noble gases
Summary
Noble gases are the Group 18 elements (He through Rn). Their filled valence shells give them the highest ionization energies in their periods and make them generally unreactive, though Xe forms compounds such as XeF₄. Boiling points rise down the group as London forces strengthen.
Detail
Meaning: Noble gases are helium, neon, argon, krypton, xenon and radon. Each has a complete valence shell (2 electrons for He, an octet for the rest), so they exist as monatomic gases at room temperature. They have the highest ionization energy in each period and very little tendency to gain electrons. Their only attractions are London dispersion forces, which grow with the size of the electron cloud, so boiling points rise down the group.
On the MCAT: Periodic-trend questions ask you to rank ionization energy across a period (the noble gas is highest) or boiling point down the group (He is lowest). Helium and neon behave nearly like ideal gases because their intermolecular forces are so weak. Xenon fluorides appear in VSEPR questions: XeF₂ is linear and XeF₄ is square planar, with the octet expanded.
Common confusion: Noble gases are often called inert, but the heavier ones, especially Xe, bond to very electronegative atoms such as F and O. Their outer electrons are farther from the nucleus and easier to remove, and their n ≥ 3 shells allow an expanded octet. Also, helium is a noble gas with 2 valence electrons, not 8.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- General Chemistry (Petrucci)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.