Revision:Bonding

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Ionic Bonding

Evidence for existence of ions

• Physical properties of ionic compounds
• Electron density maps
• Movement of ions, e.g. in electrolysis of copper chromate(VI)

(Needs expanding)

When atoms gain or lose electrons, they become charged ions. It is the attraction between oppositely charged ions which form an ionic bond. Atoms gain or lose electrons in order to try and gain a full outer shell, which is more stable.

(Dot-Cross diagram for ionic bonding)

Ionic compounds usually exist in giant ionic lattices held together by strong electrostatic attraction between oppositely charged ions. The energy holding this lattice together is called the lattice energy, and can be calculated from the energies of each of the steps needed to form the lattice from the elements:

• Enthalpy of formation of compound
• -Atomisation
• -Ionisation and electron affinity

The lattice energy calculated from this method differs to experimental values, suggesting the ionic model is not completely accurate. This is because the positively charged cations attract the electrons of the anion, so ionic bonds are polarised and are partly covalent. The polarising power of cations is affected by its charge density, and the polarisability of the anion depends on its size.

An ionic radius is the radius of an ion. Down a group, as more and more shells are being filled, ionic radii get larger. Positively charged ions, which lose electrons, generally have smaller ionic radii than negatively charged ions, which gain electrons.

Covalent Bonding

Covalent bonds are formed by the sharing of electrons between atoms. The attraction of the positively charged nucleus and the shared electrons means covalent bonds are very strong; giant atomic structures have high melting points as the covalent bonds are hard to break. (Need electron density map for simple molecules)

(Dot-Cross diagram for covalent bonding, multiple covalent bonds, and dative covalent bonds)

Metallic Bonding

Metals exist in a giant lattice of metal ions in a sea of delocalised electrons. Metallic bonding is the strong attraction of the ions and the sea of electrons. The sea of electrons mean metals are conductors. (Melting point of metals)