One possible game on atomic size could be a matching game. Here’s how it could work:

1. Prepare a set of cards, each containing the name of an element and its atomic size (in picometers or angstroms).
2. Shuffle the cards and lay them face down on a table or floor.
3. Players take turns flipping over two cards at a time, trying to find a match between an element and its atomic size.
4. If a player finds a match, they keep the pair and get a point. If not, they flip the cards back over and it’s the next player’s turn.
5. The game continues until all the matches have been found.
6. The player with the most points at the end of the game wins.

To make the game more challenging, you can include elements with similar atomic sizes or add a time limit for each turn. Additionally, you can create different levels of difficulty by using more cards or including elements with larger atomic sizes.