Intermolecular Forces Chemistry 20

Types of Forces Ionic forces Ionic forces metal + non-metal, ionic crystals metal + non-metal, ionic crystals Within an ionic crystal, each ion is attracted to several oppositely charged ions surrounding it.

Types of Forces Molecular forces (non-metals) Molecular forces (non-metals) Dipole-Dipole Forces Dipole-Dipole Forces Polar molecules Polar molecules Hydrogen Bonding Hydrogen Bonding Polar molecules Polar molecules Hydrogen bonded with fluorine, oxygen or nitrogen Hydrogen bonded with fluorine, oxygen or nitrogen London (Dispersion) Forces London (Dispersion) Forces Non-polar molecules Non-polar molecules

Intramolecular Forces In molecular compounds, each atom is covalently bonded to the other atoms in one molecule. The forces involved in these bonds act only between atoms within the same molecule and are, therefore, called intramolecular forces. Covalent bonds do not act between atoms of different molecules.

Intermolecular Forces Different types of forces (intermolecular forces or van der Waals forces) are responsible for holding molecules together in a solid (such as sugar) or a liquid (such as water).

Dipole-Dipole Forces A dipole-dipole attraction is an attractive force that exists between the molecules of a polar substance. A polar substance is made up of molecules that have a  + region and a  - region. Because these molecules have two different “poles”, they are called dipoles. When any two dipoles are near one another, the positive end of one attracts the negative end of the other.

Dipole-Dipole Forces This can take place between many different molecules at the same time (much like the attractive forces in an ionic crystal being multi-directional). Although the dipole-dipole attraction is not nearly as strong as the force between ions, it can still cause polar molecules to be attracted to each other enough to stabilize a solid crystal, such as table sugar.

Hydrogen Bonding One specific dipole-dipole interaction (called hydrogen bonding) is so strong and so important in both water and biological molecules that it is given a category of its own.

Hydrogen Bonding When a hydrogen atom is covalently bonded to a highly electronegative atom (such as fluorine, oxygen or nitrogen), the electronegative atom draws the electrons away from the hydrogen. The proton is said to be “unshielded” meaning that another molecule’s lone pair can approach this proton much more closely. Because hydrogen has no electrons other than the bonding electrons, the positive proton that makes up the hydrogen nucleus is nearly bare.

Hydrogen Bonding Since the electrostatic attractive force becomes stronger as the charges come closer together, this attractive force is often much stronger than the other types of dipole – dipole interactions. Any molecule with an –OH group or a –NH group in any part of its structure will show hydrogen bonding. Because water consists entirely of oxygen and hydrogen atoms and has two O-H bonds, hydrogen bonding is an important factor in the structure and properties of water.

Hydrogen Bonding All hydrogen atoms in water carry a  + charge while all oxygen atoms carry a  - charge. A single oxygen atom of one water molecule can be hydrogen bonded to as many as six hydrogen atoms in different water molecules at the same time.

Hydrogen Bonding Individual hydrogen bonds are much weaker than covalent bonds. However, when many hydrogen bonds are acting on each water molecule at the same time, they have a significant effect on the properties of water. Strong surface tension High specific heat capacity Solvent properties

Dissolving Salt NaCl crystal structure NaCl in water

Hydrogen Bonding The hydrogen bonding of water molecules, together with the bond angle between the oxygen and hydrogen atoms, gives snowflakes their characteristic six-sided shape.

Hydrogen Bonding The hydrogen bonds in ice are longer than they are in water, making ice less dense than liquid water. It is the orientation of the hydrogen bonds in ice that gives them their maximum strength.

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