The shape of the lesson
By the end of the lesson, learners can
- Three particles, one table: Build any atom or ion in the simulator: pick a proton number, add neutrons, then add or remove electrons.
- Beams in an electric field: Fire a beam of each particle between two charged plates.
- Counting p, n and e: Two numbers on the symbol give you everything.
- Z (bottom) = protons = electrons in the neutral atom.
- A (top) = protons + neutrons, so neutrons = A − Z.
- Ions: the charge changes electrons only. Positive means electrons lost.
- Fields: deflection follows the sign of the charge and the size of charge/mass.
One picture every idea returns to
Protons (relative charge +1, relative mass 1) and neutrons (0, 1) sit in the nucleus, so the nucleus carries nearly all the mass and all the positive charge. Electrons (−1, 1/1836) fill the space around it. Proton number Z counts protons; nucleon number A counts protons plus neutrons. Neutrons = A − Z, and electrons = Z − (charge on the ion).
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Mostly empty space.. An atom is a tiny, dense, positively charged nucleus wrapped in a cloud of electrons about ten thousand times wider. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Three particles, one table | Build any atom or ion in the simulator: pick a proton number, add neutrons, then add or remove electrons. Project tab 1 of the lab (Atomic structure lab, atom and ion builder) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Beams in an electric field | Fire a beam of each particle between two charged plates. Project tab 7 of the lab (Atomic structure lab, beam deflection) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Counting p, n and e | Two numbers on the symbol give you everything. | Connect-Extend-Challenge |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 18 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Counting p, n and e, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The charge on an ion changes the electron count only. | Protons and neutrons are untouched: an ion is still the same element with the same nucleus. And the nucleon number on a symbol is a whole-number count for one atom; it is not the relative atomic mass in the Periodic Table, which is an average over isotopes. |
Particles in the atom: worksheet
6 questions, 18 marks. Written in the style of the papers; no past paper question is reproduced.
Complete the table for the three ions.
| Ion | protons | neutrons | electrons |
|---|---|---|---|
| 3919K+ | |||
| 7935Br− | |||
| 5626Fe2+ |
Describe the distribution of mass and charge within an atom.
Separate beams of protons, neutrons and electrons are passed between two oppositely charged plates. Describe and explain the path of each beam.
An alpha particle is a helium nucleus, 42He2+. State the numbers of protons, neutrons and electrons it contains, and compare its deflection in an electric field with that of a beam of protons moving at the same speed.
An atom X has 17 protons and 18 neutrons. Identify X, write its nuclide symbol, and give the formula and electron count of the ion it forms most readily.
The nucleon number of a chlorine atom is always a whole number, yet the relative atomic mass of chlorine in the Periodic Table is 35.5. Explain the difference.
Answer key and marking notes
- K+: 19 protons, 39 − 19 = 20 neutrons, 19 − 1 = 18 electrons.
- Br−: 35 protons, 79 − 35 = 44 neutrons, 35 + 1 = 36 electrons.
- Fe2+: 26 protons, 56 − 26 = 30 neutrons, 26 − 2 = 24 electrons.
- The nucleus contains the protons and neutrons, so nearly all the mass is concentrated in the nucleus.
- The nucleus carries all the positive charge (from the protons); neutrons are uncharged.
- The electrons, with negligible mass, occupy the space around the nucleus and carry all the negative charge.
- Protons are deflected towards the negatively charged plate because they are positively charged.
- Electrons are deflected towards the positively charged plate because they are negatively charged.
- The electron beam is deflected much more than the proton beam: the size of the charge is the same but the electron's mass is about 1/1836 of the proton's, so its charge-to-mass ratio is far larger.
- Neutrons are not deflected: they have no charge, so the field exerts no force on them.
- 2 protons, 4 − 2 = 2 neutrons, 2 − 2 = 0 electrons.
- It is deflected in the same direction as the protons (towards the negative plate) because it is positive.
- It is deflected less: its charge-to-mass ratio is 2/4 = 0.5, half that of a proton (1/1), so the same field bends it about half as much.
- 17 protons means Z = 17, which is chlorine.
- A = 17 + 18 = 35, so the symbol is 3517Cl.
- Chlorine gains one electron to form Cl−, which has 17 + 1 = 18 electrons.
- Nucleon number counts the protons and neutrons in one particular atom, so it must be a whole number (35 or 37 for chlorine).
- Relative atomic mass is a weighted average over the natural mixture of isotopes (about 75% 35Cl and 25% 37Cl), so it need not be a whole number.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which statement about the nucleus of an atom is correct?
- AIt carries all the positive charge but occupies only a tiny fraction of the atom's volume
- BIt contains every particle in the atom that has mass
- CIt always contains equal numbers of protons and neutrons
- DIt carries a relative charge equal to the nucleon number
An atom of uranium-238 has proton number 92. How many neutrons does it contain?
- A146
- B92
- C238
- D330
A species contains 16 protons, 18 neutrons and 18 electrons. Which is it?
- A34S2−
- B34S
- C34Ar
- D32S2−
How many electrons are there in one sulfate ion, SO42−? (Proton numbers: S = 16, O = 8.)
- A50
- B48
- C46
- D98
Which species contains more electrons than neutrons? (Proton numbers: O = 8, F = 9, Na = 11, Mg = 12.)
- A16O2−
- B19F−
- C23Na+
- D24Mg
A beam of 1H+ ions passes between two charged plates and is deflected through a small angle. Beams of other particles travel at the same speed through the same field. Which beam is deflected through about half the angle, in the same direction?
- A2H+ ions
- B3H+ ions
- C1H− ions
- Dneutrons
An ion X2+ contains 25 electrons and 32 neutrons. What is the nucleon number of X?
- A59
- B57
- C52
- D61
Which pair of atoms contains the same number of neutrons? (Proton numbers: C = 6, N = 7, O = 8, Cl = 17, Ar = 18, Ca = 20.)
- A14C and 16O
- B14C and 14N
- C35Cl and 37Cl
- D40Ar and 40Ca
Quiz key
Protons (+1) and neutrons (0) make up the nucleus, so it holds all the positive charge, yet it is about ten thousand times narrower than the atom. Electrons do have a small mass (1/1836), so the nucleus does not hold every particle with mass. Neutron and proton numbers are often unequal (uranium-238 has 92 protons and 146 neutrons), and the nuclear charge equals the proton number, not the nucleon number.
Neutrons = nucleon number − proton number = 238 − 92 = 146. The value 92 is the number of protons (and electrons in the atom), 238 is the total number of nucleons, and 330 comes from adding the two numbers instead of subtracting.
16 protons means sulfur; 16 + 18 = 34 nucleons; 16 − 18 = −2, so the species is 34S2−. A neutral 34S atom would have only 16 electrons. Choosing argon mistakes the 18 electrons for the proton number, but the element is fixed by the 16 protons. 32S2− would have only 16 neutrons.
The neutral atoms supply 16 + 4 × 8 = 48 electrons, and a 2− charge means two extra electrons: 48 + 2 = 50. The value 48 ignores the charge, 46 subtracts the charge as if the ion were positive, and 98 is roughly the relative formula mass, not an electron count.
16O2− has 8 neutrons and 8 + 2 = 10 electrons, so electrons outnumber neutrons. 19F− has 10 neutrons and 10 electrons (equal), 23Na+ has 12 neutrons and only 10 electrons, and 24Mg has 12 of each.
1H+ has charge/mass = 1/1. 2H+ has charge +1 and mass 2, so 1/2: half the deflection, towards the same (negative) plate. 3H+ gives 1/3, a third of the angle. 1H− has the same size of charge/mass but bends towards the opposite plate. Neutrons are uncharged and pass straight through.
A 2+ ion has two fewer electrons than protons, so Z = 25 + 2 = 27 (cobalt). Nucleon number = 27 + 32 = 59. The value 57 adds the electrons to the neutrons, 52 adds protons to electrons, and 61 wrongly adds the charge on top of protons and neutrons.
14C has 14 − 6 = 8 neutrons and 16O has 16 − 8 = 8, so they match. 14C and 14N share a nucleon number but have 8 and 7 neutrons. 35Cl and 37Cl are isotopes with 18 and 20 neutrons. 40Ar has 22 neutrons but 40Ca has 20.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.