a mixture tells you a lot about what it is and how it behaves. In this lesson, you'll learn to look at the particle structure of matter and sort it into these three categories.
What Is an Element?
An element is a pure substance made of only one kind of atom. All the atoms in an element are identical — they have the same number of protons, which means they are the same element. When you look at a particle picture of an element, every single particle you see is the same. For example, pure oxygen gas () is made of oxygen atoms bonded in pairs, but each atom is oxygen. Pure iron metal is made entirely of iron atoms. The atom is the smallest unit of an element — if you break an atom apart into its protons, neutrons, and electrons, you no longer have that element anymore. Elements are the building blocks of all matter. You can find all known elements listed on the periodic table, organized by their properties and atomic structure.
What Is a Compound?
A compound is a pure substance made of two or more different elements joined together in a fixed ratio through chemical bonds. The atoms are actually bonded to each other, not just sitting next to one another. Here's the crucial point: a compound has completely different properties from the elements that make it up. Table salt (sodium chloride, NaCl) is a perfect example. Sodium is a soft, silvery metal that reacts violently with water. Chlorine is a poisonous yellow-green gas. But when sodium and chlorine atoms bond together in a 1:1 ratio, they form table salt — a white crystalline solid that is safe to eat. The compound has new properties that neither element alone possesses. In a particle picture of a compound, you will always see atoms of at least two different kinds bonded together in a definite pattern that repeats.
What Is a Mixture?
A mixture is a combination of two or more different substances that are physically combined but not chemically bonded. The key word is 'physically' — the substances are just mixed together side by side. Each substance in a mixture keeps its own properties because the particles are not bonded. A salad is a mixture: lettuce, tomatoes, and dressing are all in the same bowl, but they are still lettuce, tomatoes, and dressing. Salt water is a mixture of salt (a compound) and water (another compound) — the salt dissolves but the particles do not form new bonds with the water. If you looked at salt water under a very high-powered microscope showing individual particles, you would see salt particles and water particles near each other but not bonded together. Mixtures can be separated into their parts using physical methods like filtering, evaporating, or dissolving — no chemical reaction needed. In a particle picture of a mixture, you see different kinds of particles side by side, not bonded to form a single repeating structure.
How to Identify Each Type from a Particle Picture
When you're shown a particle picture and asked to classify it, ask yourself three questions in order. First: Are all the particles the same kind of atom? If yes, it is an element. Second: Are there two or more different kinds of atoms bonded together in a fixed repeating pattern? If yes, it is a compound. Third: Are there different substances present side by side, not bonded together? If yes, it is a mixture. Remember that elements and compounds are both pure substances — they have one definite composition and set properties. Mixtures are not pure; they can have variable composition (you can make salt water more or less salty). Also, don't confuse the number of atoms in a particle with the number of different kinds. Oxygen gas () has two atoms per molecule, but both are oxygen, so it is an element, not a compound.
Key terms
- Element.
- A pure substance made of only one kind of atom; the smallest unit is the atom itself.
- Compound.
- A pure substance made of two or more different elements chemically bonded in a fixed ratio, with properties different from its elements.
- Mixture.
- A combination of two or more substances physically mixed together but not chemically bonded; each substance keeps its own properties.
- Atom.
- The smallest unit of an element; made of protons, neutrons, and electrons.
- Chemical bond.
- A force that holds atoms together in a compound by sharing or transferring electrons.
- Pure substance.
- Matter with a definite, fixed composition and consistent properties (elements and compounds are pure substances).
- Physical combination.
- Mixing substances without forming new chemical bonds, as in a mixture.
Worked example
A scientist observes three particle pictures under a microscope. In Picture A, all particles are single atoms of the same element. In Picture B, you see atoms of two different elements bonded together in a repeating 1:2 pattern. In Picture C, you see water molecules (H₂O) and salt particles (NaCl) clustered together but not bonded to each other. Classify each picture as an element, compound, or mixture, and explain your reasoning.
Picture A: This is an element. All the particles are atoms of the same kind. Since only one type of atom is present and no atoms are bonded together in groups, this is a pure substance made of a single element. Picture B: This is a compound. You observe two different elements bonded together in a fixed pattern (1:2 ratio). The atoms are chemically bonded, not just placed side by side. Even though you can see two different kinds of atoms, they form a new substance with new properties — this is the definition of a compound. Picture C: This is a mixture. The water molecules and salt particles are different substances present in the same space, but they are not bonded to each other. Water molecules have their own properties, salt particles have their own properties, and they remain separate. You could separate them using physical methods (like evaporation). The key is that no new chemical bonds form between the two substances.
Practice questions
A student is given a particle picture showing six atoms, all bonded together in pairs. Three of the pairs are made of two identical atoms, and three pairs are made of two different atoms. How many elements, compounds, and mixtures are shown in this picture?
Answer: Three elements and three compounds (or six pure substances total, no mixtures)
Each pair of identical atoms is an element on its own (three elements). Each pair of different atoms bonded together is a compound (three compounds). Nothing is mixed together without bonding, so there are no mixtures. Remember that in this picture, the six pairs are separate from each other — they are not bonded to form one structure, so each pair is its own pure substance.
Table salt (NaCl) is made when sodium atoms bond with chlorine atoms. Sodium is a reactive metal, and chlorine is a poisonous gas. Yet table salt is safe to eat. Explain why table salt is a compound and not a mixture, and explain why its properties are so different from those of sodium and chlorine.
- It is a mixture because sodium and chlorine are both present.
- It is a compound because the atoms are bonded together in a fixed ratio, and a compound has new properties unlike its elements.
- It is an element because it appears white and solid.
- It is a mixture because you can taste salt in food.
Answer: It is a compound because the atoms are bonded together in a fixed ratio, and a compound has new properties unlike its elements.
When sodium and chlorine atoms form chemical bonds in a 1:1 ratio, they create a new substance with completely new properties. This is what makes it a compound, not a mixture. If it were a mixture, the sodium and chlorine would still be separate and would keep their original properties — which they don't. The compound NaCl is safe and edible because its bonded structure is entirely different from the poisonous chlorine gas or reactive sodium metal that created it.
A student mixes sand and water in a beaker. The sand sinks to the bottom and the water stays on top. Is this a mixture or a compound? Explain how you know, and describe one way the student could separate the two substances.
Answer: This is a mixture because the sand and water are physically combined but not chemically bonded. Sand keeps its properties (solid particles) and water keeps its properties (liquid). The student could separate them by filtering — pouring the mixture through a fine mesh that traps the sand but lets the water pass through.
Sand and water are two different substances that do not form new bonds when mixed. Each substance remains unchanged with its original properties. Because no chemical bonding occurs, this is a mixture. Mixtures can be separated using physical methods (filtering, evaporating, settling), which is impossible with compounds. If the substances were chemically bonded, you would need a chemical reaction to separate them.
FAQ
- How do I know if atoms are bonded or just sitting next to each other?
- In a particle picture, bonded atoms appear connected — drawn with lines or held closely in a fixed, repeating pattern. If you see different particles just near each other without a connecting pattern, they are mixed, not bonded. The best clue is whether the arrangement repeats with the same structure. Bonded atoms in a compound always follow the same pattern throughout.
- Can a compound look like it might be a mixture?
- Yes, which is why you have to think carefully. For example, a sugar solution looks like one liquid, but it is a mixture of water and dissolved sugar. However, table salt (NaCl) looks like a solid crystal, but it is a compound because the sodium and chlorine atoms are bonded in a 1:1 ratio throughout. Particle pictures make it much easier because you can actually see whether atoms are bonded.
- If I have three kinds of atoms all bonded together, is that still one compound?
- If all three kinds of atoms are bonded together in a fixed ratio throughout the structure, yes — it is still one compound. Ammonia (NH₃) contains nitrogen and hydrogen bonded in a 1:3 ratio, and it is a single compound. Mixtures have multiple substances side by side, but compounds have all their atoms bonded in one structure.
- Why do we need to know the difference between elements, compounds, and mixtures?
- Understanding these differences helps you predict how matter will behave. Elements and compounds have fixed, unchanging properties. Mixtures can vary — more salt in salt water makes it saltier. Also, compounds form through chemical reactions and require chemical reactions to break apart, while mixtures separate easily through physical means. Scientists use this classification to understand reactions, design materials, and separate substances.
Learn this with a teacher, not a page
The Crimsora tutor teaches Elements, Compounds & Mixtures live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.