Bonding Basics & Ionic Formation
You'll find that Group 0 elements (noble gases) already have full outer electron shells, which is why they're so stable. Meanwhile, Groups 1-3 are metals that love to lose electrons, whilst Groups 4-7 are non-metals that prefer to gain them.
The magic number here is 8 electrons in the outer shell - atoms will do almost anything to achieve this stable arrangement. When metals meet non-metals, something brilliant happens: they swap electrons to both get what they want.
Ionic bonding occurs when metals lose electrons to become positively charged ions, whilst non-metals gain these electrons to become negatively charged ions. Take sodium chloride (table salt) - sodium gives up one electron to become Na+, and chlorine gratefully accepts it to become Cl-.
Quick Tip: Remember AEI for exam questions - describe the Atom first, then what happens to the Electron, then the Ion that forms. This structure will get you full marks every time!