Safety and evidence boundary
Use supplied notation, numerical models, and approved simulations unless a separately supervised laboratory protocol is provided.
Explicit science instruction
Atomic number Z equals proton number. Changing proton count changes the element. Mass number A equals protons plus neutrons, so neutrons = A − Z.
A neutral atom has equal proton and electron counts. A 2+ ion has two fewer electrons than protons; a 1− ion has one more electron.
Particle accounting should reproduce both the written mass number and ionic charge. Writing the relationships before substituting numbers makes sign errors easier to catch.
Vocabulary: atomic number; mass number; proton; neutron; ion.
Worked example with reasoning
For ²⁷Al³⁺: protons = 13, neutrons = 27−13 = 14, electrons = 13−3 = 10. The counts reproduce the written +3 charge.
Guided question
Determine protons, neutrons, and electrons for ³⁵Cl⁻ and explain which count determines elemental identity.
Independent work
- Complete particle counts for ²³Na, ²⁴Mg²⁺, ¹⁶O²⁻, and ⁴⁰Ca²⁺.
- Write a charge audit for each.
- Explain why neutron changes can preserve element identity while proton changes cannot.
Misconception check
Mass number is not the periodic-table atomic mass; those are different quantities.
Formative understanding check
Can ordinary ion formation turn sodium into neon?
Reveal worked response
No. Ion formation changes electron count, not proton count.
Public self-check only.
Mentor evidence conference
Ask Archaemenes to inspect your particle accounting for sign errors.
Archaemenes may challenge reasoning and help plan corrections, but does not grant official mastery.
Teacher guidance
Foundation: Use a labeled Z/A/charge organizer.
Core: Solve mixed neutral-atom and ion examples with charge audits.
Extended: Create valid notation from supplied particle counts and defend every symbol.