The Periodic Table
Newlands (1864) arranged elements in atomic weights, but many elements had not been discovered. He noticed periodicity.
Mendeleev (1869) left gaps in his periodic table to allow for the discovery of other elements.
The Modern Periodic Table
Modern periodic table is arranged in terms of electronic structure
Rows = Periods - energy level is filled as you go across the table.
Columns = Groups - with more energy levels of electrons as go down the the table. All elements in a group have same number of electrons in outer shell, e.g. Group 1 = 1 electron is outer shell.
The number of electrons in the outer shell determine it's chemical properties.
Group 1 (Alkali metals) Electron is further away from the influence of the nucleus and is more easily lost.