How the table is arranged
Today the elements are arranged in order of atomic number (number of protons), not mass.
- A period is a row across. There are 7 periods. Elements in the same period have the same number of electron shells.
- A group is a column down. There are 18 groups. Elements in the same group have the same number of valence electrons, so they have similar chemical properties.
Reading one box
Each box usually shows:
- The symbol, one or two letters. The first is a capital, the second lower case: Co is cobalt, but CO is carbon monoxide.
- The atomic number (whole number, smaller).
- The relative atomic mass (often a decimal, larger).
- The name.
Some symbols come from Latin names: Na (natrium, sodium), K (kalium, potassium), Fe (ferrum, iron), Cu (cuprum, copper), Ag (argentum, silver), Au (aurum, gold), Pb (plumbum, lead).
Metals, non-metals and metalloids
A staircase line runs from boron down to astatine. Metals are to the left, non-metals to the right.
| Property | Metals | Non-metals |
|---|---|---|
| Look | Shiny (lustrous) | Dull |
| State at room temperature | Solid (except mercury) | Many are gases. Bromine is a liquid |
| Conduct heat and electricity | Yes, well | No (except graphite, a form of carbon) |
| Bend or hammer into shape | Yes: malleable and ductile | No, solids are brittle |
| Ions formed | Positive | Negative |
Metalloids such as silicon and germanium sit along the staircase and have some properties of both. Silicon is a semiconductor, which is why it is used in computer chips.
Families of elements
| Group | Family name | Valence electrons | Behaviour |
|---|---|---|---|
| 1 | Alkali metals (Li, Na, K) | 1 | Soft, very reactive, react with water to make hydrogen and an alkali. Stored under oil |
| 2 | Alkaline earth metals (Mg, Ca) | 2 | Reactive, but less than group 1 |
| 3 to 12 | Transition metals (Fe, Cu, Zn, Ag) | varies | Hard, strong, high melting points, coloured compounds, used as catalysts |
| 17 | Halogens (F, Cl, Br, I) | 7 | Reactive non-metals, exist as pairs (), form salts with metals |
| 18 | Noble gases (He, Ne, Ar) | full shell | Unreactive gases. Used in signs and light globes |
- Atomic number
- Mendeleev left gaps
- Rows
- Same number of shells
- Columns
- Same valence electrons
- Similar properties
- Metals: left
- Non-metals: right
- Metalloids: staircase
- Group 1: alkali metals
- Group 2: alkaline earth
- Group 17: halogens
- Group 18: noble gases
Quick summary
- Elements are arranged by atomic number. Mendeleev made the first useful table and left gaps for undiscovered elements.
- Periods are rows (same number of shells). Groups are columns (same valence electrons, similar properties).
- Metals are on the left, non-metals on the right, metalloids on the staircase.
- Group 1 alkali metals, group 2 alkaline earth metals, groups 3 to 12 transition metals, group 17 halogens, group 18 noble gases.
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