Development of the periodic table

Question 1

What did Mendeleev arrange the periodic table by

Answer 1

-atomic weight
-but also accounted for properties

Question 2

What were elements in the 1800s arranged by

Answer 2

-physical and chemical properties
-atomic weight

Question 3

Why did Mendeleev leave gaps in the periodic table

Answer 3

-to make sure elements with similar properties stayed in the same groups
-he knew there were more elements to discover

Question 4

What was discovered that confirmed Mendeleev theory in the 20th century

Answer 4

Isotopes, isotopes of the same element have different masses so you can’t put them into the periodic table based on that