What is the mass number of boron?

Question

Cameron Andrews · Accepted Answer

Well it depends on the isotope....

#Z=5#, and thus WE KNOW that we got the element boron. To a first approx. #""^11B# is 80% abundant, and clearly (why?) this isotope has 6 nucular neutrons and thus a mass number of #11#.....and #""^10B# is 20% abundant, and clearly (why?) this isotope has 5 nucular neutrons and thus a mass number of #10#; and a neutron is simply a massive nuclear particle with ZERO electronic charge. Do you agree?

Serenity Allen · Answer

The mass number of the most common isotope of boron is 11.

Please see the more detailed explanation below.

This is a slightly tricky question, because a mass number is for a particular isotope of an element, not the element itself. Element number 5 is boron (B). That means every atom of boron has 5 protons. The most common isotope (about 80% of naturally occurring boron) of boron has 6 neutrons too, for a total of 11 nucleons (protons + neutrons). Its mass number is 11. Some naturally occurring boron (about 20%) has 5 neutrons, for a total of 10 nucleons, and a mass number of 10. It is also possible to make isotopes with more than 6 or less than 5 neutrons. The mass number of the most common isotope of boron is 11.