Building Blocks for Understanding: The Periodic Table — AQA Combined Science: Synergy
This topic covers how the periodic table is organised and the properties of metals, non-metals and key groups.
Atomic number and the periodic table
Elements are arranged in order of increasing atomic number.
- Groups are vertical columns; elements share the same number of outer-shell electrons and have similar chemical properties.
- Periods are horizontal rows; the period number gives the number of occupied electron shells.
Dmitri Mendeleev arranged elements and left gaps for undiscovered elements, even predicting their properties — the breakthrough that led to the modern table. The discovery of isotopes later explained why ordering by atomic number works.
Metals and non-metals
- Metals (left and centre) lose electrons to form positive ions.
- Non-metals (right) tend to gain or share electrons.
Metals are typically shiny, conductive, malleable and have high melting points; non-metals are typically dull, poor conductors and have lower melting points.
Group 0 — the noble gases
- Have a full outer shell, so they are unreactive (inert).
- Exist as single atoms.
- Boiling points increase down the group as atoms get larger.
Group 1 — the alkali metals
- Have one outer-shell electron.
- React with water to produce hydrogen + a metal hydroxide (an alkali).
- Reactivity increases down the group — the outer electron is further from the nucleus and lost more easily.
Group 7 — the halogens
- Have seven outer-shell electrons; exist as diatomic molecules (e.g. Cl₂).
- Reactivity decreases down the group — it gets harder to gain an electron as atoms get bigger.
- A more reactive halogen displaces a less reactive one from a solution of its salt.
- Melting and boiling points increase down the group.
Exam tips
- Explain reactivity trends using the distance of the outer electron from the nucleus.
- Group 1 reactivity increases down the group; Group 7 reactivity decreases down the group.
- Noble gases are unreactive because of their full outer shell.
- Know why Mendeleev's table was a breakthrough (gaps and predictions).