Elements and the Periodic Table
1. What Is an Element?
Learning outcomes
- I can define an element as a substance made of only one type of atom.
- I can explain why elements cannot be broken down into simpler substances by chemical means.
- I can distinguish between elements, compounds, and mixtures.
- I can identify elements from their names, symbols, and atomic structures.
- I can classify common substances as elements or non-elements using scientific evidence.
Introduction
Everything around us is made of matter, from the air we breathe to the water we drink and the devices we use every day. Scientists have discovered that all matter is built from a relatively small number of basic substances called elements. These elements are the building blocks from which all compounds and mixtures are formed.
Understanding elements is one of the foundations of chemistry. By learning what an element is and how it differs from compounds and mixtures, we can better understand how substances are formed, how chemical reactions occur, and how matter is organised.
What Is an Element?
An element is a pure substance made of only one type of atom.
Every atom in an element has the same number of protons.
Examples include:
- Hydrogen
- Oxygen
- Carbon
- Iron
- Gold
- Copper
Although atoms of the same element may have different numbers of neutrons (forming isotopes), they are still the same element because they all contain the same number of protons.
Figure 1. An element is made entirely of one type of atom.
Elements Are Pure Substances
A pure substance contains only one type of particle.
For an element:
- Every atom is the same type.
- The composition is always the same.
- The properties are consistent.
For example:
A piece of pure copper contains only copper atoms.
A sample of pure helium contains only helium atoms.
Pure substances have fixed properties such as:
- Melting point
- Boiling point
- Density
Why Can't Elements Be Broken Down Chemically?
Elements are the simplest chemical substances.
They cannot be broken down into simpler substances by chemical reactions.
For example:
Water can be broken down into:
- Hydrogen
- Oxygen
However:
Hydrogen cannot be chemically broken down into anything simpler because it is already an element.
Only nuclear reactions can change one element into another by altering the number of protons in the nucleus.
Ordinary chemical reactions do not change one element into another.
Figure 2. Compounds can be broken down chemically into elements, but elements cannot be broken down further by chemical means.
Elements, Compounds, and Mixtures
Although these terms are related, they describe different kinds of matter.
| Element | Compound | Mixture |
|---|---|---|
| One type of atom | Two or more elements chemically bonded. | Two or more substances physically mixed |
| Pure substance | Pure substance | Not a pure substance |
| Cannot be chemically broken down. | Can be chemically broken down | Can be separated by physical methods |
| Example: Gold | Example: Water | Example: Air |
Understanding these differences is essential in chemistry.
Figure 3. Elements, compounds, and mixtures differ in the types of particles they contain.
Element Names and Symbols
Each element has:
- A unique name.
- A unique chemical symbol.
Symbols contain one or two letters.
The first letter is always capitalised.
Examples:
| Element | Symbol |
|---|---|
| Hydrogen | H |
| Oxygen | O |
| Carbon | C |
| Nitrogen | N |
| Iron | Fe |
| Copper | Cu |
| Gold | Au |
| Silver | Ag |
Some symbols come from Latin names.
For example:
- Fe comes from ferrum (iron).
- Au comes from aurum (gold).
- Ag comes from argentum (silver).
Atomic Structure and Elements
Every element is identified by its atomic number.
The atomic number equals the number of protons in the nucleus.
For example:
| Element | Protons (Atomic Number) |
|---|---|
| Hydrogen. | 1 |
| Carbon | 6 |
| Oxygen | 8 |
| Sodium | 11 |
| Iron | 26 |
No two different elements have the same number of protons.
This is what makes each element unique.
Figure 4. Elements are identified by the number of protons in their atoms.
Classifying Common Substances
Scientists use evidence to determine whether a substance is an element, a compound, or a mixture.
Elements
Examples:
- Gold (Au)
- Copper (Cu)
- Oxygen (O₂)
- Nitrogen (N₂)
- Helium (He)
Although oxygen and nitrogen usually exist as molecules containing two atoms, each molecule contains only one type of atom, so they are still elements.
Compounds
Examples:
- Water (H₂O)
- Carbon dioxide (CO₂)
- Sodium chloride (NaCl)
These contain atoms of different elements chemically bonded together.
Mixtures
Examples:
- Air
- Seawater
- Soil
- Salad
- Brass (an alloy)
These contain different substances that are physically mixed rather than chemically bonded.
Why Elements Are Important
Elements are the building blocks of all matter.
They:
- Form compounds.
- Build living organisms.
- Make up rocks and minerals.
- Produce metals used in technology.
- Participate in every chemical reaction.
There are currently 118 known elements, and every substance around us is made from one or more of them.
Figure 5. A wide variety of elements combine to form the materials found throughout our world.
Worked Example
Question
Classify each substance as an element, compound, or mixture.
| Substance | Classification |
|---|---|
| Oxygen gas (O₂) | Element |
| Water (H₂O) | Compound |
| Air | Mixture |
| Gold (Au) | Element |
| Salt water | Mixture |
| Carbon dioxide (CO₂). | Compound |
Real-World Connection
Many everyday products depend on the unique properties of different elements. Copper is used for electrical wiring because it conducts electricity well, aluminium is used in aircraft because it is strong and lightweight, and silicon is used to manufacture computer chips. Understanding the properties of elements allows scientists and engineers to design new technologies and materials.
Did You Know?
Although scientists have discovered 118 elements, only about 90 occur naturally in significant amounts on Earth. The remaining elements have been created artificially in laboratories, and many exist for only fractions of a second before they decay into other elements.
Key Terms
Atom – The smallest particle of an element that retains its chemical properties.
Atomic number – The number of protons in the nucleus of an atom.
Chemical symbol – A one- or two-letter abbreviation representing an element.
Compound – A pure substance made of two or more different elements chemically bonded together.
Element – A pure substance consisting of only one type of atom.
Mixture – Two or more substances physically combined but not chemically bonded.
Proton – A positively charged particle found in the nucleus of an atom.
Pure substance – A material containing only one type of particle with a fixed composition.
Key Takeaways
- An element is a pure substance made of only one type of atom.
- Every atom of an element has the same number of protons, which is its atomic number.
- Elements cannot be broken down into simpler substances by ordinary chemical reactions.
- Each element has a unique name and chemical symbol.
- Elements differ from compounds and mixtures in the types of particles they contain.
- All matter is ultimately built from one or more of the 118 known chemical elements.