Final answer:
Methane and ammonia are gases at room temperature because they have small molecules with weak intermolecular forces, allowing their kinetic energy to overcome these when at room temperature and atmospheric pressure.
Step-by-step explanation:
Compounds like methane, CH4, and ammonia, NH3, are gases at room temperature due to their molecular structures and forces between the molecules. Methane, a simple hydrocarbon with a tetrahedral shape, and ammonia, which has a pyramidal structure, are both made up of small, non-polar molecules or molecules with a small dipole moment. These characteristics result in very weak intermolecular forces, specifically London dispersion forces for methane and dipole-dipole interactions for ammonia. The strength of these intermolecular forces is much weaker than those found in larger or more polar molecules.
At room temperature and atmospheric pressure, the kinetic energy of these small molecules is enough to overcome the weak forces holding them together, which is why they exist as gases. In contrast, larger or more polar molecules with stronger intermolecular forces, such as water, exhibit higher boiling points and are liquid at room temperature. Thus, the physical state of a substance at a given temperature and pressure depends on the balance between the kinetic energy of its molecules and the strength of attractions between them.