INTERMOLECULAR FORCES. Introduction: The physical properties of melting point, boiling point, vapor pressure, evaporation, viscosity, surface tension, and solubility are related to the strength of attractive forces between molecules.
The physical properties of melting point, boiling point, vapor pressure, evaporation, viscosity, surface tension, and solubility are related to the strength of attractive forces between molecules.
These attractive forces are called Intermolecular Forces. The amount of "stick togetherness" is important in the interpretation of the various properties listed above.
There are four types of intermolecular forces. Most of the intermolecular forces are identical to bonding between atoms in a single molecule. Intermolecular forces just extend the thinking to forces between molecules and follows the patterns already set by the bonding within molecules.
Two oppositely-charged particles flying about in a vacuum will be attracted toward each other, and the force becomes stronger and stronger as they approach until eventually they will stick together and a considerable amount of energy will be required to separate them.
They form an ion-pair, a new particle which has a positively-charged area and a negatively-charged area.
There are fairly strong interactions between these ion pairs and free ions, so that these the clusters tend to grow, and they will eventually fall out of the gas phase as a liquid or solid (depending on the temperature).
d8 means :
Whoops! Chromium breaks the sequence. In chromium, the electrons in the 3d and 4s orbitals rearrange so that there is one electron in each orbital. It would be convenient if the sequence was tidy - but it\'s not!
A transition element is defined as one which has partially filled d orbitals either in the element or any of its compounds. Zinc (at the right-hand end of the d-block) always has a completely full 3d level (3d10) and so doesn\'t count as a transition element.
Electric current is the flow of electrons in a wire. In metals, the outer electrons of the atoms belong to a ‘cloud’ of delocalised electrons. They are no longer firmly held by a specific atom, but instead they can move freely through the lattice of positive metal ions. Normally they move randomly. However, when the wire is connected to a cell, they are pushed away from the negative terminal and drawn to the positive one. The cloud of electrons drifts through the wire. The drift velocity of the cloud is about 3 mm s-1. The electrons within the cloud are still moving randomly (at much higher speeds) - rather like a swarm of bees leaving a hive.
Animation showing electrons moving randomly and then the movement of electrons through a wire
The positive metal ions in a metal structure are packed closely together in a symmetrical geometric arrangement. They don’t move from their position in the lattice but they are constantly vibrating. If a metal is heated, the positive metal ions vibrate more vigorously. These ions collide with neighbouring ions and make them vibrate more vigorously too. In this way, the energy is passed, or conducted, through the metal.
A cool lattice. If we heat the left hand end, then the energy will be carried along by conduction.
Let\'s look at just a few electrons.
How a metal conducts by the movement of free electrons.
Metals are good conductors of heat. There are two reasons for this:
A. Outermost electrons wander freely through metal. Metal consists of cations held together by negatively-charged electron "glue.“
B. Free electrons can move rapidly in response to electric fields, hence metals are a good conductor of electricity.
C. Free electrons can transmit kinetic energy rapidly, hence metals are good conductors of heat.
D. The layers of atoms in metal are hard to pull apart because of the electrons holding them together, hence metals are tough. But individual atoms are not held to any other specific atoms, hence atoms slip easily past one another. Thus metals are ductile. Metallic Bonding is the basis of our industrial civilization.
If the difference in electronegativity is not so great, however, there will be some degree of sharing of the electrons between the two atoms. The result is the same whether two ions come together or two atoms come together:
Dipole - Dipole Interactions
Liquid water has a partially ordered structure in which hydrogen bonds are constantly being formed and breaking up.
This animation shows how water molecules are able to break the forces of attraction i.e. the hydrogen bonds to each other and escape as the gas molecule. This is what is happening inside the gas bubble as it is rising to the surface to break and release the water gas molecules.
The greater the forces of attraction the higher the boiling point or the greater the polarity the higher the boiling point.
The increase in boiling point happens because the molecules are getting larger with more electrons, and so van der Waals dispersion forces become greater.
If you repeat this exercise with the hydrides of elements in Groups 5, 6 and 7, something odd happens.
The molecules which have this extra bonding are:
Notice that in each of these molecules:
However complicated the negative ion, there will always be lone pairs that the hydrogen atoms from the water molecules can hydrogen bond to.
More normal behavior is seen in dimethyl ether (CH3)2O which has no hydrogen bonds possible
The water strider is an insect of characteristic length 1cm and weight 10 dynes that resides on the surface of ponds, rivers, lakes and the open ocean. Its weight is supported by the surface tension force generated by curvature of the free surface. Its body and legs are covered by thousands of hairs that render its legs effectively non-wetting (Andersen 1982). The water strider propels itself by driving its central pair of legs in a sculling motion. In order for it to move, it must transfer momentum to the underlying fluid. It was previously assumed that this transfer occurs exclusively through capillary waves excited by the leg stroke (Denny 1993). Our experiments reveal that, conversely, the strider transfers momentum to the fluid principally through dipolar vortices shed by its driving legs. The strider thus generates thrust by rowing, using its legs as oars, and the meniscii beneath its driving legs as blades.
The more red / blue differences, the more polar the molecule. If the surface is largely white or lighter color shades, the molecule is mostly non-polar.
A. Water molecules are asymmetrical. The positively-charged portions of one are attracted to the negatively-charged parts of another. It takes a lot of energy to pull them apart. Hence:
B. The asymmetrical charge distribution on a water molecule makes it very effective in dissolving ionically-bonded materials. However, it is not an effective solvent of covalently bonded materials (oil and water don\'t mix). Hence:
C. When water freezes, it assumes a very open structure and actually expands. Most materials shrink when they freeze and sink in their liquid phases. Implications:
These instantaneous dipoles may be induced and stabilized as an ion or a polar molecule approaches the non-polar molecule.
Interactions between ions, dipoles, and induced dipoles account for many properties of molecules - deviations from ideal gas behavior in the vapor state, and the condensation of gases to the liquid or solid states.
In general, stronger interactions allow the solid and liquid states to persist to higher temperatures.
However, non-polar molecules show similar behavior, indicating that there are some types of intermolecular interactions that cannot be attributed to simple electrical attractions. These interactions are generally called dispersion forces.
The diamond crystal bond structure gives the gem its hardness and differentiates it from graphite.
You might argue that carbon has to form 4 bonds because of its 4 unpaired electrons, whereas in this diagram it only seems to be forming 3 bonds to the neighbouring carbons. This diagram is something of a simplification, and shows the arrangement of atoms rather than the bonding.
This animation shows how graphite becomes diamond under extreme heat and pressure
Building the orbital model
Because each carbon is only joining to three other atoms, when the carbon atoms hybridise their outer orbitals before forming bonds, they only need to hybridise three of the orbitals rather than all four. They use the 2s electron and two of the 2p electrons, but leave the other 2p electron unchanged.
Each carbon atom now looks like the diagram below. This is all exactly the same as happens in ethene.
Remember:A sigma bond is formed by the end-to-end overlap between atomic orbitals.
This is easily explained. Benzene is a regular hexagon because all the bonds are identical. The delocalisation of the electrons means that there aren\'t alternating double and single bonds.
This is accounted for by the delocalisation. As a general principle, the more you can spread electrons around - in other words, the more they are delocalised - the more stable the molecule becomes. The extra stability of benzene is often referred to as "delocalisation energy".
Department of Chemistry
University of Missouri-Rolla
Jim Clark 2005