Lesson overview
Inside the atom: What makes sodium sodium?
Take one sodium atom apart a particle at a time: take away an electron, add a neutron, add a proton. Predict each time whether it will still be sodium, then find where the 12th electron goes in magnesium.
Suggested sequence
- Lesson 1 · What makes sodium sodium? Protons, neutrons and electrons; proton number and mass number. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Electrons in shells. Electrons in shells, the outer shell and the Periodic Table. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: What makes sodium sodium?
- Describe the structure of an atom and state the relative charges and masses of a proton, a neutron and an electron
- Use the proton number and the mass number to find the numbers of protons, neutrons and electrons in atoms and ions
Part 2: Electrons in shells
- Determine the electronic configuration of atoms and ions with proton number 1 to 20
- Use the electronic configuration to find the Group and the Period, and state that the noble gases have a full outer shell
Misconceptions to listen for
- “The electron has no mass at all.” — It has a tiny mass, 1/1840 of a proton's. It's so small that we leave it out of the mass number, but it isn't zero.
- “The neutron decides the element, because it's in the nucleus.” — Neutrons are in the nucleus, but adding one only made a heavier sodium atom. Only the number of protons decides the element.
- “The atom has 19 neutrons.” — 19 is the mass number: protons and neutrons together. Take away the 9 protons to find the neutrons.
- “Adding two electrons changes the element, because it changes the charge.” — It changes the charge, from 0 to −2, but the nucleus still has 11 protons. It would be an ion of sodium, not a new element.
- “Adding a neutron changes the element, because the mass number changes.” — The mass number goes from 23 to 24, but the proton number stays 11. In the experiment the atom stayed sodium.
- “The charge is −2, because two electrons are missing.” — Missing electrons leave the protons' positive charge uncancelled. Two more protons than electrons gives +2.
- “The atom is neutral because neutrons cancel the charges.” — Neutrons have no charge, so they can't cancel anything. The atom is neutral because it has as many electrons (−1) as protons (+1).
- “Potassium has 39 electrons.” — 39 is the mass number. The number of electrons in an atom equals the proton number, 19.
- “Every atom has at least one neutron.” — Most hydrogen atoms have none: mass number 1 minus proton number 1 is 0. It's the only kind of atom without neutrons.
- “H+ has 2 electrons, because + means one is added.” — A + charge means fewer electrons than protons. One proton and no electrons gives +1.
Scope
Atomic structure (2.2 Core 1–6): protons, neutrons and electrons with their relative charges and masses, proton number and mass number, the electronic configuration of the first 20 elements and their ions, the full outer shell of the noble gases, and how outer-shell electrons and occupied shells match the group and the period. Isotopes and relative atomic mass, ionic bonding and the trends of the Periodic Table are left to their own modules.