Development of period table & modern periodic table

Development

• In early 1800s elements were arranged in order of atomic weight (relative atomic mass)- some elements were in the wrong groups because properties weren’t taken into account

• Dmitri Mendeleev 1869

  • put the elements in order of atomic weight but switched the order if properties meant it should be changed.

  • Left gaps in the table to make sure that elements with similar properties stayed in same group, indicated that not all elements had been discovered.

Modern

• Elements laid out in order of increasing atomic (proton) number. There are repeating patterns in properties of elements

• Vertical columns are called groups

• Group number tells you how many electrons are in the outer shell