Structure & Properties of Matter Lesson # 7: Bond & Molecular Polarity

Bond Polarity Review Non-Polar Covalent Bond – atoms in a covalent bond share electrons equally. This occurs in molecular elements. Polar Covalent Bond – atoms that attract shared pairs of electrons are not shared equally because one atom attracts them more strongly than the other atom. VIDEO – Crash Course!

Testing for Polarity Polarity was tested by putting hydrogen fluoride molecules in an electric field, and finding that they oriented themselves so that the hydrogen oriented itself toward the negative pole, and the fluorine oriented itself toward the positive pole.

Example – Hydrogen Fluoride We show this partial charge separation using the Greek letter delta (lowercase) – δ. Since fluorine has a stronger attraction for electrons (which are negative), it gets the partial negative charge, and hydrogen has less attraction for electrons (meaning the electrons spend less time around hydrogen leaving it lacking electrons), it gets the positive charge. This charge separation is called a dipole. H – F δ+ δ-

Electronegativity Electronegativity defines how strongly an atom attracts a pair of electrons it shares with another atom in a covalent bond. The electronegativity values were created by Linus Pauling. Most scientists consider the scale arbitrary, running from 0.7 at the lowest end with cesium, and 4.0 at the highest end with fluorine. There are slightly different values depending on which chemist you ask! But, Pauling won the Nobel prize for it, so I guess that counts for something!

Electronegativity Ranges If two atoms share electrons equally, it is because they have the same electronegativity, and their difference is zero. ΔEN = 0. This covalent bond would be non-polar. ΔEN < 0.5 = non-polar covalent ΔEN 0.5-1.7 = polar covalent (both ionic and covalent character) ΔEN > 1.7 = ionic

Molecular Polarity Bonds can be polar, and so can entire molecules. Water, for example, is a polar molecule. Putting a negatively charged balloon near water makes it bend. Positive hydrogens attract the negative charge. Since oxygen in water is more electronegative then the hydrogens, and it is not a symmetrical molecule, it is said to have a net dipole (or dipole moment), with electrons spending more time around the oxygen and lone pairs of electrons.

Water’s Net Dipole

Determining Molecular Polarity Any molecule with non-polar bonds will be a non-polar molecule, regardless of shape. A molecule with polar bonds can be either a non-polar or polar molecule, depending on the arrangement of atoms in a molecule.

Example: Carbon Dioxide Linear shape Negative charge distribution is equal around the two oxygens, and positive charge is on carbon. Since the polarities of these bonds are equal and opposite, they cancel each other out.

Example: Ammonia Trigonal pyramidal shape Positive charge distributed around the three hydrogens, and the net dipole is towards the negative nitrogen and lone pair of electrons.