Valence electrons and electron dot diagrams for atoms

Answer Key - Grade 10 Science Worksheet - Building Confidence

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

1. 1.

   Hydrogen has 1 valence electron and oxygen has 6. How many valence electrons are in one H₂O molecule, Ari?

   8 electrons
2. 2.

   Carbon has 4 valence electrons and hydrogen has 1. How many valence electrons are in one C₂H₂ molecule, Bo?

   10 electrons
3. 3.

   Carbon has 4 valence electrons and hydrogen has 1. How many valence electrons are in one C₂H₄ molecule, Esme?

   12 electrons

4. Read this first

   Electrons in each shell (real data, periodic table)

   element	atomic number	electrons by shell
   carbon	6	2-4
   nitrogen	7	2-5
   sodium	11	2-8-1
   phosphorus	15	2-8-5
   chlorine	17	2-8-7
   potassium	19	2-8-8-1
   gallium	31	2-8-18-3

5. 4.

   How many valence electrons does gallium, with shells 2-8-18-3, have, Jun?

   • 18, the biggest shell in the list
   • 3, the outer shell only
   • 31, every electron it has
6. 5.

   Potassium (2-8-8-1) and sodium (2-8-1) react alike. Why, Luz?

   • Both have 1 valence electron in their outer shell
   • Both have the same total number of electrons
   • Both have eight electrons in their second shell
7. 6.

   An oxygen atom has 6 valence electrons. How many single (unpaired) dots does its dot diagram have, Milo?

   • 6, one for each valence electron
   • 4, the dots that sit in the two pairs
   • 2: six dots make two pairs and two singles
8. 7.

   Carbon has 4 valence electrons, drawn as 4 single dots. How many hydrogen atoms bond to one carbon in methane, Pia?

   • 2, because carbon sits in period 2
   • 4, one for each single dot, giving CH₄
   • 1, because hydrogen has one valence electron
9. 8.

   Sodium (2-8-1) loses its outer electron. What results, Ren?

   • Na⁺ with 2-8, the same arrangement as neon
   • Na⁺ with 2-8-1, the same shells as before
   • Na⁻ with 2-8-2, a full outer shell
10. 9.

    Magnesium (2-8-2) and chlorine (2-8-7) join. What is the formula, Tala?

    • MgCl: Mg gives 1 electron to one Cl
    • Mg₂Cl: two Mg for one Cl, since Mg has 2 outer electrons
    • MgCl₂: Mg gives away 2 electrons and each Cl takes 1
11. 10.

    Oxygen (2-6) gains electrons to fill its outer shell. What ion forms, Zee?

    • O²⁺, after losing 2 electrons
    • O⁶⁻, after gaining 6 electrons, its valence count
    • O²⁻, after gaining 2 electrons to reach 2-8