Why water is a liquid while ammonia is a gas
In short: The difference between a drinkable liquid and a gas often comes down to invisible geometry. By sharing electrons to fill their outer shells, atoms create covalent bonds. However, the way these molecules interact with one another determines whether they flow as a liquid or float away as a gas.
Covalent bonding occurs when non-metal atoms share electrons to achieve full valence shells. This pattern is visible across the second row of the periodic table. Carbon, having four electrons in its outer shell, forms four bonds with hydrogen to create methane. Moving right, nitrogen has five electrons and forms three bonds to create ammonia. Oxygen has six electrons and forms two bonds with hydrogen to create water. Each atom seeks a full shell of eight electrons, but the specific number of available electrons dictates the unique structure of each molecule.
The shape of these molecules is governed by the behavior of electrons. In ammonia, nitrogen possesses a lone pair of electrons—two electrons occupying a position not involved in bonding. Because electrons are negatively charged, these lone pairs take up space and repel the chemical bonds more strongly than the bonds repel each other. This repulsion makes ammonia less symmetric than methane. Similarly, oxygen in a water molecule has two lone pairs, which cause the molecule to adopt a distinct bent shape.
These internal geometries dictate the intermolecular forces—the attractions between separate molecules. While covalent compounds generally have weak intermolecular forces, water is unique. Because oxygen's lone pairs are slightly attracted to the hydrogen atoms in neighboring molecules, the forces in water are stronger than those in ammonia or methane. These forces are just strong enough to keep water liquid at room temperature and pressure. If these attractions were any weaker, water would be a gas, fundamentally altering the conditions for life as we know it.
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Sources
- Summary by Intellect · source: Covalent bonding in methane, ammonia & water | Properties of Matter | Chemistry | FuseSchool
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