We have to determine the mass number of boron atom that contains 6 neutrons.

**Recall** that the symbol of an element is written is this way:

**Atomic number (Z): **

• represents the identity of the element

• reveals the number of protons

• in a neutral element:

$\overline{){\mathbf{number}}{\mathbf{}}{\mathbf{of}}{\mathbf{}}{\mathbf{protons}}{\mathbf{}}{\mathbf{=}}{\mathbf{}}{\mathbf{number}}{\mathbf{}}{\mathbf{of}}{\mathbf{}}{\mathbf{electrons}}}$

**Mass number (A)**

• reveals the number of protons and neutrons

$\overline{){\mathbf{mass}}{\mathbf{}}{\mathbf{number}}{\mathbf{}}{\mathbf{=}}{\mathbf{}}{\mathbf{number}}{\mathbf{}}{\mathbf{of}}{\mathbf{}}{\mathbf{protons}}{\mathbf{}}{\mathbf{+}}{\mathbf{}}{\mathbf{number}}{\mathbf{}}{\mathbf{of}}{\mathbf{}}{\mathbf{neutrons}}}$

In a naturally occurring sample, 19.8% of boron atoms have
five neutrons and 80.2% have six neutrons.

What is the mass number of a boron atom containing 6 neutrons?

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