Reading the table
Rows are periods: each new row starts a new electron shell. Columns are groups, and elements in the same group behave alike because their outer electrons match, which is why sodium and potassium both react with water. The two rows below the table are the lanthanides and actinides, which belong in period 6 and 7; they sit apart only so the table fits on a page.
Where the numbers come from
Atomic weights are the IUPAC abridged standard atomic weights. A number in brackets, like [97] for technetium, means the element has no stable isotopes and shows the mass number of its longest-lived isotope instead. Electronegativity uses the Pauling scale. Melting and boiling points are at normal atmospheric pressure. For the heaviest elements only a few atoms have ever been made, so most of their properties are unknown, and their electron configurations are predictions.
Search and colours
Type a name, a symbol or an atomic number to pick out an element, or a word like “noble” or “halogen” to light up a whole family. Colour by state to see the two elements that are liquid at room temperature, bromine and mercury, or by block to see how the s, p, d and f orbitals shape the table.