An atom achieves a positive charge when it loses one or more electrons. This can occur in a number of ways, including:

-Photoionization occurs when an atom absorbs a photon of light with enough energy to knock an electron out of the atom.

-Collisional ionization occurs when an atom collides with another atom or molecule with enough energy to knock an electron out of the atom.

-Electron capture occurs when an atomic nucleus captures an electron from its surrounding environment.

When an atom loses an electron, it becomes positively charged because it has more protons than electrons. The positive charge of an atom is equal to the number of protons in the atom minus the number of electrons.