Question

instructions: determine the number of valence electrons in an atom of each element. 7) helium — 8) magnesium — 9) sulfur — 10) hydrogen — 11) krypton — 12) aluminum —

Explanation:

Step1: Recall valence electron rules

Valence electrons are in the outermost shell. For main - group elements (Groups 1 - 2, 13 - 18), the group number (using 1 - 18 numbering) gives the number of valence electrons (except for He which has 2 valence electrons as it has a full first shell).

Step2: Determine group numbers

• Helium (He) is in Group 18. But it has only 2 electrons in total and its first (and only) shell is full. So it has 2 valence electrons.
• Magnesium (Mg) is in Group 2. So it has 2 valence electrons.
• Sulfur (S) is in Group 16. So it has 6 valence electrons.
• Hydrogen (H) is in Group 1. So it has 1 valence electron.
• Krypton (Kr) is in Group 18. It has a full outer shell with 8 valence electrons.
• Aluminum (Al) is in Group 13. So it has 3 valence electrons.

Answer:

1. helium: 2; 8) magnesium: 2; 9) sulfur: 6; 10) hydrogen: 1; 11) krypton: 8; 12) aluminum: 3