Early scientists ordered elements by atomic weight (relative atomic mass). Protons had not yet been discovered.
Newlands noticed similar properties at every eighth element: his law of octaves. He left no gaps, so some elements with different properties were grouped together.
Mendeleev mainly used atomic weight, but changed the order where needed to keep elements with similar properties together.
He left gaps for undiscovered elements and predicted their properties from the surrounding elements.
A simplified column showing the gap below aluminium.
Later discoveries, including gallium, matched Mendeleev’s predictions. This gave scientists confidence in his table.
A metal below aluminium.
A low melting point.
A metal below aluminium.
Melts at about 30°C.
Today, elements are ordered by increasing atomic number: the number of protons.
Later discoveries about atomic structure explained why atomic-weight order sometimes needed changing. Isotopes help explain this: an element’s average mass depends on its isotopes, not just its proton number.
Early tables: atomic weight.
Mendeleev: gaps, similar properties and predictions.
Evidence: discoveries matched predictions.
Modern table: atomic number.
Find the pattern. Check the evidence.
Think about how the table improved.
One point for each answer you get right first time.
No timer. Take your time and use a clue whenever you need one.