Atomic Structure: Protons, Neutrons and Electrons
Inside the Atom
This KS3 chemistry lesson explores Atomic Structure: Protons, Neutrons and Electrons. Students explore the structure of an atom, identify subatomic particles and their charges, and connect atomic number and mass number to the composition of an atom. It provides a clear introduction to the topic and can be used for classroom teaching, homework or revision.
Lesduur is: 55 minOnderdelen in deze les
Inside the Atom
This KS3 chemistry lesson explores Atomic Structure: Protons, Neutrons and Electrons. Students explore the structure of an atom, identify subatomic particles and their charges, and connect atomic number and mass number to the composition of an atom. It provides a clear introduction to the topic and can be used for classroom teaching, homework or revision.
Atoms, KS3 Science, Chemistry, Subatomic Particles, Protons, Neutrons, Electrons, Atomic Number, Mass Number, Nucleus, MCQs, Open Questions, Polls, Emoji Polls, Exit Tickets
What words do you know that make up the insides of an atom?
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What is an atom?
An atom is the smallest particle of an element that can still be recognised as that element
All substances are made from atoms
Atoms are extremely small, but they have an internal structure
Emphasise that atoms are not the smallest possible particles in nature, but they are the smallest particles of an element in basic chemical models.
The two main parts of an atom
Nucleus: tiny central region
Electron shells: regions around the nucleus where electrons are found
Most of the atom's mass is in the nucleus
Keep the model simple and GCSE-friendly: electrons occupy shells around the nucleus. Avoid going into orbitals.
Protons, neutrons and electrons
Protons are found in the nucleus
Neutrons are found in the nucleus
Electrons are found in shells around the nucleus
These subatomic particles explain both the mass and the charge of an atom
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Relative charge and mass
Relative mass
Proton - 1
Neutron - 1
Electron - very small, about 1/1836
Relative charge
Proton - +1
Neutron - 0
Electron - -1
Students often confuse charge and mass tables. Stress that proton and neutron masses are similar, while electron mass is tiny by comparison.
Why atoms are usually neutral
In a neutral atom, the number of protons equals the number of electrons. The positive and negative charges balance, so the total charge is zero.
This is the key link needed before introducing ions. Neutrons do not affect charge.
Why are atoms usually neutral?
Protons equal electrons
Neutrons equal electrons
Protons equal neutrons
Electrons have no charge
Checks the key idea from slides 5-6 that positive proton charge balances negative electron charge. A common mistake is thinking neutrons affect charge.
Complete the summary
An atom has a tiny central nucleus and electrons in shells around it. Protons have charge +1, while neutrons have charge 0.
This anchors the core vocabulary and the charge facts from the first subtopic.
Atomic number
The atomic number tells you the number of protons in an atom
Every element has its own atomic number
In a neutral atom, atomic number also tells you the number of electrons
Make the link to the periodic table if helpful, but do not widen into unrelated periodic trends.
Mass number
The mass number is the total number of protons and neutrons in the nucleus
Electrons are not included because their mass is so small
Mass number = protons + neutrons
Students may wrongly use relative atomic mass here. Keep mass number as a whole number for one atom.
Working out particle numbers
Protons = atomic number
Neutrons = mass number - atomic number
Electrons in a neutral atom = atomic number
Example: if atomic number = 11 and mass number = 23, then protons = 11, neutrons = 12, electrons = 11
Model the subtraction carefully. A common error is using addition for neutrons.
Simple ions
An ion is a charged particle formed when an atom gains or loses electrons
Protons do not change in ordinary chemical reactions
Losing electrons makes a positive ion
Gaining electrons makes a negative ion
Keep the focus on electron gain/loss only. Do not introduce more advanced ionic bonding detail here unless needed orally.
Counting electrons in ions
Na+ has 11 protons and 10 electrons
Mg2+ has 12 protons and 10 electrons
Cl- has 17 protons and 18 electrons
The charge tells you how many electrons have been lost or gained
Students often change proton number when they see an ion charge. Re-state that proton number identifies the element and stays fixed.
An atom has atomic number 13 and mass number 27. Neutrons = ?
13
14
27
40
Students should use neutrons = mass number - atomic number. Distractors reflect common errors: using atomic number, using mass number, or adding them.
Complete the particle rules
The atomic number gives the number of protons. The mass number is the total of protons and neutrons. An ion forms when an atom gains or loses electrons, but the number of protons stays the same.
Echoes slides 7-11 and reinforces that ion charge changes electrons, not protons.
Why does Na+ have 10 electrons, not 11?
Expected elements: in Na+ the atom has lost one electron, so it has one more proton than electrons; proton number stays 11 because the element does not change.
What are isotopes?
Isotopes are atoms of the same element
They have the same number of protons
They have different numbers of neutrons
So they have different mass numbers
Tie this directly to atomic number: same element means same proton number.
Isotopes of one element
Carbon-12 and carbon-14 are both carbon atoms because both have 6 protons. Carbon-12 has 6 neutrons, while carbon-14 has 8 neutrons.
Use the example to show that the element name stays the same because the proton number stays the same.
Relative atomic mass
Most elements exist as a mixture of isotopes
Relative atomic mass compares the average mass of the atoms of an element
It depends on the masses of the isotopes and how common they are
Do not turn this into a full weighted-average calculation unless you want to extend orally. The key idea is that isotope abundance affects the average.
Why relative atomic mass is not whole
Because an element is usually a mixture of isotopes, its relative atomic mass is often not a whole number. It is an average value, not the mass number of one single atom.
This distinction is commonly examined: mass number is for one atom; relative atomic mass is an average for the element.
What is an isotope?
Atoms of the same element with different neutrons
Atoms of different elements with the same protons
Atoms with the same mass number
Atoms with no neutrons
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Complete the isotope summary
Isotopes are atoms of the same element, so they have the same number of protons. They have different numbers of neutrons, so their mass numbers are different. Relative atomic mass is often not whole because it is an average.
Brings together isotopes and why relative atomic mass is usually not a whole number.
How well do you understand the differences between atoms, ions, and isotopes?
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Glossary 1
atom - the smallest particle of an element that can still be recognised as that element
nucleus - the tiny central part of an atom containing protons and neutrons
electron shell - a region around the nucleus where electrons are found
proton - a subatomic particle with charge +1 and relative mass 1
neutron - a subatomic particle with charge 0 and relative mass 1
electron - a subatomic particle with charge -1 and a very small relative mass
atomic number - the number of protons in an atom
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Glossary 2
mass number - the total number of protons and neutrons in the nucleus
neutral atom - an atom with equal numbers of protons and electrons
ion - a charged particle formed when electrons are gained or lost
isotope - atoms of the same element with different numbers of neutrons
relative atomic mass - the average mass of the atoms of an element compared with a standard
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