This lesson turns particle knowledge into number patterns: atomic number, mass number and the idea of a neutral atom.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Write a first response before reading. Then compare it with your answer at the end.
The most important atom number in this lesson is the number of protons.
Atomic number means proton number. If the proton number changes, the element changes. That is why scientists treat proton number as the identity of the element.
This corrects a common misconception that neutrons decide the element type.
Mass number is a different count from atomic number, so the two labels must not be mixed up.
At Stage 4 level, mass number equals protons plus neutrons. Electrons are not included in this count. Students should practise this repeatedly with common examples such as carbon, oxygen and sodium.
The main goal is using the rule accurately, not memorising many elements.
Neutral does not mean no particles. It means the positive and negative charges balance.
If an atom is neutral, the number of protons equals the number of electrons. This relationship is useful in simple worked examples and prepares students for later chemistry without actually teaching bonding.
At this stage, students only need the basic equality rule.
A strong student uses the three counts together: protons, neutrons and electrons.
If you know proton number, you know atomic number. If you know protons and neutrons, you can find mass number. If the atom is neutral, electrons match protons.
This makes atomic-structure questions much easier and reduces random guessing.
Copy the three core number rules so they stay separate in your notes.
Atomic number = number of protons.
Mass number = protons + neutrons.
In a neutral atom, protons = electrons.
Complete a table for three simple atoms showing proton number, neutron number, electron number, atomic number and mass number.
Explain to a partner why two atoms with different mass numbers can still be the same element if they have the same proton number.
1. What does atomic number count?
2. What does mass number count?
3. A neutral atom has 8 protons. How many electrons does it have?
4. An atom has 6 protons and 7 neutrons. What is its mass number?
5. Which change would make an atom a different element?
Explain the difference between atomic number and mass number.
A neutral atom has 11 protons and 12 neutrons. State its atomic number, mass number and electron number.
Explain why changing neutron number does not automatically make a new element.
1: C. Atomic number counts protons.
2: B. Mass number is protons plus neutrons.
3: A. A neutral atom has equal protons and electrons, so it has 8 electrons.
4: D. Mass number is 6 + 7 = 13.
5: B. Changing proton number changes the element identity.
Atomic number is the number of protons, while mass number is the number of protons plus neutrons. They are different counts and should not be treated as the same thing.
Atomic number = 11 because there are 11 protons. Mass number = 23 because 11 + 12 = 23. Electron number = 11 because the atom is neutral.
Changing neutron number does not automatically create a new element because element identity is determined by proton number. If the proton number stays the same, the atom still belongs to the same element.
Atomic number tells you the proton number.
Mass number counts protons and neutrons together.
Neutral atoms have equal protons and electrons.
Neutron number alone does not define the element.