The Periodic Table
Arrangement and periodic trends
Elements are arranged in order of proton number. Vertical columns are groups: elements in a group have the same number of outer electrons and similar reactions. Horizontal rows are periods: elements in a period have the same number of electron shells. Metals are on the left, non-metals on the right; across a period, elements change from metallic to non-metallic.
≈ 30 minGroup I and group VII
Group I (alkali metals): soft, low-density metals that react with water to make an alkaline hydroxide and hydrogen. Reactivity increases down the group because the outer electron is further from the nucleus and lost more easily. Group VII (halogens): diatomic non-metals (, , , ) that get darker and less reactive down the group. A more reactive halogen displaces a less reactive one from its salt.
≈ 40 minTransition elements and noble gases
Transition elements (the central block, e.g. Fe, Cu, Ni, Cr) are hard, dense metals with high melting points. They form coloured compounds, have ions with variable oxidation numbers (iron(II) and iron(III)) and are often catalysts (iron in the Haber process). Noble gases (group VIII/0) have full outer shells, so they are unreactive and exist as single atoms; helium is used in balloons, argon in light bulbs and welding.
≈ 25 min