Question

What is the mass number of boron?

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Solution
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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.

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