How we learned what’s inside atoms
| Year | Scientist | Discovery | Model |
|---|---|---|---|
| ~1808 | John Dalton | Atoms are indivisible spheres; each element has its own kind | Solid sphere |
| 1897 | J. J. Thomson | Discovered the electron | "Plum pudding": electrons in a positive sphere |
| 1911 | Ernest Rutherford | Gold foil experiment: most alpha particles passed through, a few bounced back | Tiny dense positive nucleus; mostly empty space |
| 1913 | Niels Bohr | Electrons in fixed energy levels | Planetary model |
| 1932 | James Chadwick | Discovered the neutron | |
| 1920s | Schrödinger, Heisenberg | Electrons described by probability clouds (orbitals) | Quantum mechanical model |
Rutherford’s experiment was done in Manchester, after he had spent several years doing Nobel-winning research at McGill University in Montreal.
Atomic structure
| Particle | Symbol | Charge | Relative mass | Location |
|---|---|---|---|---|
| Proton | p⁺ | +1 | 1 | Nucleus |
| Neutron | n⁰ | 0 | 1 | Nucleus |
| Electron | e⁻ | −1 | 1/1836 | Orbitals around nucleus |
- Atomic number (Z) = number of protons
- Mass number (A) = protons + neutrons
- Neutrons = A − Z
Notation: ¹²₆C means mass number 12, atomic number 6 (6 protons, 6 neutrons).
Isotopes
Isotopes are atoms of the same element with different numbers of neutrons.
| Isotope | Protons | Neutrons | Notes |
|---|---|---|---|
| Carbon-12 | 6 | 6 | About 99% of carbon |
| Carbon-13 | 6 | 7 | Stable, about 1% |
| Carbon-14 | 6 | 8 | Radioactive; used in radiocarbon dating |
The atomic mass on the periodic table is a weighted average of all naturally occurring isotopes. That’s why chlorine’s is 35.45 (about 76% Cl-35 and 24% Cl-37).
Electron configuration
Electrons occupy energy levels (shells) and sublevels (s, p, d, f), filling the lowest energy first.
| Sublevel | Max electrons |
|---|---|
| s | 2 |
| p | 6 |
| d | 10 |
| f | 14 |
Filling order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p…
Electron configuration examples:
- Oxygen (8): 1s² 2s² 2p⁴
- Sodium (11): 1s² 2s² 2p⁶ 3s¹ (or [Ne] 3s¹)
- Chlorine (17): 1s² 2s² 2p⁶ 3s² 3p⁵
Valence electrons are in the outermost shell. For main-group elements, the number of valence electrons matches the last digit of the group number (group 1: 1; group 17: 7). Elements in the same group have similar chemistry because they have the same number of valence electrons.
Periodic trends
| Trend | Across a period (left → right) | Down a group (top → bottom) | Reason |
|---|---|---|---|
| Atomic radius | Decreases | Increases | Across: more protons pull electrons closer. Down: more shells. |
| Ionization energy | Increases | Decreases | Across: electrons held more tightly. Down: outer electrons farther and shielded. |
| Electronegativity | Increases | Decreases | Same reasons as ionization energy |
| Metallic character | Decreases | Increases | Metals lose electrons more easily lower and further left |
- Fluorine is the most electronegative element (3.98 on the Pauling scale).
- Francium and cesium are the least electronegative and most metallic.
- Noble gases have very high ionization energies and full outer shells, which is why they’re so unreactive.
Ions
- Metals lose electrons to form cations (positive ions): Na → Na⁺ + e⁻
- Non-metals gain electrons to form anions (negative ions): Cl + e⁻ → Cl⁻
- Cations are smaller than their atoms; anions are larger.
Why trends matter
Periodic trends let chemists predict how elements will behave without testing each one. Group 1 metals get more reactive going down because their single valence electron is easier to remove. Potassium reacts with water more violently than sodium, and cesium even more so.
Common mistakes
- Thinking atoms get bigger going across a period. They get smaller.
- Confusing mass number and atomic mass. Mass number is a whole number for one isotope; atomic mass is an average.
- Forgetting that 4s fills before 3d.
Try it yourself
- How many protons, neutrons and electrons are in ³⁹₁₉K?
- Write the electron configuration for magnesium (Z = 12).
- Rank Na, Cl and S by increasing atomic radius.
- Which has higher ionization energy, lithium or cesium? Why?
(Answers: 19 p, 20 n, 19 e; 1s² 2s² 2p⁶ 3s²; Cl < S < Na; lithium, because its outer electron is closer to the nucleus)
Quick recap
- Atomic models evolved from Dalton’s spheres to quantum orbitals.
- Atomic number = protons; mass number = protons + neutrons.
- Isotopes differ in neutrons.
- Electron configuration determines valence electrons and chemistry.
- Radius decreases across and increases down; ionization energy and electronegativity do the opposite.