I mean, Wolfram is a language--it's words are infinitely combinable. According to answers to my last question, the only way I can find out if an expression is atomic is actually use the expression.
But I can hardly test an infinity of expressions to find all the ones that are atoms and all the ones that are not. And I'm not interested in complete list of all expressions that are atomic. I just want a list of the kinds of atoms. Like, I know integers are atomic, and strings too. But what else?
You can use EntityValue to find out what symbols can be atomic:
EntityValue[EntityClass["WolframLanguageSymbol", "Atomic"], "CanonicalName"]
{"AggregationLayer", "Association", "Audio", "BasicRecurrentLayer", "BatchNormalizationLayer", "BooleanFunction", "BoundaryMeshRegion", "ByteArray", "CatenateLayer", "ColorProfileData", "Complex", "ConstantArrayLayer", "ConstantPlusLayer", "ConstantTimesLayer", "ContrastiveLossLayer", "ConvolutionLayer", "CrossEntropyLossLayer", "Dataset", "DeconvolutionLayer", "Dispatch", "DotLayer", "DropoutLayer", "ElementwiseLayer", "EmbeddingLayer", "FlattenLayer", "GatedRecurrentLayer", "Graph", "Image", "Image3D", "ImageAugmentationLayer", "Integer", "LinearLayer", "LocalResponseNormalizationLayer", "LongShortTermMemoryLayer", "MeanAbsoluteLossLayer", "MeanSquaredLossLayer", "MeshRegion", "NetChain", "NetDecoder", "NetEncoder", "NetEvaluationMode", "NetExtract", "NetFoldOperator", "NetGraph", "NetInitialize", "NetMapOperator", "NetModel", "NetNestOperator", "NetPairEmbeddingOperator", "NetPort", "NetPortGradient", "NetReplacePart", "PaddingLayer", "PartLayer", "PoolingLayer", "Rational", "RawData", "Real", "ReplicateLayer", "ReshapeLayer", "ResizeLayer", "SequenceLastLayer", "SequenceMostLayer", "SequenceRestLayer", "SequenceReverseLayer", "SoftmaxLayer", "SparseArray", "SpatialTransformationLayer", "String", "StructuredArray", "SummationLayer", "ThreadingLayer", "TotalLayer", "TransposeLayer", "UnitVectorLayer"}
