From Dobereiner's triads to Mendeleev's gaps and Moseley's atomic numbers: how the periodic table was built, how new elements are named, and how the s, p, d and f blocks follow from electron configurations.
3 lessons, about 1 h 25 min3 simulations11 problems5 practice questionsPython in every lesson
By 1865 chemists knew 63 elements and needed order. Dobereiner found triads, Newlands an octave pattern, and Lothar Meyer a periodic curve of physical properties. Mendeleev arranged the elements by atomic mass and by their properties, left gaps for unknown elements, and predicted their properties. Gallium and germanium proved him right.
Moseley found that the frequency of X-rays from an element depends on its atomic number, not its atomic mass. So the modern periodic law says that the properties are a periodic function of the atomic number. The long form has 18 groups and 7 periods. New heavy elements get a temporary IUPAC name built from the digits of the atomic number.
The period of an element is the shell its outermost electrons are in. The block is the subshell that its last electron enters: s, p, d or f. From the outer configuration you can find the group: 1 or 2 for the s-block, 13 to 18 for the p-block, and 3 to 12 for the d-block. Metals fill most of the table, non-metals sit at the top right, and metalloids lie between.
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