Question

A neutral atom of beryllium (Be) has an average mass of 9 amu and 4 electrons. How many neutrons does it have?

13
5
9
4

Answer 1

It has 5 neutrons

Step-by-step explanation:

The fact that it is a neutral atom tells us that the number of electrons and protons is the same:

• The problem tells us there are 4 electrons, thus there are 4 protons.

The average mass of an atom is the sum of the number of protons and the number of neutrons:

• Mass = # Protons + # Neutrons

With the above information in mind we can calculate the number of neutrons:

• 9 amu = 4 Protons + # Neutrons

• # Neurons = 5 Neutrons