Ammonia \(\left(\mathrm{NH}_{3}\right)\) is very soluble in water, but nitrogen trichloride \(\left(\mathrm{NCl}_{3}\right)\) is not. Explain.

Understanding the polarity of molecules is key to grasping why certain substances dissolve in water while others do not. Molecules are comprised of atoms bonded together, and each atom has a certain ability to attract electrons, known as electronegativity. When atoms with different electronegativities form a bond, the electrons are not shared equally. This uneven sharing creates a dipole, with a partial negative charge on the more electronegative atom and a partial positive charge on the less electronegative atom.

For example, ammonia (H3)) has a nitrogen atom, which is more electronegative than the hydrogen atoms it's bonded to. This results in the nitrogen end of the molecule having a slight negative charge, while the hydrogen end has a slight positive charge, making ammonia a polar molecule. On the other hand, in nitrogen trichloride (Cl3)), the electronegativity difference between nitrogen and chlorine is relatively small, making it less polar than ammonia.

This concept of molecular polarity is fundamental, as it dictates how molecules will interact with each other, particularly with solvents like water.

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs between molecules when a hydrogen atom bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine is attracted to another electronegative atom in a nearby molecule. This attraction forms a 'bridge' between molecules and is much stronger than a typical van der Waals interaction, but weaker than covalent or ionic bonds.

For substances such as ammonia, hydrogen bonding is a critical factor in its high solubility in water. The nitrogen atom of ammonia can form hydrogen bonds with the hydrogen atoms of water molecules, as both exhibit significant polarity. This interaction leads to ammonia being readily soluble in water. In contrast, nitrogen trichloride does not benefit from these strong hydrogen bonding interactions due to its decreased polarity, hence its lower solubility.

The 'Like Dissolves Like' principle is straightforward: polar substances tend to dissolve well in polar solvents, while nonpolar substances dissolve better in nonpolar solvents. Water, being a highly polar solvent, readily dissolves compounds that are also polar because the similar charges attract, allowing the solvent molecules to surround and disperse the solute molecules effectively.

Considering this principle helps to explain why ammonia, a polar molecule, would dissolve in water—a polar solvent—due to their ability to form favorable interactions through hydrogen bonding and dipole-dipole forces. On the contrary, nonpolar or less polar substances, which lack the ability to establish these strong interactions with water, like nitrogen trichloride, do not dissolve as readily.

Molecular geometry plays a crucial role in determining the polarity of a molecule and its interactions with other substances. Molecules with an uneven distribution of electron density due to a certain three-dimensional shape can exhibit a net dipole moment, contributing to their polarity.

Both ammonia and nitrogen trichloride have a pyramidal shape with a central atom (nitrogen) at the apex. However, the polarity is not solely determined by shape; the type and arrangement of surrounding atoms are equally important. Ammonia's three hydrogen atoms cause a significant dipole moment due to the large electronegativity difference with nitrogen. Despite nitrogen trichloride's similar geometry, the closer electronegativity values of nitrogen and chlorine atoms make it less polar. Thus, molecular geometry, in concert with the nature of the atoms involved, determines the solubility of a molecule in water by influencing its polarity and ability to engage in specific interactions like hydrogen bonding.