Chlorine's atomic mass is listed as 35.45 rather than a whole number because chlorine:

1. A listed atomic mass is an average across the naturally occurring forms of an element. 2. Different isotopes of one element differ in neutron count and therefore in mass. 3. The atomic mass of chlorine is 35.45 because chlorine is composed of several isotopes. 4. The average lands between the whole-number masses of those isotopes, weighted by abundance. 5. Atomic number is fixed for an element, so it cannot be unstable. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 2 'The Chemical Foundation of Life', sections 2.1-2.3_