Compounds and Bonding
1. What Is a Compound?
Learning outcomes
- I can define a compound as a substance formed when two or more elements chemically combine.
- I can distinguish between elements, compounds, and mixtures.
- I can interpret simple chemical formulas to identify the elements present in a compound.
- I can explain why compounds have properties that differ from the elements that form them.
- I can classify substances as elements, compounds, or mixtures using scientific evidence.
Introduction
Very few pure elements are found by themselves in nature. Instead, most elements combine with other elements to form compounds. Water, table salt, carbon dioxide, and sugar are all examples of compounds that are essential for life and are used every day.
Compounds are created when atoms of different elements become chemically bonded together in fixed proportions. Once bonded, the elements form a completely new substance with properties that are often very different from those of the elements that formed it. Understanding compounds is one of the key foundations of chemistry because nearly all materials around us are compounds.
What Is a Compound?
A compound is a pure substance formed when two or more different elements are chemically combined in fixed proportions.
The atoms in a compound are joined together by chemical bonds.
Examples include:
- Water (H₂O)
- Carbon dioxide (CO₂)
- Sodium chloride (NaCl)
- Ammonia (NH₃)
Each compound has:
- A fixed chemical formula.
- A fixed composition.
- Unique physical and chemical properties.
Figure 1. Compounds are formed when atoms of different elements become chemically bonded together.
Compounds Are Pure Substances
Like elements, compounds are pure substances.
This means they:
- Have a fixed composition.
- Have consistent properties.
- Contain only one type of particle (molecules or formula units).
For example:
Every water molecule contains:
- Two hydrogen atoms.
- One oxygen atom.
This ratio is always the same.
Changing the ratio would produce a different substance.
How Compounds Form
Compounds form during chemical reactions.
During these reactions:
- Atoms rearrange.
- Chemical bonds form.
- New substances are produced.
Example:
Hydrogen gas reacts with oxygen gas.
Hydrogen + Oxygen → Water
Although hydrogen and oxygen are both gases, the compound they form is a liquid at room temperature.
This demonstrates that compounds often have very different properties from the elements that compose them.
Figure 2. Chemical reactions join atoms together to form compounds with new properties.
Elements, Compounds, and Mixtures
It is important to distinguish between these three types of matter.
| Element | Compound | Mixture |
|---|---|---|
| One type of atom | Two or more different elements chemically bonded. | Two or more substances physically mixed |
| Pure substance | Pure substance | Not a pure substance |
| Fixed composition | Fixed composition | Variable composition |
| Cannot be chemically broken down. | Can be chemically broken down into elements | Can be separated by physical methods |
| Example: Gold | Example: Water | Example: Air |
Understanding these differences helps scientists classify matter correctly.
Understanding Chemical Formulas
A chemical formula shows:
- Which elements are present.
- How many atoms of each element are present.
Examples:
| Formula. | Elements Present |
|---|---|
| H₂O | Hydrogen, Oxygen |
| CO₂ | Carbon, Oxygen |
| NaCl | Sodium, Chlorine |
| NH₃ | Nitrogen, Hydrogen |
| CH₄ | Carbon, Hydrogen |
The subscript tells us how many atoms of that element are present.
Example:
H₂O
contains:
- 2 hydrogen atoms.
- 1 oxygen atom.
Figure 3. Chemical formulas show the types and numbers of atoms present in a compound.
Why Compounds Have Different Properties
When elements combine chemically, their atoms are arranged in a completely new way.
As a result, the compound often has properties that are very different from those of its elements.
Example: Water
Hydrogen:
- Colourless gas.
- Highly flammable.
Oxygen:
- Colourless gas.
- Supports combustion.
Water:
- Liquid.
- Extinguishes fires.
- Safe to drink.
The compound has completely different properties from either element.
Example: Sodium Chloride
Sodium:
- Soft, highly reactive metal.
- Reacts violently with water.
Chlorine:
- Poisonous green gas.
Sodium chloride:
- White crystalline solid.
- Common table salt.
- Safe to eat in small amounts.
Again, the compound's properties are completely different from those of its elements.
Figure 4. Compounds often have properties that are very different from the elements that form them.
Classifying Substances
Scientists use evidence to determine whether a substance is an element, compound, or mixture.
Elements
Examples:
- Gold
- Oxygen
- Copper
- Helium
Contain only one type of atom.
Compounds
Examples:
- Water
- Carbon dioxide
- Glucose
- Sodium chloride
Contain different elements chemically bonded together.
Mixtures
Examples:
- Air
- Seawater
- Soil
- Salad
- Brass
Contain substances that are physically mixed rather than chemically bonded.
Scientists classify matter by examining its composition and properties.
Why Compounds Are Important
Compounds make up most of the materials around us.
Examples include:
- Water
- Rocks
- Plastics
- Medicines
- Foods
- Fuels
- Living organisms
Nearly every biological process depends on compounds.
Without compounds, life on Earth would not exist.
Figure 5. Many everyday materials are compounds formed from combinations of different elements.
Worked Example
Question
Classify each substance.
| Substance | Classification |
|---|---|
| Gold (Au) | Element |
| Water (H₂O) | Compound |
| Air | Mixture |
| Carbon dioxide (CO₂). | Compound |
| Iron (Fe) | Element |
| Salt water | Mixture |
Real-World Connection
Many of the materials we rely on every day are compounds. Medicines are carefully designed compounds that treat disease, fertilisers are compounds that supply nutrients to crops, and concrete is made using compounds that give buildings their strength. Chemists continually develop new compounds to improve technology, medicine, agriculture, and environmental protection.
Did You Know?
The human body is composed almost entirely of compounds, not pure elements. Water makes up about 60% of an adult's body mass, while proteins, carbohydrates, lipids, DNA, and minerals are all compounds made from combinations of different elements. Only tiny amounts of a few elements, such as oxygen gas in the lungs or iron in the blood, exist in relatively pure forms.
Key Terms
Chemical bond – The force that holds atoms together in a compound.
Chemical formula – A combination of chemical symbols showing the types and numbers of atoms in a compound.
Chemical reaction – A process in which substances are transformed into new substances through the breaking and forming of chemical bonds.
Compound – A pure substance made of two or more different elements chemically bonded together in fixed proportions.
Element – A pure substance consisting of only one type of atom.
Mixture – Two or more substances physically combined but not chemically bonded.
Molecule – A group of atoms held together by chemical bonds.
Pure substance – A material with a fixed composition and consistent properties.
Subscript – A small number written below the line in a chemical formula indicating the number of atoms of an element.
Key Takeaways
- A compound is a pure substance formed when two or more different elements chemically combine.
- Compounds have a fixed composition and are held together by chemical bonds.
- Chemical formulas show which elements are present and how many atoms of each are in a compound.
- Compounds often have properties that are very different from the elements that form them.
- Elements, compounds, and mixtures can be distinguished by examining how their particles are arranged and whether substances are chemically bonded.
- Most of the materials found in living organisms and in everyday life are compounds.