MINISTRY OF EDUCATION SECONDARY ENGAGEMENT PROGRAMME GRADE 10 CHEMISTRY WEEK 8 Lesson 1 Topic: Structure and Bonding Sub-topic: Structure and Properties of Simple Ionic and Covalent Compounds Objectives: After reading and looking at related diagrams students will: describe ionic crystals, simple molecular crystals and giant molecular crystals. make diagrammatic representations of sodium chloride, graphite and diamond. Content: Ionic Crystals An ionic crystal consists of ions bound together by electrostatic attraction. The arrangement of ions in a regular, geometric structure is called a crystal lattice and varies depending on the ions’ sizes or the radius ratio (ratio of radii of the positive to the negative ion). Example: NaCl Structure of Ionic Crystal Structure of NaCl lattice Model of NaCl lattice 90 Properties of Ionic Crystals Property Explanation form crystals An ionic crystal is a regular structure, with the cation and anion alternating with each other and forming a three-dimensional structure based largely on the smaller ion evenly filling in the gaps between the larger ion. Eg. NaCl High melting and boiling points. High temperatures are required to overcome the attraction between the positive and negative ions. Therefore, a lot of energy is required to melt ionic compounds or cause them to boil. Conducts electricity when dissolved in water When ionic compounds are dissolved in water the dissociated ions are free to conduct electric charge through the solution. Molten ionic compounds also conduct electricity. Solubility Ionic compounds are usually soluble in polar solvents such as water, in which ions are free to move and interact with the solvent molecules. They are usually insoluble in non-polar solvents which do not allow free, movement of ions and interactions with solvent molecules. Molecular Crystals 1. Simple Molecular Crystals Liquids and solids composed of molecules are held together by van der Waals (or intermolecular) forces, and many of their properties reflect this weak binding. Intermolecular Forces Van der Waals Force All covalent molecules, whether polar or non-polar, develop temporary or instantaneous dipoles. This results from the uneven distribution of electronic charge lead to positive and negative charges within molecules. Van der Waals forces are weak attractions between oppositely charged ends of molecules with temporary dipoles. Example: Helium molecule 91 Diagram Showing Induced Dipole When a helium molecule with an instantaneous dipole is brought close to another helium molecule that is not polarized, it is attracted to the instantaneous dipole. This causes the uncharged non-polar helium molecule to develop induced dipoles by attracting or repelling its electrons. Hence a resultant attractive force (Van der Waals Force – London dispersion). Dispersion forces exist between all molecules, but they are the only forces that exist between nonpolar molecules. Polar molecules and ions can also induce dipoles in nonpolar molecules. This effect partially accounts for the solubility of molecular oxygen (nonpolar) in water and the ability of blood (which contains Fe cations) to bind oxygen. Hydrogen Bonds The hydrogen bond is a weak attraction between the slightly positive hydrogen atom in one molecule and the slightly electronegative atom in another polar molecule of the same type or a different type. Because of water’s polarity, individual water molecules are attracted to one another. Example: water and ammonia 92 Hydrogen Bonding in Water Hydrogen Bonding in Ammonia Properties of Simple Molecular Crystals Property Explanation Low melting point and boiling point Because of the weak intermolecular attractive forces between molecules, little energy is required to separate the molecules. Electrical conductivity There are no charged particles (ions or electrons) delocalized throughout the molecular crystal lattice to conduct electricity. They cannot conduct electricity in either the solid or molten state. Molecular crystals tend to dissolve in non-polar solvents such as alcohol. They are usually insoluble in polar solvents such as water. However, some may dissolve in water as a result of forming hydrogen bonds with it. Solubility 2. Giant Covalent Solids Giant molecular solids are a class of extended-lattice compounds in which each atom is covalently bonded to its nearest neighbors. This means that the entire crystal is, in effect, one giant molecule. These compounds exist as macromolecules in which strong covalent bonds extend in three dimensions. Examples of giant molecular solids are polymers, silicon dioxide, diamond, and graphite. 93 Properties of Giant Molecular Solids Property Very high melting and boiling points Solubility Variable electrical conductivity Explanation Very high melting points and boiling points because their very strong covalent bonds are not easily broken. Macromolecular crystals are very stable and are insoluble in a polar or non-polar solvent. Diamond does not conduct electricity, whereas graphite contains free electrons so it does conduct electricity. Silicon is a semiconductor – it is midway between non-conductive and conductive. References: 1. https://courses.lumenlearning.com/boundless-chemistry/chapter/types-of-crystals/ 2. https://courses.lumenlearning.com/introchem/chapter/allotropes-of-carbon/ 3. http://www.scienceskool.co.uk/intermolecular-forces.html 4. https://www.easybiologyclass.com/how-hydrogen-bond-is-formed-in 5. water/https://chemistryskills.com/hydrogen-bonding/ 94
