Why is the arm often described as a sort of lever?

because the bones in the arm often pivot just like a lever does.

periodic trends is an atom that gets larger as the number of electronic shells increase, the radius of atoms increases as you go down a certain group in the periodic table of elements. in general, the size of an atom will decrease as you move from left to right of a certain period.

i think it is true since you can test the answer and can use observations to complete it.