Molecules, Formulas & Extended Structures
Learn to read chemical formulas, match them to particle models, and distinguish between molecules and extended structures like salts and metals.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Molecules, Formulas & Extended Structures, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
When you see H2O written in chemistry, those numbers tell a real story about what's inside water. Every chemical formula is a code that describes how many atoms of each element are bonded together. Some substances, like water and carbon dioxide, are made of separate molecules floating around. Others, like table salt and diamond, form extended networks with no individual molecules at all. Understanding how to decode a formula and recognize what kind of structure a substance has is the foundation for understanding how chemicals behave.
How to Read a Chemical Formula
A chemical formula uses element symbols and subscript numbers to show exactly what atoms make up a substance. The element symbol comes first — H for hydrogen, O for oxygen, C for carbon — and the subscript number right after tells you how many atoms of that element are present. If there is no number written, it means there is one atom of that element.
For example, H2O means two hydrogen atoms bonded to one oxygen atom. CO2 means one carbon atom bonded to two oxygen atoms. O2 means two oxygen atoms bonded to each other. The formula is always read left to right, and every number matters.
Subscripts are never multiplied across the formula. If you see Ca(OH)2, the parentheses show that the OH group appears twice, so there is one calcium atom, two oxygen atoms, and two hydrogen atoms total. This is different from CaOH2, which is not how it is written. Learning to read formulas carefully helps you picture exactly what particles make up the substance.
For example, H2O means two hydrogen atoms bonded to one oxygen atom. CO2 means one carbon atom bonded to two oxygen atoms. O2 means two oxygen atoms bonded to each other. The formula is always read left to right, and every number matters.
Subscripts are never multiplied across the formula. If you see Ca(OH)2, the parentheses show that the OH group appears twice, so there is one calcium atom, two oxygen atoms, and two hydrogen atoms total. This is different from CaOH2, which is not how it is written. Learning to read formulas carefully helps you picture exactly what particles make up the substance.
Matching Formulas to Particle Models
Once you can read a formula, you can predict what a particle model should show. If the formula is H2O, the model must display two hydrogen atoms connected to one central oxygen atom — always the same arrangement. That one group of bonded atoms is one molecule of water.
When you match a formula to a model, count the atoms of each type in the diagram and verify they match the subscripts in the formula. If the model shows six atoms arranged in pairs (three pairs), and the formula is H2O, something is wrong — you would actually be looking at three separate water molecules.
Particle models can show a single molecule or many molecules together. The key is recognizing that each complete unit — each set of atoms bonded as the formula describes — is one molecule. If the model shows ten separate H2O units, the formula is still just H2O, but the picture represents ten molecules of it.
When you match a formula to a model, count the atoms of each type in the diagram and verify they match the subscripts in the formula. If the model shows six atoms arranged in pairs (three pairs), and the formula is H2O, something is wrong — you would actually be looking at three separate water molecules.
Particle models can show a single molecule or many molecules together. The key is recognizing that each complete unit — each set of atoms bonded as the formula describes — is one molecule. If the model shows ten separate H2O units, the formula is still just H2O, but the picture represents ten molecules of it.
Molecular Substances: Separate Particles
Water, carbon dioxide, and oxygen gas are all made of molecules — distinct, separate particles where specific atoms are bonded together in a fixed ratio. Each molecule is complete on its own. Water molecules do not permanently stick to each other; they slide past each other, which is why water can pour and flow.
In a molecular substance, the smallest piece that still acts like the substance is one molecule. One H2O molecule is the tiniest bit of water that is still water. When billions of H2O molecules cluster together, you get liquid water or ice, but they remain separate units.
Many common substances are molecular: table sugar (C12H22O11), ethanol (C2H5OH), and ammonia (NH3). Molecular substances often have low melting and boiling points because the molecules are not locked in place — they move around easily. Understanding that these substances consist of discrete molecules helps explain why they evaporate, dissolve in water, or melt when heated.
In a molecular substance, the smallest piece that still acts like the substance is one molecule. One H2O molecule is the tiniest bit of water that is still water. When billions of H2O molecules cluster together, you get liquid water or ice, but they remain separate units.
Many common substances are molecular: table sugar (C12H22O11), ethanol (C2H5OH), and ammonia (NH3). Molecular substances often have low melting and boiling points because the molecules are not locked in place — they move around easily. Understanding that these substances consist of discrete molecules helps explain why they evaporate, dissolve in water, or melt when heated.
Extended Structures: No Separate Molecules
Not all substances are made of molecules. Table salt (NaCl) is different. Sodium and chlorine atoms bond in a repeating pattern that extends in all directions — there is no point where you can draw a circle around a single "NaCl unit" and say "this is one molecule."
Instead, table salt forms an extended structure: a three-dimensional grid of sodium and chlorine ions held together by electrical attraction. Every sodium ion is surrounded by chlorine ions, and every chlorine ion is surrounded by sodium ions. The pattern repeats over and over. Even a tiny grain of salt contains millions of ions all locked into this structure.
Diamond and metal atoms also form extended structures. Diamond is a network of carbon atoms, each bonded to four other carbons in an endless lattice. Metals like copper or iron consist of metal atoms packed together in a repeating arrangement. Because these structures have no separate molecules, they tend to be hard, have high melting points, and conduct electricity differently than molecular substances do.
Instead, table salt forms an extended structure: a three-dimensional grid of sodium and chlorine ions held together by electrical attraction. Every sodium ion is surrounded by chlorine ions, and every chlorine ion is surrounded by sodium ions. The pattern repeats over and over. Even a tiny grain of salt contains millions of ions all locked into this structure.
Diamond and metal atoms also form extended structures. Diamond is a network of carbon atoms, each bonded to four other carbons in an endless lattice. Metals like copper or iron consist of metal atoms packed together in a repeating arrangement. Because these structures have no separate molecules, they tend to be hard, have high melting points, and conduct electricity differently than molecular substances do.
Why the Difference Matters
Recognizing whether a substance is made of molecules or an extended structure explains real behavior. Water boils at 100 degrees Celsius because individual H2O molecules have enough energy to break free and become a gas. Salt does not have a boiling point in the same way — the entire ionic structure must break apart, which requires much more energy.
Diamond is one of the hardest substances on Earth because breaking it apart means breaking countless carbon-to-carbon bonds throughout the entire structure, not just separating discrete molecules. This difference between separate particles and continuous networks shapes how materials behave in cooking, manufacturing, and industry.
When you read a chemical formula, you now know to ask two questions: How many atoms of each type are bonded together? And is this substance made of separate molecules, or is it part of an extended network? Those two answers tell you almost everything about what the substance can do.
Diamond is one of the hardest substances on Earth because breaking it apart means breaking countless carbon-to-carbon bonds throughout the entire structure, not just separating discrete molecules. This difference between separate particles and continuous networks shapes how materials behave in cooking, manufacturing, and industry.
When you read a chemical formula, you now know to ask two questions: How many atoms of each type are bonded together? And is this substance made of separate molecules, or is it part of an extended network? Those two answers tell you almost everything about what the substance can do.
Key terms
- Chemical formula.
- A combination of element symbols and subscript numbers showing the types and quantities of atoms in a substance.
- Subscript.
- A small number written below the line, placed after an element symbol to show how many atoms of that element are present.
- Molecule.
- A distinct group of atoms bonded together in a fixed arrangement, with a definite composition that can exist separately.
- Molecular substance.
- A substance composed of separate molecules that can move independently, such as water or carbon dioxide.
- Extended structure.
- A continuous three-dimensional network of atoms or ions bonded together with no separate molecular units, such as salt or diamond.
- Ionic bond.
- An attraction between oppositely charged ions, found in extended structures like table salt.
- Element symbol.
- The one or two letter abbreviation for an element, such as H for hydrogen or O for oxygen.
Worked example
A student looks at a particle model showing three separate groups of atoms. The first group has two hydrogen atoms bonded to one oxygen atom. The second group is identical. The third group also shows two hydrogen atoms bonded to one oxygen atom. Write the chemical formula for one of these groups, and explain how many molecules are shown in the model.
Step 1: Identify the atoms in one group. The description says each group has two hydrogen atoms and one oxygen atom.
Step 2: Write the formula. Hydrogen comes first (it often does), so write H, then the subscript 2. Oxygen comes next, so write O with no subscript (which means one oxygen atom). The formula is H2O.
Step 3: Count the molecules. The model shows three separate groups, and each group is an independent H2O particle. Three separate groups means three molecules.
Answer: The chemical formula is H2O. The model shows three molecules of water (or three H2O molecules). Each molecule is a distinct unit made of two hydrogen atoms and one oxygen atom bonded together.
Step 2: Write the formula. Hydrogen comes first (it often does), so write H, then the subscript 2. Oxygen comes next, so write O with no subscript (which means one oxygen atom). The formula is H2O.
Step 3: Count the molecules. The model shows three separate groups, and each group is an independent H2O particle. Three separate groups means three molecules.
Answer: The chemical formula is H2O. The model shows three molecules of water (or three H2O molecules). Each molecule is a distinct unit made of two hydrogen atoms and one oxygen atom bonded together.
Practice questions
Which chemical formula correctly represents a molecule containing one carbon atom and two oxygen atoms?
- C2O
- CO2
- C2O2
- 2CO
Answer: CO2
CO2 means one carbon atom (no subscript after C means one) and two oxygen atoms (subscript 2 after O). The formula C2O would be two carbon and one oxygen. C2O2 would be two of each. 2CO is written with a coefficient in front, which is a different notation used when balancing chemical equations, not for describing a single molecule.
A particle model shows a large grid of sodium ions and chloride ions locked in a three-dimensional repeating pattern, with no way to identify a separate "one unit" of the substance. Explain whether this substance is made of molecules, and describe the type of structure it forms.
Answer: This substance is not made of molecules. It forms an extended structure (also called an ionic lattice or extended network). The sodium and chloride ions are bonded in a continuous pattern with no separate molecular units.
Table salt (NaCl) is the classic example of a substance with an extended structure. Because the ions are arranged in a repeating three-dimensional grid with no breaking point, there is no such thing as one separate NaCl molecule. Every ion is bonded to multiple ions of the opposite charge. This is different from H2O, where you can identify one molecule as a complete, separate unit. The extended structure property explains why salt has a very high melting point — the entire ionic network must be broken apart, not just individual molecules separated.
A student writes four chemical formulas: O2, N2, H2O, and CO2. Which two of these represent molecular substances, and which two would also be found in extended structures in nature?
Answer: O2 and CO2 (and N2) represent molecular substances because they form discrete molecules. H2O and CO2 are also found in extended structures — water freezes into ice crystals (extended hydrogen bonding networks), and carbon dioxide forms solid dry ice.
All four formulas represent molecular substances in their common form — they have separate molecules. However, the question asks which are also found in extended structures. When water freezes, the H2O molecules form a crystal lattice held together by hydrogen bonds in a repeating pattern. When carbon dioxide is cooled enough, it becomes dry ice, which is also a crystalline extended structure. This shows that the formula alone does not always tell you the state of a substance — temperature and pressure matter too. Oxygen gas (O2) remains molecular even when liquefied or solidified under normal conditions.
FAQ
- What does the little number after an element symbol mean?
- The subscript number tells you how many atoms of that element are in one particle (molecule or formula unit) of the substance. If there is no number, it means there is exactly one atom of that element. In H2O, the 2 means two hydrogen atoms per water molecule.
- Is salt made of molecules?
- No. Table salt (NaCl) is made of sodium ions and chloride ions locked together in an extended three-dimensional grid. There is no such thing as one separate "NaCl molecule" because the ions are not bonded in a way that creates distinct units. The whole crystal is one extended structure.
- How do I know if a substance is molecular or an extended structure?
- Substances that are gases or liquids at room temperature, or that have low melting points, are usually molecular. Water, carbon dioxide, and sugar are molecular. Substances that are solids with very high melting points, like salt, diamond, and metals, are usually extended structures. However, the safest way is to look at the particle model or description — if you can circle a specific group of atoms and call it one particle, it is likely molecular.
- Can a substance have both a molecular formula and also form extended structures?
- Yes. Water's molecular formula is H2O, which describes one molecule. But when water freezes into ice, the H2O molecules form a crystalline extended structure held together by hydrogen bonds. The formula stays the same, but the way the particles are arranged changes based on temperature and pressure.
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