Development of the periodic table

What were the 2 obvious ways to categorise elements in the early 1800’s?

1. Their physical and chemical properties

2. Their atomic weight

Why were elements organised by their atomic weight in the early 1800’s?

• Scientists had not discovered protons or the atomic numbers

• They could only measure atomic weight

• When this was done the periodic pattern was noticed between the properties of the elements ( where the name ‘periodic table’ came from )

• The table was incomplete and some elements were placed incorrectly because it didn’t take into account their physical or chemical properties

How did Dimitri Mendeleev overcome the problems of old periodic table?

• rearranged 50 elements

• Mainly in order of atomic weight

• Switched the order if their properties meant it should be switched

Why were their gaps in his periodic table and what did they lead to?

• Gaps were left to ensure the elements with similar properties stayed int he same groups

• Some of these properties indicated undiscovered elements

• This allowed Dimitri to predict what their properties might be

What later discovery confirmed Dimitri was right to not place elements in a strict order?

The discovery of isotopes-ensured different masses of the same elements were not placed in the wrong order because of their weight when their properties didn’t change