Ionic Bonding

IONIC BONDING Formation of ionic compounds When ionic compounds are formed, they do not form individual molecules, but instead form giant three dimensional networks called crystal lattices. They are highly structured to minimise lattice energy - maximising attractions and minimising repulsions. An example of this is shown below of sodium chloride As you can see here, each of the sodium ions are connected to six chlorine ions, and vice versa. This means it has a coordination number of 6, as each atom is bonded to six other atoms. Many different lattice structures exist (e.g. zinc sulfide has a coordination number of 4), however what is common amongst all ionic compounds is the high level of structure and organisation of the ions. The ionic bonding lattice is simple and therefore results in limitations of this model. The limitations include: 1. It doesn't explain the forces of attraction between the ions or movement of electrons to form ions. 2. It only shows the outermost layer of the compound. Structures of ionic compounds lonic compounds form through electrostatic bonding between pairs of oppositely charged ions (electrically charged atoms). The type of ion each atom forms can be explained by the Octet Rule: The preferred ion occurs when a full outer shell of 8 valence electrons is attained through the loss or gain of electrons. Metals (Groups 1-3) lose electrons to obtain a full outer shell of electrons, and form positively + charged ions e.g. Na'. These are called cations. Non-metals (Groups 5-7) accept electrons to obtain a full outer shell of electrons to form a negatively charged ions e.g. Cl. These are called anions. Thus, ionic compounds are often composed of metal cations and non-metal anions. As the loss or gain of electrons produces charged ions, the opposite charges attract one another to form the highly structured 'lattices'. A compound is not stable unless the charge balance is equal In the example above, sodium (Na) needs to lose 1 electron to have a full outer shell and chlorine (CI) needs to gain one electron to have a full outer shell. When sodium (Na) loses one electron and donates it to chlorine, it becomes a positively charged cations, with a 1+ charge. When chlorine gains one electron from sodium to have a full outer shell, it becomes a negatively charged anion, with a 1- charge. Properties of ionic compounds lonic compounds all share similar properties, as they all have a similar structure. These include: ● very high boiling and melting points, due to the strong attraction between the oppositely charged ions (cations and anions). ● conduct electricity when dissolved in water, or when molten, as there are free moving charged particles (anions and cations). When solid, they do not conduct electricity (as there are no free moving charged particles, as they are held in position in the crystal lattice) and so act as insulators. ● hard, due to the strong forces of attractions between oppositely charges cations and anions ● brittle, when a strong force is applied to the structure, it shatters, as the ions move relative to one another and like charges will be next to each other and result in a repulsion. The crystal shatters. ● volatile, refers to the tendency of a substance to vaporize. For ionic compounds the electrostatic forces of attraction are strong and so the volatility of these compounds is very low (they require a lot of energy to vaporize) ● solubility, some ionic compounds dissolve in polar substances such as water (molecules of water are termed polar because their molecules have partial charges). The partial charges in a water molecule are able to interact with the charged ions and cause them to separate. They are not, however, soluble in non polar organic solvents (such as oil based solvents). They form structured crystal lattices, unlike other amorphous solids.