Question

why do some atoms tend to form positive ions while others form negitive ions

Answer 1

The outer-most shell of electrons can be filled, making the atom chemically inert
(He, Ar, Xe, etc.) or they can have one or a few electrons, easily lost, so they tend to become net positive ions (e.g., Na+)...or they can be one or a few short of having a filled outer shell, so they tend to pick up electrons and become negative ions (e.g, Cl-)..