4.1.5Predicting and Identifying Reactions and Products
Predicting Reactivity — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 6 of 7
Part 1 of 4Reactivity patterns in the periodic table
Metals in Groups 1 and 2
- Metals in Groups 1 and 2 These are the most reactive metals, and their reactivity increases going down each group.
- Their atoms lose their outer electrons more easily the further those electrons are from the nucleus.
Non-metals in Group 7
- Non-metals in Group 7 The halogens are reactive non-metals, but their reactivity decreases going down the group.
- A larger atom attracts an extra electron less strongly, so gaining one is harder.
Group 0
- Group 0 The noble gases have full outer shells and are generally unreactive.
Compound formation
- Compound formation Reactive metals combine with reactive non-metals to form ionic compounds, for example sodium and chlorine give sodium chloride.
Part 2 of 4Group 1 compared with transition metals
| Property | Group 1 metals | Transition metals |
|---|---|---|
| Hardness and strength | Soft and easy to cut | Hard and strong |
| Density | Low; the first three float on water | High |
| Melting point | Low, for example potassium melts at about 64 °C | High, for example titanium melts at about 1700 °C |
| Ion charge | Always 1+ | Variable, for example Fe2+ and Fe3+ |
| Reactivity | Very high; react quickly with water, oxygen and halogens | Much lower; react slowly or not at all |
Part 3 of 4Reaction with oxygen: a case study
1Cut sodium or potassium
- Cut sodium or potassium The freshly cut surface is bright and shiny.
2Oxygen in the air reacts
- Oxygen in the air reacts The metal combines with oxygen to form a metal oxide on the surface.
3Dull coating within seconds
- Dull coating within seconds The shine disappears because the surface is now covered by the dull oxide.
4Compare with iron
- Compare with iron Iron takes weeks to form rust and needs water as well as oxygen, which shows the transition metal is far less reactive.
Part 4 of 4Using the patterns to identify an element
1
Look at the clues given: a soft metal that floats on water and reacts violently suggests Group 1.
2
Use the ion charge to decide the group: an element that only ever forms 1+ ions is an alkali metal, while variable charges point to a transition metal.
3
Compare the data with neighbouring elements in the same group and extend the trend, for example a melting point that should lie between two known values.
4
State your prediction and support it with the pattern, such as increasing reactivity down the group.
5
Remember that unreactive gases with full outer shells belong to Group 0.
Key terms
| Term | Meaning |
|---|---|
| Transition metal | A metal in the central block of the periodic table, typically hard, dense and with variable ion charges. |
| Alkali metal | A very reactive, soft Group 1 metal that forms 1+ ions. |
| Halogen | A reactive non-metal in Group 7. |
| Variable charge | The ability of an element to form ions with more than one possible charge, as iron does. |
| Ionic compound | A compound formed when a metal transfers electrons to a non-metal, forming oppositely charged ions. |
Avoid these mistakes
1
Assuming reactivity increases down every group: it increases down Groups 1 and 2 but decreases down Group 7.
2
Saying transition metals form only one type of ion: many have more than one charge.
3
Describing transition metals as soft and light: that describes Group 1; transition metals are hard and dense.
4
Claiming iron rusts as quickly as sodium tarnishes: rusting is slow and needs both oxygen and water.
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