The relative atomic mass of cobalt ($Co$) is $58.9$ and that of nickel ($Ni$) is $58.7$. Even though nickel has a lower atomic mass, Mendeleev placed cobalt before nickel in his periodic table.

Explain why Mendeleev reversed the order of these two elements.

Mendeleev deliberately left gaps in his periodic table rather than forcing every known element into a continuous sequence.

Explain how leaving these gaps eventually led to other scientists accepting his periodic table.

Astatine ($At$) is located in Group 7, directly below iodine ($I$) in the periodic table.

Predict:

1. The chemical formula of an astatine molecule.
2. The state of matter of astatine at room temperature ($20^{\circ }C$).