Question

Why do some atoms form positive ions and others form negative ions?

Answer 1

Some atoms have a stronger attraction to electrons, and they have more strength to pull the electrons away from their neighboring atoms, and now because they have an extra electron, they're negative. And vice versa, some atoms are just so weak in terms of retaining their electrons that it just got ripped of, and are now positive.

This tendency to attract electron is called electronegativity, and it increases as you go up and to the right of the periodic table. In contrast, the strength of an atom to attract the electrons decrease as you go down and to the left of the periodic table (with the exception of hydrogen).