Development of the periodic table

Question 1

In the past, what property would they use to categorize elements?

Answer 1

Their Atomic weight

Question 2

Explain the two things that Dmitri Mendeleev did to change the periodic table

Answer 2

1. He ordered the elements in atomic weight but switched a few elements around In order for them to match with other elements of similar chemical and physical properties.
2. Mendeleev left gaps in the table for undiscovered elements so that when discovered, they could fit in a group with similar properties.
   He also did this to be able to predict their properties before being found.

Question 3

Isotopes: What are they and what does it tell us about the periodic table?

Answer 3

Isotopes are the same elements with different masses.
This tells us That Mendeleev was correct about leaving gaps in the table.