Why do you think there are many hydrocarbon compound

because it includes all compunds with h and c

Explanation:

The notation is actually written in this way (shown in the attached picture). The X is the symbol of the element. So, for potassium, the symbol is K. The A represents the mass number of the isotope. In this case, that would be equal to 39. The Z represents the atomic number of potassium, which is 19. Therefore, the symbol for the isotope is:  ³⁹₁₉K