Atomic Nodes are the lowest-level most basic building blocks of Substance Designer. Everything else can be constructed using these nodes. If you'd break down any graph to it's most core operations, it would consist of only these atomic nodes.
All Atomic Nodes are listed in the child pages of this page. However, some Atomic Nodes are more used and more useful than others, so to give you a quick idea of where to get started, take a look at the following ones:
Placing Atomic Nodes
Atomic Nodes can be placed in 4 different ways. They are always color coded and have icons that stay consistent across all methods.
The Node Bar
The node bar sits right above the graph view and provides easy, color coded access to the Atomic nodes.
The Spacebar/Tab Menu
If you press Spacebar or Tab (Tab is only available since Designer 2018.3.0) in the Graph view, you get a list that displays the Atomic nodes by default. It can also be used to search the Library for Library Nodes.
This menu tends to be the fastest and most versatile way to place nodes.
The Add Node Menu
Right-clicking on the Graph and choosing the first option, "Add Node" presents a similar menu to the Spacebar/Tab method. You can not choose Library Nodes from this menu however.
The Library has a section for the Atomic Nodes, where all nodes are accessible just as all the other Library Nodes.