Rare gases are stable because they

Step 1: Analyze the question The question asks why rare (noble) gases such as helium, neon, and argon are stable, meaning why they rarely react with other elements.

Step 2: The concept Chemical stability depends on the arrangement of electrons in the outermost shell (valence shell). Atoms react by gaining, losing, or sharing electrons to achieve a full outer shell.

• Noble gases already have a full outer shell.
• For neon, argon, krypton, xenon, and radon, this is 8 electrons, called an octet.
• Helium is the exception. Its full shell holds 2 electrons (a duplet).

Because their outer shells are complete, they have no tendency to gain, lose, or share electrons.

Step 3: Evaluate each option

• A. Equal number of protons and neutrons: Wrong. This is not related to stability in this sense. Argon-40, for example, has 18 protons and 22 neutrons.
• B. More electrons than protons: Wrong. Atoms are electrically neutral, so electrons equal protons. An excess of electrons would make an ion.
• C. Chemically active: Wrong. This is the opposite of the truth. Noble gases are chemically inert.
• D. Have octet structure: Correct. A complete outer shell of 8 electrons gives them a stable electronic configuration.

Step 4: Answer The rare gases are stable because they have an octet structure (a full outermost shell), which is option D.

Exam tip: Remember "Full shell = Fulfilled = Stable." When you see a question about noble gas stability, look for words like octet, duplet, complete outer shell, or inert. Other elements react in order to become like the noble gases.