IGCSE Double Award Extended Coordinated Science

Transcription

1 IGCSE Double Award Extended Coordinated Science Chemistry 3.5 & Covalent Bonds Covalent Bond You need to know what covalent bonding is. Like ionic bonds, covalent bonds are another type of chemical bond - Covalent bonds occur between nonmetals - And is the sharing of electron pairs Like ionic bonding, the atoms try to gain full outer shells, but instead of completely losing or gaining, they share. Dot-and-cross diagrams may also be used to represent covalent bonding, - But the circles overlap to indicate electron pair sharing - We can look at chlorine molecule ( Cl 2 for a clear example - Almost like a Venn diagram, the electrons that are in between the overlap are shared by both chlorines. - And it is also clear because of the sharing, both atoms have a full electronic outer shell. - Like shown above, a covalent bond diagram may be simplified with a line, - With the line representing a single covalent bond. Also like ionic bonds, more than just two atoms can form covalent bonds, like in water, H 2 O - Some elements can form more than one covalent bonds - The maximum number of covalent bonds an element can make = 8 - group number - So for oxygen, ( 8-6 = 2, oxygen can form two covalent bonds - Each oxygen atom forms two covalent bonds - one to each hydrogen atom - You can check that all the atoms have full outer shells - Oxygen has full outer shell with 8 electrons - Hydrogen has full outer shell with 2 electrons (shell no.1 is full with two electrons

2 Sometimes, two atoms may have more than one covalent bonding (multiple covalent bonding: - Double covalent bonds - Triple covalent bonds - Quadruple covalent bonds This is different to the bonding in water. Molecules like water is NOT a double covalent bond. - Even though oxygen has two covalent bonds, they are to two different atoms. - Multiple covalent bonds must be to the same atom, for example, like in an oxygen molecule. Of course, some molecules have triple bonds, in the case of a nitrogen molecule ( N 2. - Nitrogen is in group 5, so the maximum number of bonds it can form is 8-5 = 3. - Since in a nitrogen molecule, there are only two nitrogen atoms, all the bonds are to each other like so: Drawing Covalent Dot and Cross Diagrams You need to know how to draw dot and cross diagrams for the following molecules or similar molecules: - Hydrogen ( H 2, chlorine ( Cl 2, methane ( CH 4, water ( H 2 O, hydrochloric acid ( HCl, ammonia ( NH 3 - Oxygen ( O 2, nitrogen ( N 2, ethene (C 2 H 4, carbon dioxide (CO 2, These are not specified in the syllabus but if you are feeling confident and want more practice, try these: - Phosphorus (III chloride ( PCl 3 - Hydrazine ( N 2 H 4 - Ethane ( C 2 H 6 - Propane ( C 3 H 8 - Propene ( C 3 H 6

3 Types of Structures The molecules shown above like, water, oxygen, ammonia are all simple molecular covalent compounds - This means they exist as molecules shown in the dot-and-cross diagrams and formulae - H 2 O actually means that a single water molecule has two hydrogens and one oxygen. - Unlike ionic lattices where NaCl did not mean the molecule has one sodium and chloride ion. However, some covalent structures also have a giant lattice structure (giant covalent structure - Cases like diamond (covalently bonded carbons Giant Covalent Structures You need to know the typical properties of a giant covalent structure which are: - Very high melting point - Covalent bonds are very strong, and each atom forms four bonds. - Very strong - Because of the very strong covalent bonds between the atoms it is hard to break - Particles are not charged - Because there is no loss or gain of electrons - Not soluble in water - Because it is not charged, for a molecule to be soluble in water, it must be charged. - Variable conductivity of electricity and heat - Different compounds have different conducting properties - Diamond does not conduct heat or electricity - Graphite conducts both heat and electricity

4 You need to know the two giant covalent structure allotropes of carbon: diamond and graphite Carbon is able to form maximum of four covalent bonds, and it often likes to bond with another carbon atom - When many atoms of carbon bonds to itself, it creates a giant covalent structure. There are two types of giant covalent structures of carbon you should know: - Diamond and graphite - They are allotropes of carbon - Allotropes are same elements with a different physical form (structures are different - Diamond - Structured by carbon atoms forming four covalent bonds with four other carbon atoms. - All four electrons of a carbon atom are used and held in a bond. - Diamond has all the properties of a typical giant covalent structure. - Diamond does not conduct electricity - Because there are no free electrons to flow and cause a current - Because all the electrons are held in place in the covalent bonds - Graphite - Structured by carbon atoms forming three covalent bonds with three other carbon atoms. - Only three out of four electrons of a carbon atom are used and held in a bond. - The last electron is free to move around the structure of graphite between the layers. - Does not have the typical properties of a giant covalent structure: - Made up of multiple layers, with intermolecular forces acting between them to hold in place - Intermolecular force is a weak force that attracts any two molecules - The graphite layers can slide over each other, making it a good lubricant - Can conduct electricity, because of all the free electrons between layers that are free to move - and current can flow using these free electrons. - Weak vertically across the layers, because the single layers are very soft.

5 You need to know the giant covalent structure of silicon dioxide ( SiO 2 - The 3D shape is very similar to diamond, but in the place of carbon atoms are silicon atoms, - And between the silicon atoms are oxygen atoms. - Silicon dioxide also have similar properties to diamond, - Very high melting point - Very hard - Does not conduct electricity - Insoluble in water Differences between Ionic and Covalent Compounds You need to know the differences in properties between ionic compounds and covalent compounds. Property Ionic Compounds (Giant Ionic Lattice Simple Molecular Covalent Giant Covalent Lattice Volatility Low High Low Solubility High in water Variable Low Conductivity of electricity Low ( High if in liquid state Low Variable Melting/Boiling Points High Low High Physical Hard and brittle Soft and brittle Hard and brittle

6 The syllabus says you should be able to, (SO check if you can: - State that non-metallic elements form non-ionic compounds using a different type of bonding called covalent bonding. - Draw dot-and-cross diagrams to represent the sharing of electron pairs to form single covalent bonds in simple molecules, exemplified by (but not restricted to H2, Cl2, H2O, CH4 and HCl. - Draw dot-and-cross diagrams to represent the multiple bonding in N2, C2H4 and CO2. - Describe the giant covalent structures of graphite and diamond. - Relate their structures to the use of graphite as a lubricant and of diamond in cutting. - Describe the structure of silicon(iv oxide (silicon dioxide. - Describe the differences in volatility, solubility and electrical conductivity between ionic and covalent compounds.