Periodic Table and the atom

Question 1

What did Mendeleev discover

Answer 1

Mendeleev left gaps for undiscovered elements creating octaves so elements would be in groups with similar chemical properties
When we discovered new elements they fit into the gaps Mendeleev left

Question 2

Groups

Answer 2

Columns of elements with the same number of electrons on the outer shell

Question 3

Periods

Answer 3

Atoms with the same number of electron shell in horizontal rows