Yes it is diffusion ..
explanation :
diffusion is the movement of particles from high concentration to low concentration ..
as the smoke rises at one corner of the room soon it spreads all over the room which is simply diffusion ..
"Then" is used in an "if-then" scenario, to dictate chronological order. In other words, "If you jump off, then you will fall," or "he was attacked and then retreated."
"Than" is used for comparisons. In other words, "The egg was bigger than the pea," or "I'm stronger than you."
