Question

What is the largest monatomic anion? Is it $B{i^{3 - }}$ ?

Answer 1

Chemical elements have electrons in their shells and try to attain completely filled electronic configuration by gaining or losing electrons. Chemical elements turn into ions in this process. Anions are the negatively charged ions, higher is the charge present on it higher will be the size due to the addition of an extra electron.

Complete answer:
Periodic table consists of chemical elements arranged in the vertical columns and horizontal rows. The vertical columns are known as groups and the horizontal rows are known as periods. The elements present in the group $17$ are known as halogens. Halogens are the most electronegative elements and have non-metallic character.
Bismuth is an element belonging to the nitrogen family and it is a metal. Metal loses electrons. Thus, $B{i^{3 - }}$ exists as a cation but not an anion.
Astatine is a chemical element belonging to the halogen family and it is non-metal. Non-metal tries to attract electrons from another element. It can exist as an anion and it is the largest monatomic anion. It has ionic radius of $227pm$
Astatine ion is a largest monatomic ion that can exist from the elements in the periodic table. Thus, $B{i^{3 - }}$ is not a largest monatomic ion.

Note:
Due to the addition of an extra electron to the neutral atom the radius will increase in case of anions, in case of cations as an electron is removed from the shell from the neutral atom and leads to the decrease in size. Cation has small size compared to neutral atom and anion has large size compared to neutral atom.