Year 7 Science · Unit 2 · Lesson 12

Atoms — Protons, Neutrons, Electrons

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Learning Goals

Use causal reasoning to explain why the proton number defines an element's identity.
Apply atomic structure knowledge to explain Rutherford's gold foil experiment results.
Evaluate what atomic structure tells us about the nature of matter.

Warm Up

Because… chain

Fill in the missing effects in each box. Each cause leads to the next idea in the chain. The first box in each row is given.

The number of protons in the nucleus determines what element an atom is.

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If you add a proton to a nucleus, the number of protons changes.

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Adding or removing electrons does NOT change the proton count.

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Neutrons add mass but have no charge and do not change the element.

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Overall outcome: what is the only thing that makes one element different from another?

Your Turn

Real-world context

In 1909, New Zealand–born physicist Ernest Rutherford and his colleagues Hans Geiger and Ernest Marsden conducted their famous gold foil experiment at the University of Manchester. They fired tiny, positively charged alpha particles at an extremely thin sheet of gold. They expected the particles to pass straight through, based on the then-accepted "plum pudding" model of the atom. Instead, a small number of the particles bounced almost straight back — a result Rutherford described as "almost as incredible as if you fired artillery shells at tissue paper and they came back and hit you."

(a) Using what you know about atomic structure (protons, neutrons and the nucleus), explain WHY some alpha particles bounced back. What does this tell us about where the positive charge and mass of an atom is concentrated?

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(b) Most alpha particles passed straight through the gold foil without deflecting at all. What does this tell us about the structure of atoms? Use the idea of where electrons are located to support your answer.

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Show What You Know

1. Oxygen has 8 protons. Write its electron arrangement and explain why the oxygen atom is electrically neutral.

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2. A student says: "If I add 3 more electrons to a sodium atom, it will become aluminium." Is this correct? Explain your answer using what you know about what determines an element's identity.

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Wrap Up

In one sentence, what was the main idea of this lesson?