* Atomic Weight vs. Atomic Number: The atomic weight (also called atomic mass) is the average mass of an atom of an element. It's primarily determined by the protons and neutrons in the nucleus.
* Isotopes: Chlorine has two major isotopes: chlorine-35 (17 protons and 18 neutrons) and chlorine-37 (17 protons and 20 neutrons). The atomic weight of 35.5 is the weighted average of these isotopes, taking into account their relative abundance in nature.
* Electrons' Role: Electrons contribute a very small amount of mass compared to protons and neutrons. While electrons do play a crucial role in chemical bonding and determining an element's chemical properties, their contribution to the atomic weight is negligible.
In summary: The atomic weight of chlorine is 35.5 because it's a weighted average of the masses of its isotopes, which are determined by the number of protons and neutrons in their nuclei. Electrons have a minimal impact on the atomic weight.
