Development of the periodic table

Question 1

Until quite recently, what were the two obvious ways to categorise elements ?

Answer 1

• Physical and chemical properties
• atomic weight

Question 2

Why were elements arranged in atomic weight ?

Answer 2

• scientist had no idea of atomic structure or of protons, electron or neutrons ( 20th century when elements arranged in atomic number )
• known elements arranged in atomic weight - and see that there’s a periodic patten = periodic table
• Early periodic table was not complete and some elements placed in wrong group because elements placed in order of atomic weight and not take into account their properties

Question 3

How did Mendeleev overcome some of his problems of the early periodic table ?

Answer 3

taking 50 known elements and arranging them into his table of elements with various gaps

Gaps were left to ensure elements with similar properties stayed in the same groups. Some of these gaps indicated existence of undiscovered elements and allowed Mendeleev to predict what their properties might be. When they were found and fitted the pattern it supported Mendeleev’s idea.

For example, Mendeleev made really good prediction about chemical, physical property of ekasilicon Which we know today as Germanium

Question 4

How did Mendeleev arrange elements ?

Answer 4

mainly in order of atomic weight but switched order if properties meant it should be changed.

Example of Te and I.
Iodine has smaller atomic weight and placed after tellurium as it has similar properties to that group.

Question 5

What discovery confirmed that Mendeleev was correct not to place elements in a strict order of atomic weight but also to take account of their properties ?

Answer 5

The discovery of isotopes

Isotope of same element have different masses but have the same chemical properties to occupy the same position on the periodic table