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 sodium atom has mass number 23 and atomic number 11. How many neutrons are in its nucleus, Ari?

   12 neutrons
2. 2.

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

   18 neutrons
3. 3.

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

   30 neutrons

4. 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.

5. 4.

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

   • 12
   • 2
   • 24
6. 5.

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

   • 12 protons + 12 electrons
   • 12 protons + 12 neutrons
   • 2 + 8 + 2 electrons
7. 6.

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

   • Its shells hold the charge in
   • Its neutrons cancel the protons
   • Its 12 protons and 12 electrons balance
8. 7.

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

   • Protons
   • Neutrons
   • Electrons
9. 8.

   Where are an atom's electrons, Pia?

   • Inside the nucleus
   • Inside the protons
   • Outside the nucleus
10. 9.

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

    • The mass number
    • The charge
    • The atomic number
11. 10.

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

    • The mass in grams
    • The number of protons
    • The number of neutrons