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1.6 Structure of the Atom

GCSE Chemistry · AQA · Chapter 1: Atomic Structure

A free GCSE Chemistry lesson for AQA (8462) from OBA School: protons, neutrons and electrons, atomic number and mass number, and how small atoms are. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Recall the relative charge, relative mass and position of protons, neutrons and electrons
  • Explain why atoms have no overall charge
  • Use atomic number and mass number to work out the numbers of protons, neutrons and electrons
  • Recall the size of atoms and nuclei and compare them using standard form

Key words

  • Proton: A positive particle in the nucleus. Relative mass 1, relative charge +1.
  • Neutron: A particle in the nucleus with no charge. Relative mass 1.
  • Electron: A negative particle in a shell around the nucleus. Relative charge −1, very small mass.
  • Atomic number: The number of protons in an atom. It identifies the element.
  • Mass number: The total number of protons and neutrons in an atom.
  • Nucleus: The tiny, positive centre of an atom, containing protons and neutrons.

Protons, neutrons and electrons

Atoms are made of three types of subatomic particle. Their masses and charges are so small that we compare them using relative values.

  • proton; Relative charge +1; Relative mass 1; Where it is found in the nucleus
  • neutron; Relative charge 0; Relative mass 1; Where it is found in the nucleus
  • electron; Relative charge −1; Relative mass very small (about 1/2000); Where it is found in shells around the nucleus

In an atom, the number of electrons is equal to the number of protons. The positive and negative charges cancel, so atoms have no overall charge.

Atomic number and mass number

All atoms of an element have the same number of protons. This number is the atomic number. Each element has a different atomic number, and the periodic table lists the elements in order of atomic number. The mass number is the total number of protons and neutrons.

protons = atomic number

electrons = atomic number (in an atom)

neutrons = mass number − atomic number

Worked example: counting particles

An aluminium atom has atomic number 13 and mass number 27.
protons = 13 · electrons = 13 · neutrons = 27 − 13 = 14

Answer: 13 protons, 14 neutrons, 13 electrons

Worked example: working backwards

An atom has 16 protons and 16 neutrons. Which element is it, and what is its mass number?
Atomic number = 16, so the element is sulfur (S). Mass number = 16 + 16 = 32.

Answer: sulfur, mass number 32

How big are atoms?

Atoms are extremely small. The radius of an atom is about 0.1 nm, which is 1 × 10⁻¹⁰ m. The radius of the nucleus is less than 1/10 000 of the radius of the atom (about 1 × 10⁻¹⁴ m). So most of an atom is empty space, but almost all of its mass is in the nucleus, because protons and neutrons are much heavier than electrons.

Worked example: comparing sizes

A carbon atom has a radius of 7 × 10⁻¹¹ m. Its nucleus has a radius of 2.7 × 10⁻¹⁵ m.
(7 × 10⁻¹¹) ÷ (2.7 × 10⁻¹⁵) = 25 926 ≈ 26 000

Answer: the atom is about 26 000 times wider than its nucleus

Exam tip: 1 nm = 1 × 10⁻⁹ m. To turn metres into nanometres, multiply by 10⁹: 1.5 × 10⁻¹⁰ m = 0.15 nm.

Common mistakes

  • Adding electrons into the mass number. Mass number = protons + neutrons only.
  • Giving the mass number as the number of neutrons. Subtract the atomic number first.
  • Saying electrons have no mass. Their mass is very small, but not zero.
  • Mixing up the two numbers. The atomic number is always the smaller one.
  • Mixing up nm and m: 0.1 nm is 1 × 10⁻¹⁰ m, not 1 × 10⁻¹ m.

Revision questions

Try these, then check your answers below each one.

Question 1. An atom has 26 protons and 30 neutrons. What is its mass number?

Answer: 56

Working: 26 + 30 = 56.

Question 2. What does the atomic number of an element tell you? (Choose from: The number of protons in an atom, The number of neutrons in an atom, The number of protons and neutrons, The number of shells.)

Answer: The number of protons in an atom

Working: The atomic number is the number of protons. Every atom of the element has this number of protons.

Question 3. True or false? Almost all of the mass of an atom is in its nucleus.

Answer: True

Working: True. Protons and neutrons (relative mass 1) are in the nucleus; electrons have a very small mass.

Related lessons

Continue in the full course

This lesson is section 1.6 (AGCCH0106) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

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