Why did chemists need a table? In this course we follow the story from Dobereiner's triads and Newlands' octaves to Mendeleev's table with its gaps, and then to the modern table, which is arranged by atomic number.
3 lessons, about 1 h 15 min3 simulations10 problems6 practice questionsPython in every lesson
Created by StemPy LabChemists looked for patterns in the elements. Dobereiner found groups of three with similar properties, called triads, where the middle mass is near the average of the other two. Newlands found that every eighth element is similar to the first. Both ideas worked only for a few elements.
In 1869 Mendeleev arranged the elements by atomic mass, in groups and periods, so that elements with similar properties were in the same group. He left gaps for elements that were not yet found, and he predicted their properties. The elements that were found later matched his predictions.
The modern periodic law says that the properties of elements repeat in order of atomic number. The modern table has 18 groups and 7 periods. The period is the number of shells, and elements in a group have the same number of outer electrons.
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