AP Chemistry Unit 3

Coulomb's law

The rule stating that force gets stronger when charges are larger, and gets weaker when the distance between charges increases.

Coulombic Attraction

The pull between opposite charges (positive protons in the nucleus and negative electrons) that holds an atom together.

Intermolecular forces (IMF)

The attractions BETWEEN atoms, ions, or molecules, that hold them together. There are 6 types. THey differ in the arrangement of the electrons.IMF explain why CO2 is a gas, H2O is a liquid, and SiO2 is a solid even though they are all made only from 3 atoms.

How to determine the type of IMF a substance will exhibit.

There are 6 types of IMF. Look at the types of atoms that are present in the same. Start by identifying the types of atoms in the sample as a metal or a nonmetal.

6 types of IMF

London DIspersion Forces (LDF)

• Occur in all substances

• Temporary dipoles

• They are the only type of IMF present in nonpolar molecules.

• LDFs are caused by the motion of electrons within an atom or molecule. The temporary dipole in one atom or molecule causes the neighboring atoms/molecules to change; the negative end of the temporary dipole repels electrons while the positive end of the dipole attracts electrons. When neighboring species’ electrons move, this forms an “induced dipole” which is electrostatically attracted to the first atom/molecule. This is usually a short lived occurrence and will return to being evenly distributed, which is why LDF is a weak force.

• Stronger when an atom or molecule has more electrons.

• Large molecules will have stronger LDF because the increase in electrons also increases their polarizability.

• The shape of the molecule also increases the LDF. Long and thin molecules are more surface area and are more polarizable than short and around molecules because they are less surface area.

• The more polarizable the molecule, the stronger the LDF.

temporary dipole

As electrons move around with the atom there is a chance that they are unequally distributed due to there being an excess of partially negative on one area and partially positive on one area in the atom.

induced dipole

When neighboring species’ electrons move, this forms an “induced dipole” which is electrostatically attracted to the first atom/molecule.

polarized

The distortion of an electron cloud within an atom, ion, or molecule, creating an uneven spread of electron density and partial positive and negative charges.

Relation to Polarity: While a polar molecule (like water) has an inherent, permanent separation of charge, polarization can happen to any electron cloud—even in nonpolar molecules—due to outside influences

eletronegativity

how greedy an atom is for electrons

electrostatic attraction/force

the attractive or repulsive forces between electrically charged particles

dipole

A dipole happens when two bonded atoms share electrons unequally.

A separation of positive and negative electrical charges within a bond or a molecule.

Dipoles can be measured by the dipole moment of the molecule.

polarizability

The ability of an atom to form a temporary dipole or an induced dipole.

“polar” “ability”

Moving down the period table, F2, CL2, Br2, I2, they change in state from being gases to liquid to solid.

Increase in electrons increases the strength of LDF is the explanation for this phenomenon. Large molecules will have stronger LDF because the increase in electrons also increases their polarizability.

Dipole—Dipole

• Occur between molecules that have a permanent dipole due to them being polar molecules.

• Molecules are polar when their electron distribution is asymmetrical. So try to sketch the molecule to determine if the electrons are symmetrical or not.

• Dipole-dipole interactions have permanent dipoles meaning greater attractions than LDF.

• The partially positive end of one molecule is attracted to the partially negative end of a different molecule.

• Dipoles can be measured by the dipole moment of the molecule.

• Dipole-dipole interactions are stronger when the substance involved is more polar.

polar and nonpolar

The more unequally distributed the electrons are the more polar it is.

The more equally distributed the electrons are, the more nonpolar it is.

Hydrogen bonding

The name can be confusing because this type of IMF is not a covalent or ionic bond, it is a type of attraction.

• Occurs in molecules that the H atoms are bonded with an N, O, or F.

• The difference in electronegativity is so large in these bonds that a large bond dipole forms.

Kinetic Molecular Theory (3.5 daily video 1)

It explains the macroscopic gas behavior.

• Models gas as a collection of tiny particles in constant, random motion.

• Gas particles are tiny and their combined volume is negligible compared to the total volume of the container.

• Constant random motion: Particles move rapidly in straight lines, changing direction only when they collide.

• No forces: Particles experience no intermolecular attractions nor repulsions, they act independently.

• Elastic collisions: Collisions between particles or container walls conserve total kinetic energy without any net loss.

• Temperature proportional to energy: The average kinetic energy of the particles depends directly on the absolute temperature in Kelvin.