Fundamentals of chemistry: web revision guide

Web companion updated November 2, 2026

Start here to review the vocabulary and calculations used in introductory chemistry. This web guide complements the original notes in the document below. Use your assigned textbook and syllabus to check the depth required for your course.

Classify the substance before doing a calculation

An element contains one kind of atom, identified by its proton number. A compound combines different elements in a fixed chemical ratio. A mixture contains substances together without requiring a fixed ratio. Oxygen gas, O₂, is an element even though its particles contain two atoms. Water, H₂O, is a compound; salt water is a mixture.

Ask two questions: is there one chemical substance or several, and which elements are present? Uniform appearance alone cannot prove purity. Dissolved salt is distributed throughout a well-mixed solution, but changing the amount of salt changes the solution's composition. Evaporating its water separates the mixture without turning the salt into new elements.

Keep atomic number, mass number and charge separate

Atomic number Z counts protons. Mass number A counts protons plus neutrons for one isotope, so neutrons = A − Z. A neutral atom has equal numbers of protons and electrons. An ion has an unequal electron count. Losing electrons produces a positive ion; gaining them produces a negative ion.

Worked example: an aluminium-27 ion has Z = 13 and charge 3+. It has 13 protons, 27 − 13 = 14 neutrons, and 13 − 3 = 10 electrons. Check that protons minus electrons gives +3. Do not subtract the charge from the neutron count.

Read every symbol and subscript

A subscript applies to the symbol immediately before it, or to an entire group in parentheses. In Ca(OH)₂, there is one calcium, two oxygen and two hydrogen atoms per formula unit. The 2 multiplies both O and H. A coefficient placed before a formula multiplies the whole formula: 3H₂O represents six hydrogen atoms and three oxygen atoms in total.

Connect a formula to a measurable mass

Add the relative atomic masses with their formula counts. Using rounded school values Ca = 40, O = 16 and H = 1 gives Ca(OH)₂ a relative formula mass of 40 + 2(16 + 1) = 74. Its corresponding molar mass is approximately 74 g/mol. Always use the atomic masses supplied in your question when they differ.

The mole is a counting unit. One mole contains exactly 6.02214076 × 10²³ specified entities. Convert mass to amount with n = m/M, where m is in grams and M is in grams per mole. For 7.4 g of Ca(OH)₂ with M = 74 g/mol, n = 0.10 mol. State whether you are counting atoms, molecules or formula units before using the Avogadro constant.

Check your work

Finish by checking the units, the particle being counted and whether parentheses were expanded. Relative formula mass has no unit; molar mass uses g/mol. A periodic-table atomic mass is an isotope-weighted average, so it should not be used as the mass number of an individual atom. Close these notes and explain one distinction in your own words before moving to practice.

Original notes document

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