An electrically neutral atom of gallium has 31 electrons and 39 neutrons. what is the mass number for an atom of gallium? . a. 31. b. 35. c. 62. d. 70

Option (D) is the correct answer.

Explanation:

Mass number is the sum of total number of protons and neutrons present in the nucleus of an atom.

On the other hand, atomic number is only the total number of protons present  in an atom.

Also, when an atom is electrically neutral in nature then its number of protons equal to the number of electrons present in it.

For example, an electrically neutral atom of gallium has 31 electrons and 39 neutrons. This means that it also contains 31 protons.

Hence, mass number of gallium will be as follows.

                Mass number = no. of protons + no. of neutrons

                                        = 31 + 39

                                        = 70

Thus, we can conclude that the mass number for an atom of gallium is 70.

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)