**Types of cohesive force**

**Ionic bonding:**transfer of electrons from one atom to another, leading to charged atoms with filled shells, e.g. Na^{+}Cl^{–}.**Covalent bonding:**unpaired valence electrons mix (hybridise) to give a lower energy state. Covalent bonding is very directional.

s, p

_{x}, p_{y}, p_{z}-> sp^{3}hybridisation, tetrahedral diamond structure**Metallic bonding:**outer atomic orbitals overlap several other atoms, so the electrons become mobile and delocalised (reducing KE). Metallic bonding is not directional, and is weaker than covalent bonding.**Hydrogen bonding:**covalent bonds between different atomic species lead to polar molecules with electrostatic attraction between δ^{+}and δ^{–}of different molecules. Hydrogen bonding is much weaker than ionic or covalent bonding.**Van der Waals forces:**the random motion of electrons in an atom leads to a temporarily induced dipole. This inturn induces a dipole on neighbouring atoms.E-field at

*p*: induces ._{2}Energy of interaction

–

**Lattice:** an infinite array of points, each with an identical environment. Each point has position **r=***n _{1}*

**a**

_{1}+*n2*

**a**+

_{2}*n*

_{3}**a**where the

_{3}**a**are

_{i}**lattice vectors**.

**Basis:** the structural unit (group of atoms) associated with one lattice point.

**Crystal**** structure:** the convolution of a lattice with a basis, e.g.

lattice x basis = crystal structure (each dot is a lattice point)

**Unit cell:** a polyhedral region of space, such that when many identical unit cells are tessellated, they form a crystal structure (i.e. the ‘building block’ of said structure). The unit cell in 3D can be specified by three vectors, **a**, **b** and **c**. Positions within the unit cell can be specified by fractional coordinates (i.e fractios of *a*, *b* and *c*).

**Conventional unit cell:**can contain more than one lattice point (is usually an easy to work with shape like a square or cube that makes the maths easier).

**Primitive unit cell:**contains only one lattice point.

**Wigner-Seitz unit cell:**contains one lattice point, and displays the same point symmetry as the lattice.