Argon gas in general chemistry
Argon gas (symbol Ar) is a noble gas found in Group 18 of the periodic table. Its most important chemical feature is its filled valence shell, which explains why argon is unusually unreactive under ordinary conditions.
Step 1: Identify argon from the periodic table
| Property | Argon (Ar) | Chemical meaning |
|---|---|---|
| Atomic number | 18 | 18 protons; a neutral atom has 18 electrons |
| Group | 18 (noble gases) | Filled valence shell; low tendency to form ions or bonds |
| Common state | Monatomic gas | Exists mainly as single atoms rather than molecules |
| Atomic (molar) mass | \(\approx 39.95\ \text{g/mol}\) | Mass of 1 mole of argon atoms |
Step 2: Build the electron configuration for argon
A neutral argon atom contains 18 electrons. Filling orbitals in the usual order (Aufbau principle) gives:
\[ \mathrm{Ar}:\ 1s^2\,2s^2\,2p^6\,3s^2\,3p^6 \]
Noble-gas shorthand: \[ \mathrm{Ar}:\ [\mathrm{Ne}]\,3s^2\,3p^6 \]
Step 3: Explain why argon gas is chemically inert
Chemical reactions often occur because atoms can lower energy by gaining, losing, or sharing electrons to reach a stable valence arrangement. Argon already has a stable configuration:
- No strong drive to gain electrons: adding an electron would place it into a higher-energy orbital beyond the filled \(3p\) subshell.
- No strong drive to lose electrons: removing an electron disrupts a complete valence shell, which is energetically costly.
- Weak bonding tendency: argon forms very few stable compounds; when interactions occur, they are typically weak dispersion forces in condensed phases.
Under standard conditions, these factors make argon gas effectively inert, which is why it is widely used as a protective atmosphere (for example, in welding or to shield reactive materials from oxygen).
Step 4: Molar mass statement (common chemistry use)
Because argon is monatomic, its molar mass is numerically equal to its atomic mass from the periodic table:
\[ M(\mathrm{Ar}) \approx 39.95\ \text{g/mol} \]