EasyChemistryHigh School (11–12)The Periodic Table › Development of the Periodic TableNumericalBoard Exam
Calcium (40.08 u), strontium (87.62 u) and barium (137.33 u) behave alike…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Calcium ( u), strontium ( u) and barium ( u) behave alike. Find the mean of the masses of calcium and barium, and by what percentage strontium differs from it. Is it a good triad?
Need a reminder? The key ideaShow
Dobereiner's triads: the atomic mass of the middle element of three similar elements is close to the mean of the other two. Newlands' law of octaves: every eighth element, in order of atomic mass, is like the first. Mendeleev's periodic law: the properties of the elements are a periodic function of their atomic masses; he left gaps and predicted unknown elements from their neighbours.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The mean. u.
Step 2. The difference. .
Step 3. Judge. Strontium is only below the mean, so the three make a good triad.
Answer: the mean is u, and strontium is below it: a good triad.
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