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Protons, neutrons and electrons, atomic and mass numbers, isotopes, relative atomic mass and how the model of the atom changed.
Specification references this topic covers: AQA 5.1.1; Edexcel C1.1, C1.2, C1.3, C1.4, C1.5, C1.6, C1.7, C1.8.
Atoms are the tiny building blocks of everything around you. Your phone, the air and your own body are all made of them. Chemistry starts with knowing what an atom looks like inside.
It matters because every other idea in chemistry builds on it. Bonding, reactions and the periodic table all depend on the particles in atoms. Once this is clear, much of the course gets easier.
By the end you will be able to name the three particles and count them in any atom or ion. You will explain isotopes, work out a relative atomic mass and describe how the model of the atom changed.
An atom has a tiny nucleus in the middle. The nucleus is made of protons and neutrons. Electrons move around the nucleus in shells.
An atom is very small, with a radius of about 0.1 nm. The nucleus is about 10 000 times smaller than the whole atom. So most of an atom is empty space.
Picture a pea in the middle of a football pitch. The pea is the nucleus and the pitch is the atom.
Each particle has a relative charge and a relative mass. Relative means compared with a proton. A proton has charge +1 and mass 1. A neutron has no charge and mass 1.
An electron has charge −1 and a very small mass, as the bars show. It is so small that we ignore it when adding up the mass of an atom.
An atom has the same number of protons and electrons. The +1 and −1 charges cancel out. So an atom has no overall charge. We say it is neutral.
The atomic number is the number of protons. It decides which element the atom is. The mass number is the number of protons plus the number of neutrons.
To find the neutrons, subtract: mass number − atomic number. Sodium-23 has atomic number 11. It has 11 protons, 11 electrons and 23 − 11 = 12 neutrons.
An ion has a different number of electrons from protons. Mg²⁺ has 12 protons but only 10 electrons, because it lost 2. Cl⁻ gained one electron. The protons and neutrons do not change.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers.
Carbon-12 has 6 protons and 6 neutrons. Carbon-14 has 6 protons and 8 neutrons. Think of two tennis balls, one filled with sand. They look the same, but one is heavier.
Isotopes have the same number of electrons, so they react in the same way. Only their masses are different.
Most elements are a mix of isotopes. The relative atomic mass, Ar, is the mean mass of the atoms of an element. It allows for how much of each isotope there is. This is why Ar is not always a whole number.
To work it out, multiply each mass number by its percentage. Add the results, then divide by 100. Chlorine is 75% chlorine-35 and 25% chlorine-37.
Ar = (75 × 35 + 25 × 37) ÷ 100 = 35.5. It is a mean, like an average test score. The more common isotope pulls the answer towards it.
Scientists change a model when new evidence does not fit it. J J Thomson found the electron. His plum pudding model was a ball of positive charge with electrons spread through it, like fruit in a pudding.
Rutherford’s team fired alpha particles at thin gold foil. Most went straight through. A few were deflected a lot and a very few bounced back. Alpha particles are positive, so something positive repelled them.
So most of an atom is empty space, with a small, dense, positive nucleus. This was the nuclear model. Bohr then put electrons in shells, and Chadwick found the neutron.
The first model came from John Dalton in the early 1800s. He pictured atoms as tiny solid spheres that could not be divided, with a different kind of atom for each element. This model had to change when scientists discovered smaller particles inside the atom, starting with the electron.
An atom has a nucleus of protons and neutrons, with electrons in shells around it. An atom is neutral.
Atomic number is the protons. Mass number is protons plus neutrons. Neutrons = mass number − atomic number.
Isotopes have the same protons and different neutrons.
Ar = (each mass × its percentage, added up) ÷ 100.
The gold foil results replaced the plum pudding model with the nuclear model.
An atom of fluorine has atomic number 9 and mass number 19. Work out the number of neutrons.
Answer: 10
Describe how the alpha scattering experiment led to the nuclear model of the atom.
Answer: Most alpha particles went straight through, showing mostly empty space, and a few were deflected, showing a small dense positive nucleus.
Magnesium has three isotopes. 79% are magnesium-24, 10% are magnesium-25 and 11% are magnesium-26. Calculate the relative atomic mass of magnesium. Give your answer to 3 significant figures.
Answer: 24.3
Atomic structure, isotopes and models of the atom practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
An atom has 17 protons. What is its atomic number?
Answer: 17
State the relative charge of a neutron.
Answer: 0
Where is most of the mass of an atom found?
Answer: the nucleus
What is the name for atoms of the same element with different numbers of neutrons?
Answer: isotopes
Which particle has a relative charge of −1?
Answer: electron
Name the particle in the nucleus that has no charge.
Answer: neutron
An atom of aluminium has atomic number 13 and mass number 27. How many neutrons does it have?
Answer: 14
A potassium atom has 19 protons and 20 neutrons. What is its mass number?
Answer: 39
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
In an experiment, alpha particles were fired at a very thin piece of gold foil. (a) State what happened to most of the alpha particles. [1 mark] (b) Explain what the result in part (a) shows about the structure of an atom. [1 mark] (c) A few alpha particles bounced back. Explain what this showed about the nucleus. [2 marks]
Answer: (a) They passed straight through. (b) Most of the atom is empty space. (c) The nucleus is positively charged, and it is small and dense, so it repelled the alpha particles.
Chlorine has two isotopes. 75% of chlorine atoms have a mass number of 35 and 25% have a mass number of 37. (a) Calculate the relative atomic mass of chlorine. [3 marks] (b) State one difference between an atom of each isotope. [1 mark]
Answer: (a) 35.5 (b) They have different numbers of neutrons.
Saying the mass number is the number of neutrons
The mass number is protons plus neutrons. Subtract the atomic number to find neutrons.
Saying isotopes have different numbers of protons
Isotopes have the same number of protons and different numbers of neutrons.
Taking a simple mean of isotope masses
Weight each mass by its percentage, then divide by 100.
Counting the electrons in an ion the same as the atom
Electrons = protons − charge. Mg²⁺ has 2 fewer electrons than protons.
Atomic structure, isotopes and models of the atom is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 4.
Saying the mass number is the number of neutrons. The mass number is protons plus neutrons. Subtract the atomic number to find neutrons.
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.
See how the papers are set: AQA Combined Science papers and Edexcel Combined Science papers.
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