In Mendeleev's 1869 periodic table, Cobalt ($Co$, relative atomic mass $58.9$) was placed before Nickel ($Ni$, relative atomic mass $58.7$), despite cobalt having a higher atomic weight than nickel.

Explain why Mendeleev made this decision to reverse the order of these two elements.

Mendeleev left distinct gaps in his periodic table and predicted the existence of elements like "eka-silicon" (germanium).

Explain how these gaps and predictions eventually led to the widespread acceptance of his periodic table.

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

Predict:

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