Question

deuterium is an isotope of hydrogen. it has a mass number of two. which describes a deuterium atom?
a. a nucleus of one proton and one neutron, which is orbited by one electron
b. a nucleus of one proton, which is orbited by one neutron and one electron

Explanation:

Hydrogen has atomic number 1, so 1 proton. Deuterium's mass - number is 2. Mass - number = protons + neutrons. So neutrons = 2 - 1=1. Atoms are neutral, so number of electrons = number of protons = 1. Protons and neutrons are in the nucleus, electrons orbit the nucleus.

Answer:

A. a nucleus of one proton and one neutron, which is orbited by one electron