- Created by: xylaic
- Created on: 05-11-15 17:06
Ionic bonding is a strong electrostatic attraction between oppositely charged ions which form a lattice once all are formed.
They have a high melting and boiling point as a lot of kinetic energy is required to overcome the strong electrostatic attraction between ions. They higher the charge and the smaller the ions, the larger the forces of attraction.
They conduct electricity when molten as they have ions that are free to move but not when solid as the ions are fixed in one place.
They are also hard and brittle.
Molecular covalent bonding
A covalent bond consists of a shared pair of electrons between two atoms.
A dative or a co-ordiate bond is when one atom donates both the pair of electrons to another atom which is capable of accepting them.
Molecular molecules are held together by intermolecular forces with little difference between melting and boiling point however they are very low.
Brittle and do not carry charged particles so they do not conduct electricity. Form molecular covalent crystals and crystalline substances e.g ice and iodine
Macromolecular covalent bonding
Giant covalent substances such as diamond and graphite which are allotropes of carbon.
Diamond has a very high melting point, is the hardest substance, is used for tipping cutting tools, strongly bonded in a tetrahedral arrangement at 109.5 degrees and does not conduct electricity.
Graphite conducts electricity due to delocalised electrons between teh layers, has a high melting point due to many covalent bonds between carbon atoms, a hexagonal arrangement at 120 degrees and the weak forces between the layers can be broken allowing them to slide over one another.
The metallic bond is the electrostatic attraction between the delocalised electrons and the positive metal ions in the lattice.
Metals conduct electricitydue to their delocalised electrons, this also allows them to conduct heat as their particles are close together.
Metals are ductile and malleable- they can be drawn out and hammered into shape. Metals also have high densities as the positive ions are packed tightly together.
Metals have high melting points as they have a regular structure with electrostatic attractions which take a lot of energy in the form of heat to overcome.
The electronegativity of an atom is defined as the ability of an atom to attract bonding pairs of electrons in a covalent bond towards itself.
Electronegativity increases across a period as there are more protons wihtout any additional shielding so the atomic radii decreases.
Electronegativity decreases down a group as there is aditional shielding although there are more protons, there is a larger atomic radius.
Electronegativity depends on the factors: distance of bonding pair from the nucleus, the shielding my inner electrons and the size of the nuclear charge.
Van der walls forces
The electrons in this molecule is not static, they are in constant motion so there is likely to be a surplus of electrons at one atom at any given time.This is known as a temporary dipole.
Van der walls forces are between a positive end of one molecule and negative end of another molecule, induced dipole. This is the weakest form of bonding against dipole and hydrogen.
The strength of the van der walls forces depend on two factors:
- The number of electrons within the molecule
- The surface area of the molecule
Dipole-dipole bonding is between molecules with permanent dipoles, it slightly increases the strength of intermollecular bonding and is stronger than van der walls forces.
Here, the permanent dipole has a huge effect on the strength of the molecule, In the H-f bonding molecule teh H atom has very little electron density around its nucleus so it can get very close to F.
This forms ver strong intermolecular dipole-dipole bonds which are called Hydrogen bonds. This is when the H is bonded to a very electronegative element such as N, O or F. Alcohols, urea, acid amides, amines and carboxylic acids contains H-bonds.
The H-bonds dramtically higher boiling poin, makes ethanol have a higher boiling point and to be miscible with water, and increases the boiling point of propanone.