In 1864, John Newlands arranged the known elements in order of relative atomic mass. He noticed that every eighth element had similar properties. He did not leave any gaps, so some elements with very different properties were placed together. Most scientists rejected his arrangement.
In 1869, Mendeleev also used relative atomic mass, but he left gaps and changed the order of some pairs of elements. He predicted an element that he called eka-aluminium. This element was discovered in 1875 and named gallium.
The table compares Mendeleev's predictions with the properties of gallium.
| Property | Predicted for eka-aluminium | Measured for gallium |
|---|---|---|
| relative atomic mass | 68 | 69.7 |
| density in g cm−3\mathrm{g\,cm^{-3}}gcm−3 | 5.9 | 5.9 |
| melting point | low | 30 °C |
| formula of oxide | Ea2O3\mathrm{Ea_{2}O_{3}}Ea2O3 | Ga2O3\mathrm{Ga_{2}O_{3}}Ga2O3 |
Give one similarity between Newlands' arrangement and Mendeleev's periodic table.
Calculate the percentage error in Mendeleev's prediction of the relative atomic mass of gallium. Use the equation:
percentage error=measured value−predicted valuemeasured value×100\displaystyle \text{percentage error} = \frac{\text{measured value} - \text{predicted value}}{\text{measured value}} \times 100percentage error=measured valuemeasured value−predicted value×100
Give your answer to two significant figures.
Explain why Mendeleev's table was accepted by scientists but Newlands' arrangement was not.
Evaluate Mendeleev's periodic table as a way of arranging the elements. Include its strengths and its limitations.
50 exam-style questions on Edexcel GCSE Chemistry 2.2 The periodic table, covering 2.2.1 Mendeleev's periodic table, 2.2.2 Periods, groups, metals and non-metals, and 2.2.3 Electronic configuration and position in the periodic table. Each one has a worked solution and a mark scheme showing where the marks go.