Atomic and Physical Properties of Group 18 Elements
The science of highly stable, closed-shell atoms and extremely weak forces.
Group 18 elements (Noble Gases) possess the highly stable, completely filled electronic configuration $ns^2 np^6$ (except Helium, which is $1s^2$). This lack of desire to gain, lose, or share electrons governs all of their atomic and physical properties. They are the ultimate "loners" of the periodic table.
1. Atomic Properties
A. Atomic Radii (The van der Waals Trap)
Atomic radii naturally increase down the group with the addition of new electronic shells. However, there is a massive conceptual trap when comparing noble gases to halogens across a period.
Across a period, atomic size decreases, meaning the Halogen (Group 17) should be the smallest, and the Noble Gas (Group 18) should be even smaller. However, data books show noble gases as larger than the corresponding halogens!
Reason: Noble gases do not form molecules, so we cannot measure their covalent radii. We can only measure their van der Waals radii (the distance between two non-bonded adjacent atoms). Halogens are measured using covalent radii (where orbitals overlap and pull atoms closer). Because van der Waals radii are inherently larger than covalent radii, noble gases appear "larger."
B. Ionization Enthalpy
Due to their completely filled and highly stable $ns^2 np^6$ configurations, noble gases exhibit the highest ionization enthalpy in their respective periods.
As you move down the group, atomic size increases. The valence electrons get further from the nucleus, reducing the effective nuclear pull. Thus, ionization enthalpy decreases down the group from Helium to Radon.
C. Electron Gain Enthalpy (Large Positive Values)
Halogens love gaining electrons (negative values). Noble gases absolutely hate gaining electrons.
Because their valence shell is completely full, adding an extra electron means forcing it into a completely new, higher principal energy level ($n+1$). This is energetically highly unfavorable. Therefore, noble gases have large positive electron gain enthalpies (energy must be supplied to force the electron in).
2. Physical Properties
A. Physical State
All noble gases are colorless, odorless, and tasteless. They exist as monoatomic gases under standard conditions because they do not form covalent bonds with each other.
B. Melting and Boiling Points (Extremely Low)
Because noble gases are neutral, monoatomic species, there are no ionic or dipole-dipole interactions between them. The only type of interatomic force present is the very weak London dispersion force (induced dipole-induced dipole interactions).
- Because these forces are so weak, very little thermal energy is needed to separate the atoms. Hence, they have extremely low melting and boiling points.
- Helium has the lowest boiling point of any known substance ($4.2 \text{ K}$).
- As you move down the group, the atoms become larger, and their electron clouds become more "polarizable." This increases the strength of the London dispersion forces. Thus, melting and boiling points increase steadily down the group.
Figure 1: Even the heaviest noble gas (Radon) boils at a temperature well below freezing ($0^\circ\text{C} = 273.15\text{ K}$).
C. Solubility in Water
Despite being non-polar, noble gases are slightly soluble in water. Their solubility increases down the group from Helium to Radon.
Reason: Water is a highly polar molecule. When it interacts with a noble gas atom, it induces a temporary dipole in the noble gas electron cloud (Dipole-induced dipole interaction). Because larger atoms (like Xe) have more diffuse, polarizable electron clouds, the induced dipole is stronger, making them more soluble than smaller atoms (like He).
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