Chlorine has an average atomic mass of 35.45 amu. the two naturally occurring isotopes of chlorine are clorine-35 and chlorne -37 why doesthis idicate that most clorine atoms contain 18 neutrons

See below.  

Step-by-step explanation:

The two isotopes are Cl-35 and Cl-37.

The average atomic mass (35.45 u) is closer to 35 u, so most of the atoms are Cl-35.

mass number = number of protons + number of neutrons

          A          =               p                +               n

         35         =              17                +               n

                   n = 35 – 17

                   n = 18

∴ Most chlorine atoms contain 18 neutrons.

it's c. elements are placed in rows based on how may valencies electrons have.