6. The answer is B, periods.
They represent the number of occupied electron shells of an element. For example, potassium (19 electrons) and calcium (20 electrons) are both on the same period, as they both have 3 occupied electron shells.
7. In his table, he arranged some known elements (which are a lot less comparing to nowadays), into columns and rows. He put the elements with the similar chemical properties into the same column.
8.
A: Atomic number. (the number of protons of that element)
B: The chemical symbol of the element.
C: The name of the element
D: The relative atomic mass of the element
9. Relative atomic mass is the average mass of the isotopes of an element. Each isotope has a different mass as the number of neutrons is different. Since each isotope of an element has a different abundance in nature, it's nearly impossible to have a whole number of relative atomic mass.
For example, Cl-35 has an abundance of around 75.4%, and Cl-37 has an abundance of 24.6%.
So the relative atomic mass of chlorine is 75.4% x 35 + 24.6% x 37 = 35.492. which is not a whole number.
