To "Expand" would imply something is getting larger. Infinity does not get larger. The number "1" is just as close to infinity as the number "1,000,000,000" because there is no "end" the infinity.
An atom was formed from sub atomic particles colliding, thats old news but i think you want to know where the first forms of energy/matter all together came from which science states always existed in a constant state of change, like god always existed to religion so did this base energy/matter. You need to imagine that nothing came about, it always was there moving around and colliding into each other like god was always there controlling his creation.
"Time" itself is also a product of the universe. To ask which atoms came "first" cant really be assigned a chronological value if it happened before the Big bang. Before then, everything was, in a way, omnipresent.