Bonding

Intramolecular forces

Forces inside molecules such as ionic or covalent bonds. These are very strong

an ionic bond

Am electrostatic attraction between two oppositely charges ions

Anion

A negatively charged ion

Cation

A positively charged ion

Structure of ionic compounds

Each ion is surrounded by 6 oppositely charged ions, these ions are attracted to each other forming a giant ionic lattice which is solid in room temperature

Properties of ionic compounds

Can conduct electricity only when molten or dissolved as then the ions are free to move and carry a charge, a large amount of energy is needed to break the strong electrostatic forces of attraction so ionic compounds have a high melting and boiling point, ionic lattices dissolve in polar substances like water as the polar molecules break the lattice down.

Covalent bond

The strong electrostatic forces of attraction between a shared pair of electrons and the nuclei of the bonded atom. Occur between two non-metals

Dative covalent bond

A shared pair of electrons which has been provided by one bonding atom only. Represented by an arrow in molecules like NH4+

Lone pairs

An outer electron pair not used for chemical bonding. This pair gives a concentrated region of charge around the atom

Average bond enthalpy

Measures the energy required to break a covalent bond

Factors affecting the strength of a covalent bond

How much the outer atomic orbitals of the bonded atoms overlap and how strongly the atomic nuclei are attracted to the bonded electrons

Special covalent bond cases

A few compounds can have less than 8 electrons in their outer shells and some compounds can use the d-orbital to expand the octet so have more than 8 electrons in their outer shells

Simple covalent compound properties

even though some covalent molecules have permanent dipoles overall covalent molecules are uncharged so can’t conduct electricity, covalent compounds with hydrogen bonds can form hydrogen bonds with water so will be soluble, non-polar covalent molecule will be insoluble

Induced dipole-dipole interactions

A type of intermolecular force which occurs as electrons in charged clouds are constantly moving quickly so can be on either side of an atom at any given moment, making that side of the atom slightly negative creating a temporary dipole. This can then pass onto a neighbouring atom as its electrons are repelled by the temporary dipole causing another induced temporary dipole to form in that atom in the opposite direction of the first temporary dipole. These two dipoles are then attracted to each other. The second temporary dipole can induce another temporary dipole in a third atom and so on.

Factors affecting induced dipole-dipole interactions (boiling points)

the amount of electrons and the molecular size of the atoms

what do induced dipole-dipole forces do to compounds

Increase their melting and boiling points and their viscosity

Permanent dipole-dipole interactions

A type of intermolecular force. These are the weak electrostatic forces of attraction between permanent dipoles in neighbouring polar molecules. These happen in addition to induced dipole-dipole interactions.

Determining boiling points

Strength of induced dipole-dipole forces between molecules determine it unless the molecule can make hydrogen bonds. If two molecules have a similar amount of electrons the strength of their induced dipole-dipole forces will be similar so if one of the substances has more polar molecules than the other it will have stronger permanent dipole-dipole interactions so a higher boiling point.

Hydrogen bonding

The strongest type of intermolecular forces. These bonds are permanent dipole-dipole forces that happen only in particular circumstances

Conditions for hydrogen bonding to be possible

A hydrogen atom has to be covalently bonded to a small electronegative atom (like O,N or F) and there has to be at least one lone pair on the electronegative atom

How hydrogen bonds affect Compounds

They make compounds soluble in water and make them have a higher melting and boiling point due to the extra energy needed to break the hydrogen bonds

How hydrogen bonds affect water

Ice is held together in a lattice by hydrogen bonds so has more of them than liquid water as hydrogen bonds are relatively long the water molecules are more spread out in ice than in water therefore ice is less dense than liquid water so can float in top of it and create an insulating layer.

Diamond

A macro molecular structure arranged in a giant covalent lattice made up of only carbon. Each carbon makes 4 covalent bonds so diamond has a rigid structure, is very hard and has a high melting and boiling point

Graphite

A macro molecular structure arranged in a giant covalent lattice made up of only carbon atoms. Each carbon makes 3 covalent bonds in a flat hexagonal sheet which make up the layers in graphite. There is a delocalised electron for every carbon atom which can travel through the layers allowing graphite to conduct electricity. There are only weak dipole-dipole forces between the layers of graphite allowing them to slide over each other making graphite a good lubricant

Graphene

Consists of single 2D sheets of graphite that are just one atom thick. These sheets are formed bout of hexagonal rings of carbon that create a very strong, rigid material that is extremely lightweight. Also contains delocalised electrons as the carbons make only 3 bonds so it can conduct electricity

how does the amount of electrons affect london forces (boiling points)

The more electrons a molecule has the stronger the induced dipole-dipole forces and more of them are created as well

how does the molecular size affect induced dipole-dipole forces (boiling point)

the bigger the molecular size of the atoms the stronger the induced dipole-dipole forces between them as it means there’s more electrons and the electrons are further from the nucleus asking it easier to form temporary dipoles