MediumChemistryHigh School (11–12)The Periodic Table › Development of the Periodic TableConceptualBoard Exam
Newlands' list, in order of atomic mass, began: H, Li, Be, B, C, N, O, F, Na, Mg, Al, Si, P, S, Cl…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Newlands' list, in order of atomic mass, began: H, Li, Be, B, C, N, O, F, Na, Mg, Al, Si, P, S, Cl, K, Ca, Cr, Ti, Mn, Fe. Show that the octave pattern works for lithium, sodium and potassium. Then explain why chemists rejected it, using chromium and iron.
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
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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 pattern works. Counting from lithium, the eighth element is sodium, and the eighth from sodium is potassium. All three are alkali metals.
Step 2. The pattern fails. Eight places after aluminium comes chromium, a transition metal not at all like aluminium. Eight places after sulfur comes iron, a metal, while sulfur is a non-metal.
Step 3. Why. Newlands left no gaps, and some masses of his time were wrong. After calcium, unlike elements were forced into the same column.
Answer: the octaves hold for light elements up to calcium, but put unlike elements together after that, as with chromium under aluminium and iron under sulfur.
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