CHEMISTRY · Stage 9 · Chapter 11: Patterns in the Periodic Table
Chemistry 11.4 The periodic table: Group 1 — Alkali Metals Physical Trends
5 min read 5 parts Lesson 4 of 7
Lesson summary
11.4 The periodic table: Group 1 — Alkali Metals Physical Trends — Alhussan International Academy — Chemistry (Stages 7–9) notes: Give the position of Group 1, describe and explain the pattern in its melting points and use data to describe trends.Position — Group 1 is the column …
- Boiling point
- Compared with other metals
- Down the group
- Attraction
- Secondary source
- Conclusion
Part 1 of 5Position and general properties
1Position1 point
— Group 1 is the column on the left of the periodic table.
- The elements are lithium, sodium, potassium, rubidium, caesium and francium.
2Metals1 point
— like other metals, the Group 1 elements conduct electricity and are shiny when freshly cut.
- A freshly cut surface tarnishes quickly in air.
3Softness
— they are softer than most other metals and can be cut with a knife.
4Uses
— lithium is used in the batteries of electric cars, and Bolivia has huge reserves of lithium in salt flats.
Part 2 of 5Physical trends down the group
| Metal | Melting point (°C) | Boiling point (°C) | Density (g/cm3) |
|---|---|---|---|
| Lithium | 181 | 1330 | 0.53 |
| Sodium | 98 | 890 | 0.97 |
| Potassium | 63 | 774 | 0.86 |
| Rubidium | 39 | 688 | 1.53 |
- Melting point — it decreases from lithium at the top to caesium at the bottom.
- Caesium melts at about 29 °C, close to room temperature on a warm day.
- Boiling point — it also decreases down the group.
- Softness — the metals get softer down the group.
- Compared with other metals — melting points are low compared with a typical transition metal such as iron (1538 °C).
Part 3 of 5Explaining the melting point pattern
- Structure — Group 1 elements have giant metallic structures in which each atom loses its one outer electron to form a positive ion.
- The electrons move around between the ions.
- Down the group — the outer electron gets further from the nucleus and the positive ions get bigger.
- Attraction — the electrostatic attraction between positive ions and negative electrons gets weaker.
- Result — as attractions get weaker, ions leave their fixed positions more easily, so the melting point decreases.
Part 4 of 5Presenting data and drawing conclusions
- Secondary source — Banjeet could not measure melting points himself, so he chose a trustworthy secondary source such as a reference table.
- Evaluate a source by asking whether it is a reliable textbook or scientific database.
- Table — the independent variable (the element) goes in the left column.
- Bar chart — it is used because the independent variable, the element, is categoric (described by words).
- Conclusion — from lithium at the top to caesium at the bottom, melting point decreases.
- Quote values: lithium melts at 181 °C and rubidium at 39 °C, a fall of 142 °C.
- Predicting — caesium should melt below rubidium, and interpolation within the data is more reliable than extrapolation beyond it.
Part 5 of 5Density
- Patternoverall density increases from lithium at the top to caesium at the bottom, because the atoms become heavier.
- Exceptionpotassium does not fit the pattern.
- Floatinglithium, sodium and potassium are less dense than water and float, whereas rubidium and caesium are denser and sink.
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