Mendeleev's gamble
Early tables ordered elements by atomic weight, and some ended up in the wrong groups. Mendeleev swapped some pairs so that elements with similar properties lined up, and left gaps for elements not yet discovered, predicting their properties. When gallium and germanium were found, they fitted.
Ordering by atomic number (protons) and the discovery of isotopes later explained why his swaps were right.
Groups and periods
- Period (row): the number of occupied electron shells.
- Group (column): the number of electrons in the outer shell. Group 0 has a full outer shell.
- Metals are on the left and in the middle; non-metals are on the right. Metals form positive ions; non-metals that form ions form negative ones.
- 2,8,3: three shells, so period 3; three outer electrons, so group 3: aluminium
Explore the table
Open the periodic table. Click sodium, then magnesium and aluminium along period 3: the atomic number goes up by one each time. Then go down Group 1 from lithium: each new period adds a shell.