The building blocks of everything
Everything around you, from the air to your sneakers, is made of tiny particles called atoms. Atoms are so small that a single grain of sand contains more than 10¹⁹ (ten billion billion) of them.
Inside an atom
| Particle | Charge | Location | Mass |
|---|---|---|---|
| Proton | Positive (+) | Nucleus | About 1 atomic mass unit |
| Neutron | None (0) | Nucleus | About 1 atomic mass unit |
| Electron | Negative (−) | Moving around the nucleus in energy levels | About 1/1836 of a proton |
The nucleus is tiny. If an atom were the size of a hockey arena, the nucleus would be about the size of a pea at centre ice. Most of an atom is empty space.
In a neutral atom, the number of electrons equals the number of protons.
Elements
An element is a pure substance made of only one kind of atom. What makes each element different is its number of protons, called its atomic number.
- Hydrogen has 1 proton.
- Carbon has 6 protons.
- Oxygen has 8 protons.
- Gold has 79 protons.
There are 118 known elements. About 90 occur naturally; the rest have been made in laboratories.
Each element has a chemical symbol of one or two letters: H (hydrogen), O (oxygen), Fe (iron, from Latin ferrum), Na (sodium, from Latin natrium).
The periodic table
The periodic table organizes all the elements by atomic number. Dmitri Mendeleev published the first widely used version in 1869. He even left gaps for elements that hadn’t been discovered yet, and predicted their properties correctly.
- Rows are called periods. Elements in the same row have the same number of electron shells.
- Columns are called groups or families. Elements in the same group have similar properties.
| Region | Properties | Examples |
|---|---|---|
| Metals (left and centre) | Shiny, conduct heat and electricity, bendable | Iron, copper, sodium |
| Non-metals (upper right) | Dull, poor conductors, brittle if solid | Oxygen, carbon, sulfur |
| Metalloids (along the staircase line) | Some properties of both | Silicon, boron |
Important groups
| Group | Name | Property |
|---|---|---|
| 1 | Alkali metals | Very reactive; react with water |
| 2 | Alkaline earth metals | Reactive, but less than group 1 |
| 17 | Halogens | Very reactive non-metals |
| 18 | Noble gases | Very unreactive |
Molecules and compounds
A molecule is two or more atoms joined by chemical bonds. An oxygen molecule (O₂) has two oxygen atoms.
A compound forms when two or more different elements combine chemically in a fixed ratio. The compound has properties completely different from its elements.
- Sodium is a soft metal that explodes in water. Chlorine is a poisonous green gas. Together they make sodium chloride (NaCl), table salt!
- Hydrogen and oxygen are both gases. Together they make water (H₂O), a liquid.
Chemical formulas
A chemical formula tells you what atoms are in a substance and how many.
| Formula | Name | Atoms |
|---|---|---|
| H₂O | Water | 2 hydrogen, 1 oxygen |
| CO₂ | Carbon dioxide | 1 carbon, 2 oxygen |
| C₆H₁₂O₆ | Glucose | 6 carbon, 12 hydrogen, 6 oxygen |
| NaCl | Sodium chloride | 1 sodium, 1 chlorine |
The small number after a symbol is called a subscript.
Elements, compounds and mixtures
| Type | Made of | Can be separated by physical means? | Example |
|---|---|---|---|
| Element | One kind of atom | No | Gold |
| Compound | Different atoms chemically bonded | No (needs a chemical reaction) | Water |
| Mixture | Substances combined physically | Yes | Salt water |
Common mistakes
- Mixing up atoms and molecules. A molecule is made of atoms.
- Thinking compounds keep the properties of their elements.
- Reading CO as carbon dioxide. CO is carbon monoxide; CO₂ is carbon dioxide.
Try it yourself
- How many atoms of each element are in C₁₂H₂₂O₁₁ (table sugar)?
- Find nitrogen on the periodic table. What is its atomic number and group?
- Classify each as an element, compound or mixture: air, iron, carbon dioxide, orange juice.
(Answers: 12 C, 22 H, 11 O; atomic number 7, group 15; mixture, element, compound, mixture)
Quick recap
- Atoms are made of protons, neutrons and electrons.
- An element has one kind of atom; its atomic number is its number of protons.
- The periodic table organizes elements into periods and groups.
- Compounds are elements chemically combined, with new properties.
- Chemical formulas show which atoms are in a substance and how many.