Trends across and down the periodic table
The properties of elements change in a regular way across a period and down a group. This is why the periodic table is useful: from the position of an element we can predict how it behaves.
| Property | Across a period (left to right) | Down a group |
|---|---|---|
| Atomic radius (size) | Decreases: more protons pull the shells in | Increases: more shells |
| Metallic character | Decreases | Increases |
| Ionisation energy (energy to remove an electron) | Increases | Decreases |
| Electronegativity (pull on shared electrons) | Increases | Decreases |
| Oxides | Basic → amphoteric → acidic | More basic |
Electronic configuration and position in the periodic table
The period of an element is the number of shells that contain electrons. The group is the number of outer electrons (group 1 to group 8; the noble gases have a full shell). Elements in the same group have the same number of outer electrons, so they react in similar ways.
| Group | Name | Outer electrons | Typical behaviour |
|---|---|---|---|
| 1 | Alkali metals (Li, Na, K) | 1 | Soft, very reactive; react with water to make hydrogen and an alkali; reactivity increases down the group |
| 2 | Alkaline earth metals (Mg, Ca) | 2 | Less reactive than group 1 |
| 7 | Halogens (F, Cl, Br, I) | 7 | Reactive non-metals; reactivity decreases down the group |
| 8 | Noble gases (He, Ne, Ar) | 8 (2 for He) | Unreactive, full outer shell |
Try itPull of the nucleus across period 3
Choose an element from sodium (Z = 11) to argon (Z = 18). The ten inner electrons shield the nucleus, so the outer electrons feel a net positive charge about Z − 10.
Key terms
- Ionisation energy
- The energy needed to remove one electron from an atom.
- Electronegativity
- How strongly an atom attracts the electrons in a bond.
- Alkali metals
- The group 1 metals.
- Halogens
- The group 7 non-metals.
Summary
- Across a period atomic size and metallic character decrease; ionisation energy and electronegativity increase.
- Down a group atomic size and metallic character increase; ionisation energy decreases.
- Period = number of shells; group = number of outer electrons.
- Group 1 reactivity increases downwards; group 7 reactivity decreases downwards.