What is the mass number of an iron atom that has 28 neutrons? Calculate the number of neutrons of 239Pu. Write the appropriate symbol for each of the following isotopes: (a) Z = 11, A = 23; (b) Z = 28, A= 64. Define, with two examples, the following terms: (a) alkali metals, (b) alkaline earth metal, (c) halogens, and (d) noble gases.