Atoms and Molecules

Why This Chapter Matters

Chemistry becomes understandable when matter is treated as an enormous population of atoms and molecules. The laws of chemical combination were discovered before atoms could be directly observed. The regularity of mass in reactions suggested discrete units that rearrange without being created or destroyed. Dalton turned that evidence into atomic theory. The practical challenge then becomes counting unimaginably small particles, which is why chemists use relative masses, formulae and the mole.

Bridge from Earlier Learning

• Elements and compounds
• Simple chemical reactions
• Mass measurement
• Ratios and multiplication

These ideas are reused and deepened in this chapter.

1. Law of Conservation of Mass

2. Law of Constant Proportions

A pure compound contains the same elements in a fixed proportion by mass. This distinguishes a compound from a mixture. Salt water can contain variable salt concentration; pure sodium chloride has a fixed composition. The law provided strong evidence that elements combine in definite particle ratios.

3. Dalton Atomic Theory

Dalton proposed that matter is made of atoms, atoms combine in simple whole-number ratios and chemical reactions rearrange atoms. Modern science refined his picture: atoms contain subatomic particles and isotopes show that atoms of one element can have different masses. Yet the central idea of discrete atomic combination remains foundational.

4. Atomic Mass and Symbols

Atoms are too light for convenient gram-scale measurement, so chemists compare them using relative atomic masses. Chemical symbols create universal shorthand: H, O, Na, Fe and so on. Capitalisation matters because symbols are precise scientific language. For school calculations, use the atomic masses supplied or standard values.

5. Molecules, Ions and Formula Units

A molecule is a bonded group of atoms acting as a unit, such as O₂ or H₂O. Ions are charged atoms or groups of atoms formed through electron loss or gain. Ionic solids such as NaCl are described by formula units showing the simplest ion ratio rather than discrete molecules in the same sense as covalent substances.

6. Writing Chemical Formulae

A neutral compound must have balanced total positive and negative charge. Identify the ions and valencies, choose the smallest whole-number ratio that makes net charge zero, and write the formula. Aluminium oxide combines Al³⁺ and O²⁻ in a 2:3 ratio, giving Al₂O₃. The cross-over method is a shortcut; charge balance is the reason.

7. Molecular and Formula Mass

Molecular mass is the sum of relative atomic masses of all atoms in one molecule. The same addition for an ionic formula gives formula-unit mass. This value in atomic mass units is not the same physical quantity as molar mass, although molar mass has the same numerical value in grams per mole.

8. Mole Concept

A mole is a counting unit, like a dozen but vastly larger. One mole contains about 6.022 × 10²³ specified entities. It connects microscopic particle count to measurable mass. If water has molar mass 18 g/mol, then 18 g water contains one mole of H₂O molecules. Always specify the entity being counted.

Compare the Ideas

Visualise the Concept

The mole bridges microscopic particle counts and measurable laboratory mass.

Formula / Relationship Sheet

Worked Examples & Scientific Reasoning

Observe Like a Scientist

Exam Thinking Strategy

Common Mistakes

Quick Revision

Focused Practice

Final Chapter Summary