The atomic mass of an element is calculated using the
(1) atomic number and the ratios of its naturally occurring isotopes(2) atomic number and the half-lives of each of its isotopes(3) masses and the ratios of its naturally occurring isotopes(4) masses and the half-lives of each of its isotopes

The atomic mass of an element is calculated using its masses and the ratios of its naturally occurring isotopes. The answer is number 3.  The rest of the choices do not answer the question above.

dynamic equilibrium is the answer.

atomic mass = 39

number of neutrons = 20

number of protons= 19

electronic configuration = 1s2 2s2 2p6 3s2 3p6 4s1

the number of electron in first shell = 2

number of electrons in n=3 shell = 8