A.1 Atomic Structure

UNIT SYLLABUS

MYP

The atom is chemistry's fundamental unit, and its structure explains everything that follows. A tiny, dense nucleus of protons (positive) and neutrons (neutral) holds nearly all the mass; electrons (negative) occupy shells around it. The proton number defines the element; the electron arrangement — filling shells 2, 8, 8 — defines how it behaves. Isotopes show that atoms of one element can differ in mass, and ions show that atoms gain or lose electrons in pursuit of stability. Master this unit and the periodic table transforms from a wall poster into a map.

Guiding Questions

? If atoms are mostly empty space, why does matter feel solid?
? What makes one element different from every other?

✓ What the IB expects you to master

Describe the nuclear atom: protons and neutrons in the nucleus, electrons in shells.
State the relative masses and charges of protons (1, +1), neutrons (1, 0) and electrons (~1/1836, −1).
Use atomic number (protons) and mass number (protons + neutrons) with notation $^A_Z X$ .
Work out numbers of protons, neutrons and electrons for atoms and ions.
Define isotopes as atoms of the same element with different numbers of neutrons.
Write electron arrangements for the first 20 elements using the 2, 8, 8 pattern.
Explain how ions form by electron loss (positive) or gain (negative).

1 Key Formulas

Mass number

A = Z + N

2 Exam Preparation & Topic Explanations

Proton–neutron–electron bookkeeping (Criterion A)

Almost every atomic-structure question is bookkeeping: protons = atomic number, always; neutrons = mass number − atomic number; electrons = protons for neutral atoms, adjusted opposite to the charge for ions (2+ means two FEWER electrons).

Electron arrangements for the first 20 elements follow 2, 8, 8, then 2 more — practise until instant.

Pro Exam Strategy

A positive ion has LOST electrons — students reversing this is the single most common error.
The nucleus never changes in chemistry — only electrons move.
Group number (1–7, 0) equals outer electrons; period equals number of shells.
Draw shell diagrams with electrons in pairs where possible, and label the nucleus contents.

3 MCQ Practice

Q1. An atom has 11 protons, 12 neutrons and 11 electrons. Its electron arrangement is:

2, 8, 1
2, 9
11
2, 8, 2

Q2. Chlorine-35 and chlorine-37 differ in their number of:

Protons
Electrons
Neutrons
Shells

Q3. When a magnesium atom (2, 8, 2) becomes an ion it:

Gains 2 electrons to form Mg²⁻
Loses 2 electrons to form Mg²⁺
Loses 2 protons to form Mg²⁻
Gains 6 electrons to form Mg⁶⁻

4 Short Answer Questions

PDF

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All questions from this topic + answer key — free, printable.

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M.2 Pure Substances, Mixtures & Separation

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A.2 The Periodic Table & Trends

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