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 C. a nucleus of one neutron, which is orbited by one electron D. a nucleus of two neutrons which is orbited by two electrons

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

Explanation:

Isotopes have the same atomic number and different mass number. The atomic number is the numbers of protons in the nucleus and defines each element while the mass number is the sum of protons and neutrons in the nucleus.

An hydrogen atom with a mass number of two means that its nucleus has one proton and one neutron because the atomic number of hydrogen is 1, otherwise it wouldn't be hydrogen, so it has 1 proton and to complete the mass number of two it must have 1 neutron.

On the other hand, the atomic model says that the nucleus is orbited by electrons. The number of electrons orbiting the nucleus is given by the atomic number. Then an atomic number of 1 means that 1 electron orbits the nucleus with one proton.

also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of deuterium, called a deuteron, contains one proton and one neutron, whereas the far more common protium has no neutron in the nucleus.