Hydride Anomaly
Boiling point (°C) vs Period
| Formula | M (g/mol) | mp (°C) | bp (°C) | ΔHvap | ΔHfus | Liq. range | Tags |
|---|---|---|---|---|---|---|---|
| H₂O | 18.0 | 0.0 | 100.0 | 40.65 | 6.01 | 100.0 | H-bond donorH-bond acceptorpolar |
| HF | 20.0 | -83.6 | 19.5 | 7.49 | 4.58 | 103.1 | H-bond donorpolar |
| NH₃ | 17.0 | -77.7 | -33.3 | 23.33 | 5.66 | 44.4 | H-bond donorH-bond acceptorpolar |
| CH₄ | 16.0 | -182.5 | -161.5 | 8.19 | 0.94 | 21.0 | nonpolar |
Within each hydride family the heavier members follow a predictable trend: as you go down the periodic table, molar mass rises and so does the boiling point, melting point, and latent heat. A least-squares line through periods 3-5 extends smoothly to period 2 for most properties.
The anomaly residual is computed as:
where the trend is a linear fit through periods 3, 4, and 5.
Water, hydrogen fluoride, and ammonia all break this extrapolation. Their boiling points, melting points, and enthalpies of vaporization lie far above the predicted value. The common explanation is hydrogen bonding: the anomalously high electronegativity of O, F, and N creates strong intermolecular attractions that require more energy to overcome.
London dispersion forces scale roughly with molar mass:
but hydrogen bonds add a term that dominates in small, electronegative hydrides.
The scatter view plots melting point against boiling point for all enabled substances. A reference line fits boiling point against ln(molar mass) on non-H-bonding substances — water and HF jump well above it. Enable the alkane and diatomic families for a pure-dispersion baseline.
The phase lens shows a complementary view: at room temperature water is a liquid while every other hydride in groups 14-17 is a gas. Slide the temperature to watch phase boundaries shift and pin substances to keep them in a comparison table across all views.