Shapes of molecules and intermolecular forces

Shapes of molecules and intermolecular forces

Define ionic bonding

An ionic bond is the electrostatic force of attraction between positive and negative ions

Explain the solid structures of giant ionic lattices

This is a result of oppositely charged ions strongly attracted in all directions

Explain the effect of structure and bonding on the physical properties of ionic compounds, including melting and boiling points, solubility and electrical conductivity in solid, liquid and aqueous states.

The network of strong ionic bonds requires a high amount of energy to break, resulting in high melting and boiling points.

They are usually soluble in polar solvents such as water as the water molecules can break up the lattice and surround the ions to form a solution. Polar water molecules are attracted towards ions on the surface of the ionic lattice. Water molecules bond to the ions, weakening ionic bonds, until they break. These ions then become surrounded by water molecules and break free from the lattice.

In a solid state, giant ionic compounds do not conduct electricity as the ions are fixed in place and don’t move. However, they do conduct electricity when in molten state or in aqueous solution as the ions are now free to move and the sea of free electrons can carry a current.

Define covalent bonding

The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.

Explain the solid structures of simple molecular lattices, as covalently bonded molecules attracted by intermolecular forces.

Weak intermolecular forces of attraction allow the molecules to separate from each other quite easily so they have low melting points and low boiling points.

Molecules do not carry an overall charge and so they do not conduct electricity. Some molecular substances may, however, react with water to form solutions containing ions which do allow the flow of an electric current.

Molecules with an overall dipole may be soluble to varying degrees in the polar water solvent. Where hydrogen bonding with water molecules is possible this might favour solubility.

Non-polar molecules have very little solubility in water, but are soluble in non-polar solvents as the strength of the intermolecular bonding is similar.

Explain the effect of structure and bonding on the physical properties of covalent compounds with simple molecular lattice structures including melting and boiling points, solubility and electrical conductivity.

Molecular substances tend to be gases, liquids or low melting point solids, because the intermolecular forces of attraction are comparatively weak. The size of the melting or boiling point will depend on the strength of the intermolecular forces. The presence of hydrogen bonding will lift the melting and boiling points. The larger the molecule the more van der Waals attractions are possible – and those will also need more energy to break.

Most molecular substances are insoluble in water, but are often soluble in organic solvents – which are themselves molecular.

Molecular substances won’t conduct electricity: there are no charged ions or delocalised electrons to carry a current.

Describe and explain the bonding and structure in graphite.

Graphite is a giant covalent lattice that forms in layers. It is a good conductor because it has delocalised electrons and has a high melting point because strong covalent bonds have to be broken. It is soft because the weak Van der Waal bonds can break allowing the layers to slide.

Draw of ‘dot-and-cross’ diagrams of molecules and ions to describe: single covalent bonding, multiple covalent bonding, lone pairs, dative covalent (coordinate) bonding.

Single covalent bonds are shown by drawing a single dot and cross in the overlapping part of the circle, multiple covalent bonds by drawing multiple many dot and cross in the overlapping part of the circle. Lone pairs of electrons not involved in bonding are shown by drawing two dots. Dative covalent (coordinate) bonding, a shared pair of electrons which has been provided by one of the bonding atoms only, is indicated by an arrow pointing away from the donor atom.

Define dative covalent bonding

A dative covalent bond is a shared pair of electrons which has been provided by one of the bonding atoms only.

What are London forces?

Induced dipole– dipole interactions caused by an uneven distribution of electrons causes a temporary dipole on one molecule inducing an opposite dipole onto the next molecule.

Define hydrogen bonding.

The intermolecular bonding between an electron deficient H+ on one molecule and a lone pair of electrons on a hydrogen or nitrogen molecule. They are shown by a dotted line on a diagram.

Draw a hydrogen bond




Explain the anomalous properties of H2O resulting from hydrogen bonding.

The density of ice compared with water. This is because when water freezes into ice, a network of hydrogen bonds form. The H2O molecules in ice are held apart in an open lattice structure. There is lot of space in the structure which gives ice a lower density than water. When ice melts the hydrogen bonds break which allows the liquid H2O molecules to move closer together. Its relatively high melting and boiling points. Hydrogen bonds are extra forces, on top of London forces. When ice is melted or water boiled, these strong H-bonds need to be broken. More energy is needed to do this than for other molecules which do not possess H-bonding.

Explain the use of the term average bond enthalpy.

The average bond enthalpy is a measurement of covalent bond strength: the larger the value of the average bond enthalpy, the stronger the covalent bond.

Explain electronegativity.

The ability of an atom to attract the bonding electrons in a covalent bond. The trend for electronegativity is to increase as you move from left to right and bottom to top across the periodic table; this means that the most electronegative atom is Fluorine. Furthermore, if the electronegativity difference between two atoms is very large, then the bond type tends to be more ionic, however if the difference in electronegativity is small then it is a nonpolar covalent bond.

Is this bond polar?

A polar bond is a permanent dipole within molecules containing covalently-bonded atoms with different electronegativities.

Is this molecule polar?

No because the molecule is symmetrical therefore the dipoles cancel out.

Yes because the molecule contains polar bonds and is not symmetrical due to the _____ so the dipoles do not cancel each other out.