An atom has 8 electrons in its outermost shell. Explain its chemical behaviour.

In all electrically-neutral atoms, the number of electrons is the same as the number of protons. This is known as the octet rule which states that, with the exception of the innermost shell, atoms are more stable energetically when they have eight electrons in their valence shell, the outermost electron shell.

indium, and thallium have three electrons i think

it's filtration, but this is an assumption so my apologies if i got this