Quantum Mechanics demystified, a try
Ions are atoms, that do not have exact same amount of electrons on their shells than they have protons in nucleus. Thus they become electrically charged.
Negative ions, anions, have more electrons than protons, kations are positively charged ones due to having less electrons than protons.
By octet rule, atoms prefer having eight electrons (or zero ih case of e.g. H?) on their valence, outermost shell. I.e. if there is only one electron on valence shell, it is very loosely connected, and gets lost easily. Loosing may take place in collisions with particles, other atoms and molecules.
On the other hand, if there is 7 electrons of an atom on the valence shell, this atom is very keen on grasping one more electron to satisfy octet rule.
Sodium Na and chlorine Cl atoms form a simple example. When these atoms get in proximity, Na will give it’s outermost electron to Cl to fill up Cl’s valence shell. These ions attract each other to form a crystal bond.