The given sequence appears to represent a combination of chemical elements. Breaking it down:

C2N4 - Carbon (C) and Nitrogen (N)
C2N2 - Carbon (C) and Nitrogen (N)
CN3CNO - Carbon (C), Nitrogen (N), and Oxygen (O)
C2N5ON - Carbon (C), Nitrogen (N), and Oxygen (O)

It seems like the sequence is showing different combinations of Carbon, Nitrogen, and Oxygen atoms.