What's inside an atom?

Students describe the nuclear atom (protons and neutrons in the nucleus, electrons in shells), use atomic and mass numbers to count each particle, and connect the model to the evidence that replaced earlier ones (AC9S9U06, node 1, a hinge node). About 26 minutes.

Open the teacher guide: answers, solutions, slides and a worksheet.

Students will be able to

• I can describe the nuclear atom.
• I can use the atomic number and mass number to find the protons, neutrons and electrons.
• I can describe the relative mass and charge of protons, neutrons and electrons.

Before class

A concept tutorial for introducing or revising the idea; it does not replace a timeline of atomic models or a gold-foil analogy activity. Set the width with ?width=minimum, typical (default) or wider.

The hinge: students arrive with Year 7 particles that have no inside, so the model has to be extended explicitly. The prediction asks for neutrons from the two numbers before students meet the rule; the usual slips are taking the mass number as the neutrons, or adding the numbers.

Board notes: the Bohr model is drawn not to scale (the nucleus would be about 100 000 times smaller than the atom). Shells fill 2, 8, 8, 2; this lesson doesn't need shells beyond "electrons around the nucleus" (Year 10 Shells does). Protons up to 20, neutrons up to 24.

Running the lesson

1. Engage: tally 6 / 12 / 18. The 12s have mixed up mass number and neutrons.
2. Explore: ask what changed on the board when they added a neutron (mass number, not the name).
3. Explain: the two number checks are the node's success criteria; the gold foil question is the evidence.
4. In your own words: the labelled carbon-12 diagram with working is the evidence.

The hard part (on purpose)

Using atomic and mass numbers to count protons, neutrons and electrons. When a student says carbon-12 has 12 neutrons, ask what the mass number counts.

When students get stuck

• A student says carbon-12 has 12 neutrons. Ask: What does the mass number count? Fix: Protons plus neutrons. Subtract the protons (the atomic number): 12 − 6 = 6.
• A student changes the protons to make a heavier carbon. Ask: What happened to the element's name on the board? Fix: Protons decide the element. Add neutrons for a heavier atom of the same element.
• A student thinks electrons hold most of the mass because they fill the atom. Ask: What is the mass of an electron compared with a proton? Fix: About 1/1840: almost nothing. The nucleus is tiny but holds nearly all the mass.

Stretch and support

Stretch: Carbon-14 is used to date old wood and bone. How is a carbon-14 atom different from carbon-12, and how is it the same?

Support: Give the frame: 'Atomic number = protons = … . Mass number − atomic number = neutrons = … . Neutral, so electrons = … .'

Where it leads

Next: isotopes (node 2), then alpha, beta and gamma decay (node 3), half-life (node 4); decay chains are a class task (node 5). In Year 10, Shells uses the same board to arrange electrons.

In class: a timeline of atomic models (Dalton, Thomson, Rutherford, Bohr) with the evidence for each change; a "mystery box" gold-foil analogy (rolling marbles at a hidden shape).

What each step is for

• Engage: counting the parts: Asking for neutrons from the atomic and mass numbers tests the new move before students meet it; wrong options are the usual slips (mass number taken as neutrons, or added to the atomic number).
• Explore: build carbon-12: Building the atom particle by particle makes the atomic number, mass number and charge things students can count, not definitions to recite.
• Explain: protons, neutrons, electrons: Separating what each particle contributes (identity, mass, charge) and tying the nucleus to evidence is the hinge for isotopes and decay.
• Elaborate: chlorine: A bigger atom with an odd mass number checks the counting rule without the symmetry of carbon (6, 6, 6).
• Elaborate: an ion: Changing only electrons shows charge comes from the electron count while the element stays the same, ready for ions in bonding.