Atoms, Elements and Compounds | Chemistry

📖 In-Depth Theory

Atoms — The Building Blocks of Matter

All substances are made of ATOMS — the smallest particles that cannot be broken down by chemical means.

An ELEMENT is a substance made of only ONE type of atom. Elements cannot be broken down into simpler substances by chemical reactions.

There are 118 known elements — each has a unique symbol (e.g. C = carbon, O = oxygen, Fe = iron, Na = sodium).

Elements are arranged in the PERIODIC TABLE in order of atomic number.

A COMPOUND is a substance formed when two or more DIFFERENT elements are CHEMICALLY BONDED together.

The properties of a compound are completely different from the properties of its elements.

Example: sodium (reactive metal) + chlorine (toxic green gas) → sodium chloride (table salt — safe to eat).

Compounds can only be separated by chemical reactions — not physical methods.

A MIXTURE contains two or more substances that are NOT chemically bonded — each keeps its own properties.

Mixtures can be separated by physical methods (filtering, distillation, chromatography).

The proportions of substances in a mixture can vary.

Chemical Formulae and Equations

Every compound has a CHEMICAL FORMULA showing which atoms are present and in what ratio.

Examples:

H₂O — water: 2 hydrogen + 1 oxygen.

CO₂ — carbon dioxide: 1 carbon + 2 oxygen.

NaCl — sodium chloride: 1 sodium + 1 chlorine.

H₂SO₄ — sulfuric acid: 2 hydrogen + 1 sulfur + 4 oxygen.

The small SUBSCRIPT numbers tell you how many atoms of each element are present.

CHEMICAL EQUATIONS show what happens in a reaction:

Reactants → Products

Equations must be BALANCED — the same number of each type of atom on both sides.

Atoms cannot be created or destroyed in a chemical reaction (law of conservation of mass).

Balancing example:

Hydrogen + oxygen → water

2H₂ + O₂ → 2H₂O ✓ (4 H and 2 O on each side)

H₂ + O₂ → H₂O ✗ (unbalanced — 2 O on left, 1 O on right)

Comparing Elements, Compounds and Mixtures

Here is a clear comparison of the three:

ELEMENT:

One type of atom only.

Cannot be broken down chemically.

Has fixed properties.

Examples: iron (Fe), oxygen (O₂), gold (Au), sulfur (S).

COMPOUND:

Two or more elements CHEMICALLY bonded in fixed proportions.

Has new, different properties from its elements.

Can only be separated by chemical reactions.

Examples: water (H₂O), carbon dioxide (CO₂), magnesium oxide (MgO).

MIXTURE:

Two or more substances NOT chemically bonded.

Each component keeps its own properties.

Can be mixed in any proportions.

Separated by physical methods.

Examples: air, seawater, crude oil, bronze (copper + tin), ink.

⚠️ Common Mistake

A COMPOUND is NOT a mixture. In a compound, elements are CHEMICALLY BONDED — you need a chemical reaction to separate them (e.g. electrolysis to split water into hydrogen and oxygen). In a MIXTURE, substances are just mixed — physical methods separate them. Water is a compound. Saltwater is a mixture.

📌 Key Note

Element: one type of atom. Compound: two or more elements chemically bonded — new properties, chemical separation only. Mixture: not bonded — keeps own properties, physical separation possible. Equations must be balanced — atoms conserved.

🎯 Matching Activity — Element, Compound or Mixture?

Sort each substance into the correct category. — drag the symbols on the right to match the component names on the left.

Element

Drop here

Compound

Drop here

Mixture

Drop here

Compound

Drop here

Mixture

Drop here

Element

Drop here

Air — nitrogen, oxygen and argon not chemically combined

Gold (Au) — one type of atom, listed on the periodic table

Copper (Cu) — made of only copper atoms

Water (H₂O) — hydrogen and oxygen chemically bonded

Crude oil — many different hydrocarbons mixed together

Carbon dioxide (CO₂) — carbon and oxygen chemically bonded

🎯 Test Yourself

Question 1 of 4

1. What is the key difference between a compound and a mixture?

2. A student heats a mixture of iron filings and sulfur. A grey solid forms that cannot be separated using a magnet. What has formed?

3. Which equation is correctly balanced?

4. Which of these is a mixture?

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🧪 Atoms, Elements and Compounds

Atoms — The Building Blocks of Matter

Chemical Formulae and Equations

Comparing Elements, Compounds and Mixtures

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