Welcome to Atomic Structure & The Periodic Table — the foundation of all chemistry. Everything you touch, breathe, eat and drink is made of atoms, and understanding how atoms are built is the key to unlocking every other topic in this subject.
In this module you will learn how atoms are structured with protons, neutrons and electrons, how the periodic table organises elements by their properties, how electron configurations determine chemical behaviour, and how isotopes and relative atomic mass calculations work. These concepts appear in nearly every CSCA Chemistry section.
By the end you will be comfortable drawing electron shell diagrams, predicting element properties from their position in the periodic table, and performing mass-number calculations with confidence. Let's get started!
Atomic structure questions typically make up 20–25% of the chemistry paper. Expect 1–2 calculation questions (isotope Ar, moles, Mr), 1 explanation question on periodic trends, and 1 question on group properties (alkali metals or halogens). Dot-and-cross diagrams and electron configurations are frequently tested.
An atom is the smallest particle of an element that retains the chemical properties of that element. Every atom consists of a tiny, dense nucleus at the centre, surrounded by electrons that orbit in shells (energy levels).
Sub-atomic Particles
Key Numbers
In a neutral atom, the number of electrons equals the number of protons (charges balance). When an atom gains or loses electrons it becomes an ion — a cation (+) if it loses electrons, an anion (−) if it gains them.
Notation
We write an element as ᴬ_Z X where A is the mass number (top) and Z is the atomic number (bottom). For example, carbon-12 is written ¹²₆C — it has 6 protons, 6 neutrons and (when neutral) 6 electrons.
The nucleus is incredibly small compared to the atom. If the atom were the size of a football stadium, the nucleus would be a marble on the centre spot — yet it contains over 99.9% of the atom's mass.
Understanding this basic structure is essential because the number and arrangement of electrons determines how atoms bond, react and form compounds.
💡Atoms have protons (+1) and neutrons (0) in the nucleus, electrons (−1) in shells. Atomic number = protons; mass number = protons + neutrons.
📋 Key Formulas
Z = atomic number = protons | A = mass number = protons + neutrons | Neutrons = A − Z | In neutral atom: electrons = protons
📝 Worked Example 1
Example 1: An atom of sodium has atomic number 11 and mass number 23. How many protons, neutrons and electrons does it have?
Protons = Z = 11. Neutrons = A − Z = 23 − 11 = 12. Electrons = 11 (neutral atom, so electrons = protons).
📝 Worked Example 2
Example 2: A chloride ion Cl⁻ has Z = 17, A = 35. How many electrons does it have?
Neutral Cl has 17 electrons. The ⁻ charge means it gained 1 electron → 18 electrons.
📝 Worked Example 3
Example 3: An atom has 26 protons and 30 neutrons. Identify the element and its mass number.
Z = 26 → Iron (Fe). A = 26 + 30 = 56. This is ⁵⁶₂₆Fe.
🧠Always check whether the question says "atom" or "ion" — ions have a different electron count.
🧠Mass number is always a whole number. Relative atomic mass (on the periodic table) is usually a decimal because it is a weighted average of isotopes.
⚠️Confusing mass number with relative atomic mass: Mass number is for one specific atom (always a whole number). Relative atomic mass is the weighted average across all isotopes.
⚠️Forgetting that ions change electron count, not proton count: A Na⁺ ion still has 11 protons; it just lost an electron to have only 10.
🎯 Try This Yourself
An atom of potassium (K) has Z = 19 and A = 39. Calculate the number of protons, neutrons and electrons.
Open and read all sections to complete this module