unit for K-lines, in Moseley's work) to the calculated electric charge of the nucleus, i.e. the element number Z. Among other things, Moseley demonstrated that the lanthanide series (from lanthanum to lutetium inclusive) must have 15 members--no fewer and no more--which was far from obvious from known chemistry at that time.

Missing elements

After Moseley's death in 1915, the atomic

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