system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are atoms with 8 valence electrons so stable?
gpt
Think of it like having a complete set of cards (Pokemon, all cards of a suit, whatever). The outermost layer is like your pile of cards. Those players with a full outermost shell have a “full set”. They have no incentive to play the game.
However, if you’re one of those players that is missing just one card from your set, you’re going to try to win/steal that remaining card from another player.

Since your goal is to “catch them all” you have two strategies:

A: Steal that card from a player who has only one card in their hand. You steal that card (electron) from the other player (atom) and now you have a full set of cards (outermost shell). You’re happy! But because you feel guilty that you have one extra card than you’re supposed to, and the other player is angry he has one less than he should, you guys stick around one another (ions with opposing charges- ionic bond. Ex: NaCl)

B: A friend of yours who is also short one card decides to share a card with you. You both can behave like you have a full set, but you guys always stay by each other so you can continue to share the card (covalent bonding Ex: H2 hydrogen gas)