Electronegativity and polarity

What the exam asks

The commonest item is a molecule you must decide is polar or not, usually inside a bigger question about solubility or boiling point. The trap is treating polar bonds as proof of a polar molecule. CO2, CCl4 and BF3 all have polar bonds and no dipole, and each one appears as a distractor. Draw it, count the domains on the central atom, and ask whether the outer atoms are identical and whether there is a lone pair, because a lone pair is what breaks the symmetry that would otherwise cancel everything. The second trap is assuming hydrogen bonding always wins a boiling point comparison. It does not. A long enough hydrocarbon out-boils a small alcohol on dispersion forces alone, so compare sizes before you compare force types.

This topic runs a chain: electronegativity difference makes a bond polar, geometry decides whether those bond dipoles cancel, the surviving net dipole sets the intermolecular forces, and the intermolecular forces set the properties you can measure. Every question in this area is somewhere on that chain, and most of them are asking you to walk one link of it.

The link that carries the most marks is the second. A molecule can be stuffed with polar bonds and have no dipole at all, because the dipoles are vectors and a symmetric arrangement adds them to zero. Carbon dioxide has two strongly polar bonds and is nonpolar. Water has two similar bonds and is strongly polar. The difference is not the bonds, it is the shape.

Geometry comes from counting electron domains around the central atom: bonds and lone pairs both count, and lone pairs take up more room than bonding pairs, which is why they squeeze the angles down. What you carry in is the counting, not a table of angles. Given the domains, the shape follows, and given the shape, the dipole follows.

What to hold

• Electronegativity rises across a period and falls down a group, so fluorine is the most electronegative element, with oxygen next, and nitrogen and chlorine close behind it.
• A bond is polar when the two atoms differ appreciably in electronegativity, and the electron density sits nearer the more electronegative atom.
• Carbon and hydrogen have similar electronegativities, so a C-H bond is effectively nonpolar, which is why hydrocarbons are nonpolar and do not mix with water.
• Molecular shape is set by counting electron domains on the central atom: two domains give linear, three give trigonal planar, four give tetrahedral.
• Lone pairs count as domains but are invisible in the shape's name, so four domains with one lone pair is trigonal pyramidal, not tetrahedral.
• Lone pairs repel more strongly than bonding pairs, which is why the angle falls from 109.5 degrees in methane to about 107 in ammonia and about 104.5 in water.
• Bond dipoles are vectors, so a molecule with identical outer atoms and no lone pairs on the central atom has no net dipole: the dipoles cancel by symmetry.
• Hydrogen bonding needs a hydrogen bonded directly to nitrogen, oxygen or fluorine, and a lone pair on a nearby nitrogen, oxygen or fluorine to accept it.
• A hydrogen bond is an intermolecular attraction, not a bond within the molecule, and it is roughly a tenth the strength of a covalent bond.
• Dispersion forces exist between all molecules and grow with the number of electrons and with how much surface two molecules can lay against each other, so they are not the weak force by default.
• Boiling breaks intermolecular forces and leaves every covalent bond intact, which is why boiling water gives steam and not hydrogen and oxygen.
• Like dissolves like: a solute dissolves when the solute to solvent attractions can pay for the solute to solute and solvent to solvent attractions that had to be broken.