Imagine that you wanted to make a time capsule in which you would seal important artifacts from your life (pictures, poems, a lo
ck of your baby hair, etc.), to be opened by your heirs 1,000 years from now. To prevent these artifacts from decaying, you want to fill the capsule with a gas that would be least reactive. Which of these gases would you choose: oxygen gas (O2) carbon dioxide (CO2), argon gas (Ar), or hydrogen gas (H2)? (The atomic numbers of the atoms in these molecules are as follows: oxygen = 8; carbon = 6; argon = 18; and hydrogen = 2.)
Between the options given, argon gas is the least reactive. Argon is known as a <em>noble gas</em>, this means that it is located in the group 18 in the periodic table. These gases are known for their low reactivity, under most circumstances.
The <u>electronic configuration</u> of argon helps us see why that is the case: With an atomic number of 18, it has an electronic configuration of 1s² 2s² 2p⁶ 3s² 3p⁶. This means the most external orbital is filled with electrons, meaning that there are not unpaired electrons able to react with other substances.
