development of periodic table

• in early 1800s, elements were arranged by atomic weight, so many were placed in the wrong groups as properties were not taken into account
• mendeleev put the elements mainly in atomic weight but did switch that order because of properties
• gaps were left to make sure elements with similar properties stayed in the same group
• some of the gaps were undiscovered elements and mendeleev predicted their properties, when they were found it fitted pattern
• the discovery of isotopes confirmed mendeleev was correct not to place them in strict order of atomic weight