Problem: 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?

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:

$\boxed{\mathbf{number}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{protons}\mathbf{ }\mathbf{=}\mathbf{ }\mathbf{number}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{electrons}}$

Mass number (A)

• reveals the number of protons and neutrons

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