An atomic note contains exactly one idea.
Not one topic. Not one article. One claim that can stand on its own.
Bad: "Notes on WWII" (a topic, not a thought) Good: "The Molotov-Ribbentrop pact delayed Germany's eastern front by 2 years" (a claim)
Atomic notes are easier to link because they have a clear identity. You know when two notes belong together.
Related: zettelkasten, ↗ @carol / on-music-theory-as-a-language