the idea
Three particles, three jobs.
| Particle | Charge | Mass (amu) | Where | What it controls |
|---|---|---|---|---|
| Proton | +1 | ≈ 1 | nucleus | Identity. Atomic number Z. Change it and you have a different element. |
| Neutron | 0 | ≈ 1 | nucleus | Mass and stability. Different counts give isotopes. |
| Electron | −1 | ≈ 0.00055 | around the nucleus | Chemistry. Bonding, charge, and reactivity. Gain or lose them and you have an ion. |
Plant version: think of a lot code. The protons are the SKU, and change it and it's a different product. Neutrons are like a slight fill-weight variation, still the same SKU but heavier or lighter (isotopes). Electrons are the packaging, the outside layer that actually touches other things, which is why they drive all the reactions.
Atomic number (Z)
= protonsMass number (A)
= protons + neutronsNeutrons
= A − ZCharge
= protons − electronstry it
Build an atom
Protons
Neutrons
Electrons
The Bohr model (electrons in rings) is a simplification. It works for counting shells in elements 1–20, which is why the builder stops at calcium. Real electrons live in orbitals, covered in Electron Configuration.
isotopes
Why the periodic table says 35.45 for chlorine
No single chlorine atom weighs 35.45 amu. About 76% are Cl-35 and 24% are Cl-37. The table lists the weighted average, the same way an average fill weight works across a whole run.
Isotope mass (amu)Abundance (%)Contribution
how we got here
Four models, each fixing the last one's mistake
1803
DaltonAtoms are solid, indivisible spheres. Each element has its own kind.1897
ThomsonDiscovered the electron. "Plum pudding": electrons stuck in a positive blob.1911
RutherfordGold foil experiment. Most alpha particles passed straight through and a few bounced back, so the atom is mostly empty space with a tiny dense positive nucleus.1913 → 1926
Bohr → QuantumElectrons sit in fixed energy levels. Schrödinger then replaced orbits with orbitals, regions where an electron is likely to be found.practice
Count the particles
practice
Average atomic mass
exam traps
Common mistakes
- Using the average atomic mass (35.45) as the mass number. A mass number is always a whole number for one specific isotope.
- Changing the proton count to make an ion. Ions gain or lose electrons. Changing protons makes a new element.
- Getting the charge sign backwards. Losing electrons makes a positive ion (Na⁺) and gaining them makes a negative one (Cl⁻). More protons than electrons means positive.
- Forgetting to convert % to a decimal before taking the weighted average.