Chapter 7Covalent Bonding - Non-metals react with one another to attain noble gas structure. - Non-metal atoms share valence electrons. - Bonds formed between atoms by sharing electrons called covalent bond. - Molecules of non-metallic element are made up of 2 of more identical atoms. - Sharing of 2 electrons – single covalent bond a single covalent bond / single bond represented by a single line in a structural formula - 4 electrons shared – double covalent bond double bond formed with 2 pairs of electrons shared between 2 atoms represented by “ = “ in structural formula - Molecules can be made from 2 or more different types of atom linked together by covalent bonding. - These molecules called molecular compounds or covalent compounds. - Structures & properties of covalent substances covalent substance exist as simple molecules or giant structures Simple molecular structure Within each molecule, atoms are held together by strong covalent bonds. Between the molecules in the solid, there are only weak Van der Waal’s forces holding the molecules together. Strong covalent bonds “within” molecule but weak Van der Waal forces “between” molecules Since molecules loosely held together, they exist as individual or discrete molecules. Weak van der Waal forces between molecules can cause: 1. Solid to sublime on heating (Eg. iodine) 2. Gas at r.t.p. (Eg. Methane) Physical properties : 1. Volatility Many simple covalent substances (esp. made up of small molecules) are all liquids or gases at room temp. volatile or have high volatility (evaporate easily) Occurs because forces between molecules weak compared to covalent bonds within molecules very little heat energy need to overcome intermolecular forces low melting and boiling points Simple covalent substance (made up of larger molecules) is solids at room temp. as intermolecular forces of attraction are stronger between larger molecules 2. Solubility Most covalent molecules are insoluble in water and soluble in organic solvents (with some exceptions like alcohol and sugar) Some covalent molecules dissociate when they dissolve in water.
3. Electrical conductivity Most covalent elements or compounds do not conduct electricity in all states as they do not have free moving ions or electrons to conduct electricity. Exceptions like carbon (in form of graphite) conduct electricity. Other exceptions react with water to form solutions that conduct electricity.Giant molecular structure Diamond One of allotropes of carbon. Allotropes are different forms of an element. In diamond, each carbon atom joined to 4 other atoms by strong covalent bonds. / each carbon atom is bonded tetrahedral to … Have giant molecular structure. Physical property 1. Very hard substance – not easily scratched or worn out 2. Exist as solid in nature 3. Has high melting and boiling points 4. Does not conduct electricity 5. Insoluble in water Structure of diamond for its physical properties 1. All typical properties of giant covalent molecule 2. A crystal of diamond contains millions of carbon atoms joined by covalent bonds so lot energy required breaking the bonds. = So diamond is hard and difficult to melt. 3. in diamond structure, all valence electrons of carbon atom used for bonding so no free electrons to move through structure. = So diamond cannot conduct electricity Uses 1. Rare & precious – used as gemstones in jewelry 2. Depend on its hardness and high melting & boiling point – synthetic diamond produced under high pressure and temp. Used for tips of drills or cutting tools to drill, grind, polish hard surfaces Graphite Allotrope of carbon. Made of layers of carbon atoms. In each layer, each carbon atom form strong covalent bonds with 3 other atoms. Rings of 6 carbon atoms joined together form 2D layers and each layer is giant molecule. Layer of carbon atoms held by wear Van der Waal forces lie on top each other. Structure for physical properties
1. Bond within each layer strong and difficult to break and like diamond, graphite is sold. = So high melting & boiling points 2. Forces of attraction between layers of carbon weak so layers can slide over each other = So graphite is soft and slippery 3. Each carbon atom has one outer shell electron not used to form covalent bond. This electron is delocalized and move along layers from one atom to next when connected to a battery. = So graphite is good conductor of electricity. Uses Properties of graphite Uses of graphite depending on this property - Slippery Graphite is dry lubricant. Oil attack rubber but - Does not decompose at high temp. graphite does not so it is used to lubricate - does not attack rubber machine parts containing rubber. - soft (layers of carbon atoms can be rubbed Graphite baked with clay and made into pencil off easily) lead. Since it is soft, carbon layers flake off and stick to paper when we write. - good conductor of electricity Graphite used as brushes for electric motors - fairly unreactive and also for inert electrodes for electrolysis. - Chemical formula for covalent substances Some covalent elements exist as diatomic molecules. Add subscript “2” to chemical symbol. Elements not made up of diatomic molecules, formula written as chemical symbol of element.Metallic bonding - Metal atoms are held strongly to each other by metallic bonding. - In metal lattice, atoms lose their valence electrons and become positively charged. - The valence electrons no longer belong to any metal atom and are delocalized. They move freely between metal ions like a cloud of negative charge. - Lattice structure described as a lattice of positive metal ions and sea of delocalized electrons. - Physical properties Electrical conductivity Metals are good conductors of electricity due to the mobility of valence electrons in metal lattice. When metal used in electrical circuit, electrons enter one end of metal , causing a similar number to be displaced at the other end. Valence electrons move from negative terminal to positive terminal of electrical circuit so metal can conduct electricity. Malleability & ductility Valence electrons do not belong to any atom. If sufficient force applied, one layer of atoms can slide over another without disrupting the bonding.
Metallic bonds are strong but flexible and metal can be hammered into different shapes(malleable) or drawn into wires (ductile) without breaking.