All matter is made of atoms. A substance made of only one kind of atom is an element; 118 are known. Atoms join to form molecules. Inside every atom are electrons, protons and neutrons. Arranging elements by their protons gave us the periodic table. Exams ask the scientist behind each discovery, isotopes and their uses, and the families of the table.
On this page you will learn:
- Elements, Latin symbols and the laws of Lavoisier and Proust
- Electron, proton and neutron, and the atomic models
- How atoms join: ionic and covalent bonds
- Isotopes, radioactivity and carbon dating
- The periodic table: alkali metals, halogens, noble gases
Elements, symbols and the first laws
Rule in one line: in a chemical reaction, mass is neither made nor lost.
| Element | Symbol | From the Latin |
|---|---|---|
| Sodium | Na | Natrium |
| Potassium | K | Kalium |
| Iron | Fe | Ferrum |
| Copper | Cu | Cuprum |
| Silver | Ag | Argentum |
| Gold | Au | Aurum |
| Lead | Pb | Plumbum |
| Mercury | Hg | Hydrargyrum |
| Tin | Sn | Stannum |
A mole is a fixed count of particles: 6.022 × 10²³, the Avogadro number — just as a dozen means 12. One mole of a substance weighs its formula mass in grams. One mole of water (H2O, 18 u) weighs 18 g.
Inside the atom, and how atoms join
Rule in one line: protons and neutrons sit in the nucleus; electrons move round it in shells.
| Particle | Charge | Found by | Year |
|---|---|---|---|
| Electron | Negative | J. J. Thomson (cathode rays) | 1897 |
| Proton | Positive | E. Goldstein found canal rays; Rutherford named the proton | 1886 |
| Neutron | None | James Chadwick | 1932 |
- 1897Thomson's model
Electrons stuck in a ball of positive charge, like seeds in a watermelon.
- 1911Rutherford's model
Gold-foil experiment: the atom is mostly empty, with a tiny, heavy, positive nucleus.
- 1913Bohr's model
Electrons move in fixed shells (K, L, M, N) without losing energy.
- 1932Chadwick
Finds the neutron in the nucleus.
| Element | Z | Shells K, L, M, N |
|---|---|---|
| Hydrogen | 1 | 1 |
| Carbon | 6 | 2, 4 |
| Oxygen | 8 | 2, 6 |
| Sodium | 11 | 2, 8, 1 |
| Chlorine | 17 | 2, 8, 7 |
| Argon | 18 | 2, 8, 8 |
Isotopes and radioactivity
Rule in one line: isotopes have the same protons but different neutrons.
1 proton, no neutron. Almost all hydrogen.
1 proton, 1 neutron. Makes heavy water, used in nuclear reactors.
1 proton, 2 neutrons. Radioactive; it is not a mineral.
The three isotopes of hydrogen. Mineral water carries minerals such as calcium, magnesium and potassium — never tritium.
Some atoms are unstable. They break down on their own and give out rays — this is radioactivity, found by Henri Becquerel in 1896 from uranium salts. Living things take in a little radioactive carbon-14. After death no new carbon-14 comes in, and the old amount halves every 5,730 years. Measuring what is left tells the age of bones, wood and other remains of once-living things. This is radiocarbon dating.
The periodic table
Rule in one line: the modern table arranges elements by increasing atomic number.
- 1817Döbereiner's triads
Groups of three alike elements; the middle mass is the average of the other two.
- 1866Newlands' octaves
Every eighth element is like the first, as in music; works only up to calcium.
- 1869Mendeleev's table
By atomic mass; left gaps for elements not yet found, such as gallium and germanium.
- 1913Moseley
Showed atomic number is the true basis — the modern periodic law.
| Trend | Across a period (left to right) | Down a group |
|---|---|---|
| Atomic size | Decreases | Increases |
| Metallic character | Decreases | Increases |
| Valence electrons | Rise from 1 to 8 | Stay the same |
Families to know: halogens and noble gases
Rule in one line: elements in the same group behave alike, because they have the same number of outer electrons.
Li, Na, K: soft, very reactive, kept under kerosene.
Be, Mg, Ca: magnesium is in chlorophyll, calcium in bones.
F, Cl, Br, I: 'salt-makers'. Bromine is the only liquid non-metal.
He, Ne, Ar: full outer shell, so they hardly react.
The four families asked most often
| Noble gas | Common use |
|---|---|
| Helium | Balloons (light, does not burn); mixed with oxygen for patients and deep-sea divers |
| Neon | Glowing orange-red advertisement signs |
| Argon | Filling electric bulbs, often with nitrogen |
Traps and practice
Last revision — where the options try to fool you.
- Conservation of mass: Lavoisier, not Boyle or Dalton.
- Thomson: electron; Rutherford: nucleus; Chadwick: neutron; Bohr: shells; Becquerel: radioactivity.
- Tritium is a radioactive isotope of hydrogen, not a mineral.
- Carbon dating is for things that were once alive — not rocks or soil.
- H2 has a covalent bond; NaCl has an ionic bond.
- Mercury is a liquid metal but not an alkali metal; bromine is the liquid non-metal.
- Mendeleev used atomic mass; the modern table uses atomic number.
An atom has 11 protons and 12 neutrons. Find its atomic number, mass number and the element.
Show answer
Atomic number 11; mass number 11 + 12 = 23; the element is sodium.
- Law of conservation of mass
- Lavoisier
- Discovered the electron
- J. J. Thomson
- Discovered the neutron
- James Chadwick
- Discovered radioactivity
- Henri Becquerel
- Radioactive isotope of hydrogen
- Tritium
- Dates fossils
- Carbon-14
- F, Cl, Br, I
- Halogens
- Gas in filament bulbs
- Nitrogen or argon
- Lavoisier: conservation of mass; Proust: constant proportions
- Electron 1897 Thomson; neutron 1932 Chadwick
- Z = protons; A = protons + neutrons
- Hydrogen isotopes: protium, deuterium, tritium
- Carbon-14 half-life about 5,730 years
- Group 1 alkali metals; 17 halogens; 18 noble gases
- Transition metal compounds are coloured