What is Atomic Relation in First Normal Form
1NF requires that every attribute position in every tuple in every relation contains a single value of the appropriate type. The types can be arbitrarily complex. In fact, the types can be relations. (CJ Date's book Database in depth: relational theory for practitioners treats this issue in a way that's pretty easy to understand.)

"Atomic" has never really meant "indivisible", which is why that term is finally falling out of favor. Loosely speaking, "atomic" means if a value has component parts, the dbms either ignores the existence of those parts, or it provides functions to manipulate them. For example, a timestamp value has these parts.


Year
Month
Day
Hours
Minutes
Seconds
Milliseconds


That kind of value is obviously divisible, and all database management systems provide functions to manipulate those parts. They also provide a way to select a timestamp as a single value. (Which, of course, it is.)