The Periodic Table and Group Trends: Question 10

Worked solution

Step 1: Recall why a full outer shell means no reactivity

An atom reacts by losing, gaining or sharing electrons to reach a stable, full outer electron shell. If an atom’s outer shell is already full, there is nothing to gain from reacting. It has no tendency to lose, gain or share electrons at all.

Step 2: Check that neon and helium both have a full outer shell

• Neon (2,8): the outer (second) shell holds 8 electrons. The maximum number of electrons a second shell can hold is 8, so this shell is full.
• Helium (2): the outer (and only) shell is the first shell, which holds 2 electrons. The maximum number of electrons the first shell can hold is only 2 (not 8), so this shell is also full.

Even though neon’s outer shell holds 8 electrons and helium’s holds only 2, both outer shells are complete for the shell they are. This is the key idea, not the number “8” itself.

Step 3: Why the other options are wrong

• B: neon has 10 electrons in total and helium has only 2, they do not have the same total number of electrons, so this cannot be the reason both are unreactive.
• C: every neutral atom has an equal number of protons and electrons, not more protons than electrons; this option describes something that is not true and is not the reason for unreactivity.
• D: simply stating the group number does not explain why elements in that group are unreactive. It is the full outer shell that these elements happen to share, not the label “Group VIII” itself.

Final answer

• Both neon and helium are unreactive because each has a $\boxed{\text{complete (full) outer electron shell}}$, option A.