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IB DemystifiedMYP Sciences

Atomic structure

Atoms are so small that millions would fit across a full stop, yet scientists have worked out what is inside them without ever seeing one directly. Every atom has a tiny, dense nucleus of protons and neutrons, surrounded by electrons, and the numbers of these particles explain how each element behaves.

Recommended for MYP 3 · About 3 lessons · Criteria A, B, C and D

11 p12 nnucleus: 11 protons, 12 neutronselectron (in the third shell)electron arrangement: 2, 8, 1atomic number = 11mass number = 23Not to scale: a real nucleus is about 100 000 times smaller than the atom.
Figure 1. A simple model of a sodium atom.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Particles in an atom
  6. Atomic science in the real world
  7. Worked examples
  8. Assessment tips
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • describe protons, neutrons and electrons: their charge, mass and position
  • use atomic number and mass number to find numbers of particles
  • explain what isotopes are
  • write electron arrangements for the first 20 elements
  • explain how models of the atom changed as new evidence appeared
  • discuss uses of atomic science in medicine and research

Before you start

You will use these skills. If any feel shaky, review them first.

  • elements and the periodic table (see Elements, compounds and the periodic table)
  • electric charge (positive and negative)
  • percentages

Key vocabulary

Proton
A positive particle (+1, mass 1) in the nucleus.
Neutron
A neutral particle (0, mass 1) in the nucleus.
Electron
A negative particle (−1, almost no mass) in shells around the nucleus.
Atomic number
The number of protons in an atom.
Mass number
The number of protons plus neutrons.
Isotopes
Atoms of the same element with different numbers of neutrons.

Understanding the ideas

  1. What is it?

    An atom has a tiny central nucleus containing protons (positive) and neutrons (neutral), which make up almost all its mass. Electrons (negative) are arranged in shells around the nucleus. Atoms have equal numbers of protons and electrons, so they have no overall charge.

  2. Why does it happen?

    The atomic number is the number of protons and identifies the element; the mass number is protons plus neutrons. Isotopes of an element have the same number of protons but different numbers of neutrons. Electrons fill shells in order: 2 in the first, then 8, then 8 for the first 20 elements.

  3. How do we know?

    Our model of the atom has changed with evidence. Dalton pictured solid spheres; Thomson discovered electrons and proposed the "plum pudding" model; the gold foil experiment showed a tiny, dense nucleus; Bohr placed electrons in shells; and Chadwick discovered the neutron in 1932.

  4. Why does it matter?

    Understanding atoms has led to medical imaging with isotopes, new materials, and huge international experiments that explore what matter is made of.

  5. What does it connect to?

    Atomic structure explains the periodic table, chemical bonding and reactions in chemistry, and radioactivity and electricity in physics.

Particles in an atom

  • Proton: charge +1, relative mass 1, in the nucleus.
  • Neutron: charge 0, relative mass 1, in the nucleus.
  • Electron: charge −1, relative mass about 1/2000, in shells.
  • Protons = atomic number; electrons = protons (neutral atom); neutrons = mass number − atomic number.
  • Shells: 2, then 8, then 8 (for elements 1–20).

Atomic science in the real world

Hospitals use isotopes as tracers to see how organs work. Scientists and engineers from many countries, including Pakistan, work at CERN studying the smallest particles. Carbon isotopes help archaeologists date ancient remains.

Worked examples

Example 1: particle numbers

Potassium has atomic number 19 and mass number 39. Find the numbers of protons, neutrons and electrons.

  1. Protons = atomic number = 19.
  2. Electrons = 19 (neutral atom).
  3. Neutrons = 39 − 19 = 20.

Example 2: electron arrangement

Write the electron arrangement for sulfur (atomic number 16).

  1. 16 electrons.
  2. First shell 2, second shell 8, remaining 6 in the third shell.
  3. Arrangement: 2, 8, 6.

Assessment tips

Questions on atomic structure often use numbers, diagrams and the history of models. Expect to:

  • Calculate numbers of protons, neutrons and electrons.
  • Draw or write electron arrangements.
  • Explain how evidence changed the model of the atom.
  • Evaluate models and experimental data.

Common mistakes: saying isotopes have different numbers of protons; adding electrons to the mass number; putting more than 8 electrons in the second shell; and thinking the nucleus fills most of the atom.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: flame tests

Partially guided investigation · about 40 minutes · teacher demonstration or small groups

Research question
Can the colour of a flame identify which metal is in an unknown compound?
Scientific background
When metal atoms are heated, their electrons gain energy and then give it out as light of particular colours. Each metal gives its own colour, like a fingerprint.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Nichrome wire loops (or wooden splints soaked in water), Bunsen burner, samples of metal chloride salts (such as sodium, potassium, calcium and copper chlorides), dilute hydrochloric acid for cleaning the wire (teacher only).
Method
  1. Clean the wire loop and dip it into the first salt.
  2. Hold it at the edge of a blue Bunsen flame and record the flame colour.
  3. Repeat with the other salts, cleaning the loop between each.
  4. Test an unknown salt and identify the metal from its colour.

Safety. Wear eye protection, tie back hair, and keep flammable materials away from the flame. Some salts are harmful; wash hands afterwards. The teacher should handle the acid.

Then evaluate: why might sodium contamination make other flame colours hard to see?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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