Bonding and Intermolecular forces


Shapes of Molecules

Molecular shape depends of electron pairs around the central atom Molecules and molecular ions come in loads of different shapes. The shape depends on the numbers of electrons in the outer shell of the central atom. In ammonia for example the outermost shell has four pairs of electrons. Lone pairs of electrons are not shared. Electron pairs repel each other. Electrons are all negatively charged so it's pretty obvious that electron pairs will repel each other as much as they can. This sounds straightforward, but the type of electron pair affects how much it repels other electron pairs. Lone pairs repel more than bonding pairs. So, the greatest angles are between lone pairs, and bond angles between bonding pairs are often reduced because they are pushed together by lone-pair repulsion. This is known as the 'electron-pair repulsion theory'. Methane, ammonia and water have four pairs of electrons, but in ammonia one lone pair reduces the bond angle from 109.5 degrees to 107 degrees. In water, two lone pairs reduce it even more to 104.5 degrees. Single-bonded carbon atoms have their bonds arranged like a tetrahedron. When a carbon makes four single bonds the atoms around each carbon form a tetrahedral shape. The angle between any two covalent bonds is 109.5 degrees. At this angle, the bonds are as far apart from each other as possible. You need to be able to explain the shapes of several molecules and any that are basically the same. 2 electron pairs on central atom - linear molecule (e.g beryllium chloride or carbon dioxide). 3 electron pairs on central atom: no lone pairs - trigonal planar, one lone pair is non-linear or bent. In carbonates and nitrates the bonds are all midway between single and double bonds. 4 electron pairs on central atom: No lone pairs - tetrahedral, one lone pair - trigonal pyradimal, two lone pairs - non-linear or bent. 5 electron pairs - trigonal bipyradimal, 6 electron pairs - octahedral. Some central atoms can use d orbitals to expand the octet, which means they have more than 8 bonding electrons.

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Carbon Structures

Diamond is the hardest known substance. Allotropes are different forms of the same element in the same state. Carbon forms three allotropes - diamond, graphite and fullerenes. Each allotrope has a different giant molecular structure. Diamond is made up of carbon atoms. Each carbon atom is covalently conded with sigma bonds to four other carbon atoms. The atoms arrange themselves in a tetrahedral shape - its crystal lattice structure. Because of it's strong covalent bonds diamond has a very high melting point (it actually sublimes), is extremely hard, vibrations travel easily easily through the stiff lattice, so it's a good thermal conductor, it wont dissolve in any solvent, and doesn't conduct electricity. Graphite is another allotrope of carbon. Graphite has a different macromolecular structure than diamond. The carbon atoms are arranged in sheets of flat hexagons covalently bonded with three bonds each. The fourth outer electron of each carbon atom is delocalised. Graphite's structure means it has some similarities and differences from diamond. The weak bonds between the layers in graphite are easily broken so the sheets can slide over each other. The 'delocalised' electrons in graphite aren't attached to any particular carbon atom and are free to move along the sheets, so an electric current can flow. The layers are quite far apart compared to the length of the covalent bonds, so graphite is less dese than diamond. The strong covalent bonds in the sheets mean graphite also has a high melting point (it sublimes at over 3900 K). Like diamond graphite is insoluble in any solvent. The covalent bonds in the sheets are too difficult to break. The fullerenes include hollow balls. Fullerenes are molceules of carbon shaped like hollow balls or tubes. Each carbon atom forms three covalent bonds with its neighbours, leaving free electrons to conduct electricity. Fullerenes are nanoparticles. The first fullerene discovered was buckminsterfullerene with 60 carbon atoms joined to make a ball. Many fullerens are soluble in organic solvents and form brightly coloured solutions. Because they're hollow they can cage other molecules. The fullerene structure forms around another molecule, which is then trapped inside. This could be used to deliver drugs into specific cells of the body. Fullerenes are used in nanotechnology. At a very tiny scale materials often have very different properties from 'bulk' forms of the same substance. The fullerens also include tubes. A carbon nanotube is like a single layer of graphite rolled up into a tiny hollow cylinder. All the covalent bonds make carbon nanotubes very strong. They can be used to reinforce graphite in tennis rackets and to make stronger, lighter building materials. They conduct electricity so they can be used as tiny wires in circuits. The ends of a nanotube can be 'capped', or closed off, to create a large molecular cage structure. There's a debate about the safety of nanotechnology. When someone comes up with a new way to use nanotechnology there are often questions about whether it's safe. As with all practical application you weigh the benefits against the risks. A new nanotechnology could improve people's health or quality of life. Before they're used, new products and technologies are thoroughly tested to make sure they're not harmful. Some people question whether it has been tested enough. Others think we should take the risk of using naotechnologies that seem to be safe.

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Electronegativity and Polarisation

There are limitations to models of bonding. With covalent bonds, the dot-and-cross model only illustrates how the atoms in a compound share their electron pairs. It can't explain the length of the covalent bond or the overall shape of the molecule. Most bonds aren't purely ionic or covalent but somewhere in between. Bond polarisation means most compounds have ionic anc covalent properties. There's a gradual tranistion from ionic to covalent bonding. Very few compounds come close to being purely ionic. Only bonds between atoms of a single element like hydrogen can be purely covalent. 'Purely ionic' and purely covalent bonding are the extremes of a continuum. Most compounds come between these two extremes, which means they often have ionic and covalent properties, like covalent hydrogen chloride gas molecules dissolve to form ionic hydrochloric acid. Some atoms attract bonding electrons more than other atoms. The ability to attract the bonding electrons in a covalent bond is called electronegativity, usually measured using the Pauling scale. Flourine is the most electronegative at 4.0. Oxygen, nitrogen and chlorine are also strongly electronegative. Covalent bonds may be polarised by differences in electronegativity. In a covalent bond between two atoms of different electronegativities, the bonding electrons are pulled towards the more electronegative atom. This makes the bond polar. The covalent bonds in diatomic molecules are non-polar - the atoms are the same, so they have the same electronegativity. Some elements, like carbon and hydrogen, have pretty similar electronegativities, so the bonds are essentially non-polar. In a polar bond, the difference in electronegativity between the two atoms causes a dipole. A dipole is a difference in charge between the two atoms caused by a shift in the electron density in the bond. If the difference is large enough the bond becomes pretty much ionic. Polar bonds don't always make polar molecules. Some molecules with polar bonds are polar molecules - the molecule itself has a permanent dipole. Whether a molecule itself is polar depends on its shape and the polarity of the bonds. In a simple molecule the polar bond gives the whole molecule a permanent dipole. A more complicated molecule may have several polar bonds. If the polar bonds are arranged so they point in opposite directions, they'll cancel each other out. If they point in the same direction, it will be polar. Lone pairs of electrons on the central atom also have an effect on the overall polarity and may cancel out the dipole created by the bonding pairs. The length of a bond is related to its strength. In covalent bonds there isn't an attraction between the nuclei and shared electrons. The distance between the two nuclei is the distance where the attractive and repulsive forces balance each other. This is the bond length. The stronger the attraction between the atoms, the higher bond enthalpy and the shorter the bond length. If there's more attraction, the nuclei will pull close together. Example carbon double bonds are shorter than single bond due to more electrons and more attraction, and carbon triple bonds are shorter than double ones - more electrons are shared.

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Intermolecular Forces

Intermolecular forces are very weak. Intermolecular forces are forces between molecules. They're much weaker than ionic, covalent or metallic bonds. There are three types you need to know: London forces (weakest), permanent dipole-dipole interactions, and hydrogen bonding (strongest). London forces are found between all atoms and molecules. London forces cause all atoms and molecules to be attracted to each other. Electrons in charge clouds are always moving really quickly. At any particular moment, the electrons in an atom are likely to be more to one side. At this moment, the atom would have a temporary dipole. This dipole can cause another temporary dipole in the opposite direction on a neighbouring atom. The two dipole are then attracted to each other.  second dipole can cause another dipole in a third atom. Because the electrons are constantly moving, the dipoles are being created and destroyed all Even though the dipoles keep changing, the overall effect is for the atoms to be attracted to one another. Stronger London forces mean higher melting and boiling points. Not all London forces are the same strength - larger molecules have larger electron clouds, meaning stronger London forces. Molecules with greater surface areas also have stronger London forces because they have a bigger exposed electron cloud. When you boil a liqiud, you need to overcome the intermolecular forces, so that the particles can escape from the liqiud surface. You need more energy to overcome stronger intermolecular forces, so liquids with stronger London forces will have higher boiling points. Melting solids also involves overcoming intermolecular forces, so solids with stronger London forces have higher melting points. Boiling points of alkanes depend on size and shape. The smallest alkanes are gases, whilst larger alkanes are liquids, so they have higher boiling points. Alkanes have covalent bonds inside the molecules. Between the molecules, there are London forces which hold them together. The longer the carbon chain, the stronger the London forces - there's more molecular surface area and more electrons to interact. Branched-chain alkanes have smaller molecular surface area and they can't pack as closely together - so the London forces are reduced. Polar molecules have permanent dipole-dipole forces. The delta +ve and -ve charges on polar molecules cause weak electrostatic forces of attraction between molecules. The polar molecules in a polar liquid can turn around to make opposite charges attract - which is why a stream of water attracts to both +ve and -ve charged rods. Hydrogen bonding is the strongest intermolecular force. Hydrogen bonding only happens when hydrogen is covalently bonded to flourine, nitrogen or oxygen. These are all very electronegative, so they draw the bonding electrons away from the hydrogen atom. The bond is so polarised, and the hydrogen has such a high charge density because it's small, that the hydrogen atoms from weak bonds with lone pairs of electrons on the flourine, nitrogen or oxygen atoms of other molecules. Molecules with hydrogen bonding are usually organic, containing -OH or -NH groups. Water and ammonia have hydrogen bonding. Hydrogen bonding has a huge effect on the properties of substances. They have higher boiling and melting points than other similar molecules because you need more energy to break the hydrogen bonds. This is the case with water, and hydrogen flouride, which has a higher boiling point compared to other hydrogen halides. Ice has more hydrogen bonds than liqiud water, and hydrogen bonds are relatively long. So the H2O molecules in ice are further apart on average, making it less dense than water.

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Solubility is affected by bonding. For one substance to dissolve into another, bonds in the substance have to break, bonds in the solvent have to break, and new bonds have to form between the substance and the solvent. Usually a substance will only dissolve if the strength of the new bonds formed is about the same as, or greater than, the strength of the bonds that are broken. There are polar and non-polar solvents. There are two main types of solvent: Polar solvents such as water, which has hydrogen bonds between water molecules. Non-polar solvents such as hexane, which has London forces between it's molecules. Many substances are soluble in one type of solvent. and not the other. Ionic substances dissolve in polar solvents such as water. The ions are attracted to the oppositely charged ends of the water molecules. The ions are pulled away from the ionic lattice by the water molecules, which surroind the ions, known as hydration.  Some ionic substances don't dissolve because the bonding between them is too strong, like in aluminium oxide, where the bonds between the ions are stronger than the ones that would form with water molecules. Alcohols also dissolve in polar solvents such as water. Alcohols are covalent but they dissolve in water because the polar O-H bond in an alcohol is attracted to the polar O-H bonds in water. Hydrogen bonds form between the lone pairs on the delta -ve oxygen atoms and the delta +ve hydrogen atoms. The carbon chain is not attracted to water, so the longer the chain, the less soluble. Not all molecules with polar bonds dissolve in water. Halogenoalkanes contain polar bonds but their dipoles aren't strong enough to form hydrogen bonds with water. The hydrogen bonding in water is stronger than the bonds that would be formed, so they don't dissolve. Non-polar substances dissolve best in non-polar solvents. Non-polar substances like ethene have London forces between their molecules. They form similar bonds with non-polar solvents like hexane - so they tend to dissolve in them. Molecules of polar solvents such as water are attracted to each other more strongly than they are to non-polar molecules such as iodine - so non-polar substances don't tend to dissolve easily in polar solvents.

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