Count protons, neutrons and electrons in atoms, ions and isotopes

Answer Key - Grade 10 Science Worksheet - Building Confidence

Circle the best answer, or write your answer on the line.

1. 1.

   A chlorine atom has mass number 35 and atomic number 17. How many neutrons are in its nucleus, Bo?

   18 neutrons
2. 2.

   An iron atom has mass number 56 and atomic number 26. How many neutrons are in its nucleus, Esme?

   30 neutrons

3. Read this first

   Atom diagram with nuclear notation

   Magnesium-24. The nucleus holds 12 protons and 12 neutrons; 12 electrons sit in shells of 2, 8 and 2. Beside it: mass number 24 written over atomic number 12, then Mg.

4. 3.

   In the diagram, what is magnesium's atomic number, Jun?

   • 2
   • 24
   • 12
5. 4.

   In the diagram, why is the mass number 24, Kira?

   • 2 + 8 + 2 electrons
   • 12 protons + 12 electrons
   • 12 protons + 12 neutrons
6. 5.

   In the diagram, why does magnesium-24 have no overall charge, Luz?

   • Its neutrons cancel the protons
   • Its 12 protons and 12 electrons balance
   • Its shells hold the charge in
7. 6.

   Which particle's count decides what element an atom is, Milo?

   • Electrons
   • Protons
   • Neutrons
8. 7.

   Where are an atom's electrons, Pia?

   • Outside the nucleus
   • Inside the nucleus
   • Inside the protons
9. 8.

   In nuclear notation, what does the top number show, Ren?

   • The charge
   • The mass number
   • The atomic number
10. 9.

    In nuclear notation, what does the bottom number show, Tala?

    • The number of neutrons
    • The mass in grams
    • The number of protons
11. 10.

    What does the 14 in carbon-14 tell you, Zee?

    • Its number of electrons
    • Its atomic number
    • Its mass number