Little is known about the chemistry of astatine (At) from direct observation, but reasonable predictions can be made.
(a) Is astatine likely to be a gas, a liquid, or a solid?

Periodic trends refer to the predictable patterns in the properties of elements as you move across or down the periodic table. For instance, elements in the same group often exhibit similar physical and chemical properties. Understanding these trends helps predict the state of an element at room temperature based on its position in the periodic table.

Astatine (At) is a member of the halogen group, which includes fluorine, chlorine, bromine, and iodine. Halogens typically have varying states at room temperature, with fluorine and chlorine being gases, bromine a liquid, and iodine a solid. This group’s properties can provide insights into the expected state of astatine, which is likely to be a solid due to its heavier atomic mass.

The atomic structure of an element, including its atomic mass and electron configuration, influences its physical state. Heavier elements tend to have stronger van der Waals forces due to increased electron cloud size, which can lead to solid states at room temperature. Astatine's atomic structure suggests it will exhibit solid characteristics, similar to other heavier halogens.