When were the first atoms formed?
Atoms were not formed until long after the Big Bang.
We are not sure what happened before or soon after the Big Bang as the physics isn't completely understood. Soon after the Big Bang the universe was very hot. As it cooled down the elementary particles started to form. It is thought that it took over 300,000 years for the first atoms to form when protons and electrons had low enough energies to combine to form hydrogen.