Some information about Russell's atomism was discussed in in our Wittgenstein's Tractatus podcast.
I leave it to you all to explore this essay as you like, but let me give you a taste, which aligns well with what what we've seen previously of Russell, i.e. that perception grasps (by definition, it seems) something non-mental, that he believes in sense data, and (stressed more in our Wittgenstein discussion) he gives those sense data a primary place in the ontology:
Common sense believes that what we see is physical, outside the mind, and continuing to exist if we shut our eyes or turn them in another direction. I believe that common sense is right in regarding what we see as physical and (in one of several possible senses) outside the mind, but is probably wrong in supposing that it continues to exist when we are no longer looking at it. It seems to me that the whole discussion of matter has been obscured by two errors which support each other. The first of these is the error that what we see, or perceive through any of our other senses, is subjective: the second is the belief that what is physical must be persistent. Whatever physics may regard as the ultimate constituents of matter, it always supposes these constituents to be indestructible. Since the immediate data of sense are not indestructible but in a state of perpetual flux, it is argued that these data themselves cannot be among the ultimate constituents of matter. I believe this to be a sheer mistake. The persistent particles of mathematical physics I regard as logical constructions, symbolic fictions enabling us to express compendiously very complicated assemblages of facts; and, on the other hand, I believe that the actual data in sensation, the immediate objects of sight or touch or hearing, are extra-mental, purely physical, and among the ultimate constituents of matter.
I brought this up in the context of Russell's discussion of the Dedekind cut, where an irrational number like the square root of two is commonly taken as a limit that the numbers around it can never reach. Russell's point was that this is not a number at all, but rather a gap between numbers, meaning that the space between two integers is not a continuum: here's a cut between two numbers that is just not filled, i.e. it's not any particular number. You can still have infinitely many numbers, even just between 1 and 2, but however many there are, they don't amount for a continuum. While this doesn't entail that space itself is not continuous, it's certainly compatible with Russell's ultimate pluralism about matter: it's not just one, big thing a la Spinoza's God.