How do the valency and the atomic size of the element vary while going from left to right along a period in The modern periodic table?​

Atomic size decreases on going along a period from left to right because the electron enters into the same shell, therefore, attraction between nucleus and outermost electron increases and thus atomic size decreases. Valency first increases from 1 to 4 and then decreases from 4 to GO along a period from left to right.

indium, and thallium have three electrons i think

"the periodic table is arranged in order of atomic number."