There are 118 known elements, from the hydrogen in water to the gold in a medal. Learning each one separately would be overwhelming. Instead, chemists use the periodic table, a chart that arranges the elements so that ones with similar properties line up together. It works a bit like a grocery store: if you know where the cereal aisle is, you know where to find every cereal. If you know where an element sits on the table, you can predict a lot about how it behaves.
In 1869 the Russian chemist Dmitri Mendeleev wrote the properties of the roughly 63 elements known at the time on cards and arranged them in order of atomic mass. He noticed that similar properties repeated at regular intervals, so he placed similar elements in the same column. Boldly, he left gaps where no known element fit and predicted the properties of the missing elements. When germanium was discovered in 1886, its properties matched his predictions closely, which convinced other scientists his table was right. Today the table is arranged in order of atomic number (number of protons), an improvement made by Henry Moseley in 1913.
Each box on a full periodic table usually gives the atomic number, the symbol (one or two letters, the first always a capital, such as C for carbon or Na for sodium, from its Latin name natrium), the name and the atomic mass. A horizontal row is called a period; there are 7 periods. A vertical column is called a group or family; there are 18 groups, numbered 1 to 18 from left to right. Elements in the same group have similar chemical properties.
Metals, non-metals and metalloids
| Property | Metals | Non-metals | Metalloids |
|---|---|---|---|
| Where on the table | Left of the staircase line (most elements) | Right of the staircase line, plus hydrogen | Along the staircase line: boron, silicon, germanium, arsenic, antimony, tellurium |
| State at room temperature | Solid, except mercury (liquid) | Many are gases; some solids; bromine is a liquid | Solid |
| Appearance | Shiny (lustrous) | Usually dull | Can be shiny or dull |
| When hammered | Malleable (flattens) and ductile (can be drawn into wire) | Solids are brittle (shatter) | Brittle |
| Conductivity | Good conductors of heat and electricity | Usually poor conductors (insulators) | Semiconductors: conduct a little, and the amount can be controlled |
| Examples | Iron, copper, aluminium, gold, sodium | Oxygen, nitrogen, carbon, sulfur, chlorine | Silicon (computer chips), boron |
Chemical families
| Family | Group | Members | Properties | Uses |
|---|---|---|---|---|
| Alkali metals | 1 | Lithium, sodium, potassium, rubidium, cesium, francium | Soft enough to cut with a knife; react with water, more and more violently going down the group, giving off hydrogen gas; stored under oil | Lithium in rechargeable batteries; sodium compounds in table salt |
| Alkaline earth metals | 2 | Beryllium, magnesium, calcium, strontium, barium, radium | Reactive, but less than the alkali metals; harder | Magnesium in fireworks and light alloys; calcium compounds in bones, chalk and cement |
| Halogens | 17 | Fluorine, chlorine, bromine, iodine, astatine | Very reactive non-metals; form salts with metals; at room temperature fluorine and chlorine are gases, bromine is a liquid and iodine is a solid | Chlorine to disinfect drinking water and pools; fluoride in toothpaste; iodine as an antiseptic |
| Noble gases | 18 | Helium, neon, argon, krypton, xenon, radon | Colourless gases; very unreactive (only a few compounds of the heavier ones, such as xenon, are known) | Helium in balloons; neon in signs; argon in light bulbs and as a shielding gas in welding |