* Protons and Neutrons: The atomic mass of an element is primarily determined by the number of protons and neutrons in its nucleus.
* Isotopes: Chlorine has two main isotopes: chlorine-35 (with 17 protons and 18 neutrons) and chlorine-37 (with 17 protons and 20 neutrons).
* Isotope Number: The isotope number is the total number of protons and neutrons.
* Average Mass: Chlorine-35 is more abundant in nature than chlorine-37. The atomic mass of 35.5 is actually the weighted average of the masses of these two isotopes, taking into account their relative abundance.
In simpler terms: Imagine you have a bag of marbles, some are green (chlorine-35) and some are blue (chlorine-37). The green marbles are heavier than the blue ones, but there are more green marbles in the bag. The average weight of all the marbles in the bag will be somewhere between the weight of a green marble and a blue marble, just like chlorine's atomic mass is between 35 and 37.
