Atomic Structure: Protons, Neutrons and Electrons

• New lesson editor
• Chemistry
• Lower Secondary (Key Stage 3)

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?

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

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?

A

Protons equal electrons

B

Neutrons equal electrons

C

Protons equal neutrons

D

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.

nucleus

shells

+1

0

-1

1/1836

mass number

electrons

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 = ?

A

13

B

14

C

27

D

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.

atomic number

mass number

electrons

protons

neutrons

nucleus

charge

shells

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?

A

Atoms of the same element with different neutrons

B

Atoms of different elements with the same protons

C

Atoms with the same mass number

D

Atoms with no neutrons

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.

element

protons

neutrons

mass numbers

average

electrons

ion

charge

shells

nucleus

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?

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

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