EasyChemistryHigh School (9–10)GeneralThe Periodic Table › Development of the Periodic TableNumerical

The atomic masses of potassium and caesium are 39.1 u and 132.9 u.

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The atomic masses of potassium and caesium are $39.1\ \text{u}$ and $132.9\ \text{u}$ . Use Dobereiner's rule to estimate the atomic mass of rubidium, which lies between them. The real mass is $85.5\ \text{u}$ . By what percentage of the real mass is your estimate off?

Need a reminder? The key ideaShow

Dobereiner: in a triad of three similar elements, the middle atomic mass is close to the average of the other two. Newlands: in order of atomic mass, every eighth element is like the first, which worked only up to calcium.

The worked solution

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The atomic masses of potassium and caesium are 39.1 u and 132.9 u.

The worked solution

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