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 |
The trend and the gap
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 to 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.
Why water breaks the line
Water, hydrogen fluoride, and ammonia all break this extrapolation. Their boiling points, melting points, and enthalpies of vaporisation lie far above the predicted value. The common explanation is hydrogen bonding: the 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.
Two more lenses
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, and 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 to 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.
- substance dataset: boiling point, melting point, latent heat of vaporisation and fusion, molar mass, and periodic row for 38 substances across eight families.
- anomaly model: least-squares fit through periods 3, 4, 5 of each hydride family, extrapolated to period 2; residual = observed minus trend.
- scatter baseline: boiling point regressed on ln(molar mass) over non-hydrogen-bonding substances, so water and HF can be seen jumping above the dispersion line.
- phase lens: solid/liquid/gas classification at an adjustable ambient temperature, showing water alone liquid among the groups 14 to 17 hydrides at 25 C.
- calibration: extrapolated trend values and anomaly residuals checked against closed-form OLS computations, including a methane control with a small negative residual.