Presentation on theme: "By the end of this lesson, you should be able to…."— Presentation transcript:
1By the end of this lesson, you should be able to…. 14 April, 2017Covalent BondingBy the end of this lesson, you should be able to….Describe and define a covalent bondDescribe single and multiple covalent bondingUse dot and cross diagrams to represent covalent bonding
2Covalent Bonds Only occurs between non-metals Formed by a shared pair of electrons
3Why is there an attraction? A covalent bond is directional, it only acts on the atoms involved, unlike an ionic bond
5Click for another example Hydrogen and HydrogenHHH2Click for another exampleClick for animation
6Nitrogen and Hydrogen (Ammonia) NH3HClick for another exampleClick for animation
7Hydrogen and Oxygen O H2O H H Click here to return to bonding options Click for animation
8TaskThis is the dot-and-cross diagram for a water molecule. Draw dot-and-cross diagrams for the following:F2H2HClCH4SCl2
9Lone pairsWhen an electron pair is not used for bonding, it is known as a lone pair.A water molecule has 2 lone pairs.An ammonia molecule has 1 lone pair.Lone pairs can affect the chemistry of molecules in quite significant ways.
10Multiple covalent bonds Some atoms can share more than one pair of electrons to form a multiple bond.OO
13TaskDraw dot and cross diagrams for the following:C2H4HCΞNc) H2C=O
14Naming covalent compounds Two nonmetalsName each elementEnd the last element in -ideAdd prefixes to show more than 1 atomPrefixesmon penta 5di hexa 6tri 3tetra 4
15Naming Covalent Compounds Fill in the blanks to complete the following names of covalent compounds.CO carbon ______oxideCO2 carbon _______________PCl3 phosphorus _______chlorideCCl4 carbon ________chloride
16Answers CO carbon monoxide CO2 carbon dioxide PCl3 phosphorus trichlorideCCl4 carbon tetrachloride
17Dative BondingA co-ordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom.Lone pair
18Dative BondingYou can represent a dative covalent bond with an arrow, ABThe direction of the arrow, shows the direction that the electron pair has been donated.
19Dative BondingWhen an acid is added to water, oxonium ions can form (H3O+)
21Complications……(as usual) When covalent bonds form, unpaired electrons pair up to obey the Octet Rule, however this isn’t always possible because:there may not be enough available electrons to reach an Octetthere may be more than four electrons that pair up during bonding – this is called ‘Expansion of the Octet’
22Not enough electrons….Eg: Be and B both form compounds with covalent bonds (despite being metals) – they do not have enough unpaired electrons to reach an octet……
23Expansion of the octet In groups 5-7 something unusual happens. Moving down the groups means that more of their outer-shell electrons are able to take part in bonding – this can break the Octet Rulee.g. phosphorous can form two chlorides, PCl3 and PCl5 it just depends on how much chlorine is available.
24Task Draw dot-cross diagrams for the following compounds: BF3 PF5 SO2 Extension:5. BF3NH3 (This compound is formed when BF3 and NH3 react together)
25Shapes of MoleculesNow we know how covalently bonded compounds form we can look at their shape. On paper they look flat, but many are 3D.How to predict the shape of a molecule:Draw the dot and cross diagramCount the number of electrons surrounding the central atomDivide this by 2 to find out the number of electron pairs.These will arrange themselves so that they are as far apart as possible – REMEMBER: They are negative and so repel
26Use the molymods to make the following 3D structures CH4PCl3NH3CO2SF6NO2+
27Describing shapes of molecules The shape of a molecule can be described in terms of its bond lengths and bond angles.Bond length is the distance between the nuclei of two bonded atoms.bond angleBond angle is the angle between two covalent bonds.Teacher notesBond length depends on which two atoms are bonded together and on the structure of the rest of the molecule. It can be found experimentally using X-ray diffraction. Bond angle can also be measured using X-ray crystallography techniques.bond lengthCounting electrons enables the basic shape of the molecule and its approximate bond angles to be predicted.
31For the following covalent compounds do the following: Draw the dot-cross diagramPredict and draw the shape of the moleculePredict the bond angleCH4BeCl2BF3NH4+SF6SiF4
32Bonding and lone pairsA pair of electrons in a covalent bond are called a bonding pair. Pairs of electrons that are not involved in bonding are called lone pairs.bonding pairlone pairElectron pairs are clouds of negative charge, so there is mutual repulsion between them, forcing them as far apart as possible.This means the number of electron pairs around the central atom(s) determines the basic shape of the molecule.
33Effect of lone pairs on shape The number of lone pairs in a molecule is calculated by subtracting the number of bonding pairs from the total number of electron pairs in the outer principal energy level.The shape of a molecule with lone pairs is based on the basic shape for the total number of outer electron pairs, but with a lone pair replacing one of the bonds.tetrahedralpyramidalV-shapedTeacher notesStudents could be taught the ‘square planar’ molecular shape, as exemplified by xenon tetrafluoride (XeF4). In this structure, four bonding pairs are arranged in the same plane around the central atom with a bond angle of 90°, with a lone pair both above and below the central atom.V-shaped molecules may also be described as “non-linear”.replacing one bonding pair with a lone pairreplacing another bonding pair with a lone pair
34Effect of lone pairs on bond angles Teacher notesIt may be worth pointing out to students that the dotted line represents a bond extending behind the plane of the screen, and the wedge-shaped bond represents a bond extending in front of the plane of the screen.
35SiCl4 PCl4+ PCl6- SiCl62- H2S XeF4 QUESTIONS For each of the following ions/molecules, state the number of bond pairs, state the number of lone pairs, state the bond angle(s), state, or draw, the shapeSiCl4PCl4+PCl6-SiCl62-H2SXeF4
38Nitrogen and Boron can form the chlorides NCl3 and BCl3. Draw dot cross diagrams to show the bonding in both NCl3 and BCl3.Draw the shapes of both of these molecules. Show the approximate values of the bond angles and the name of the shape.Explain why the shape of both of these molecules is different.
41Strength of metallic bonding: ion charge The strength of metallic bonding depends on two factors:1. the charge on the metal ions2. the size of the metal ions.1. The charge on the metal ionsThe greater the charge on the metal ions, the greater the attraction between the ions and the delocalized electrons, and the stronger the metallic bonds. A higher melting point is evidence of stronger bonds in the substance.ElementCharge on ionMelting point (K)NaMgAl1+2+3+371923933
42Strength of metallic bonding: ion size 2. The size of the metal ionsThe smaller the metal ion, the closer the positive nucleus is to the delocalized electrons. This means there is a greater attraction between the two, which creates a stronger metallic bond.ElementIonic radius (nm)Melting point (K)LiNaKRbCs0.0760.1020.1380.1520.167Teacher notesThe ionic radius of an ion is not fixed, but depends on several factors, such as the co-ordination number of the ion.454371337312302
44Discuss the bonding, structure and properties of the following substances. Points to consider and an example follow.sodium chloridealuminiumwateriodinediamond carbongraphite carbonhydrogen chloridesubstances:
45C (diamond) C(graphite) HCl NaCl Al H2O I2C (diamond) C(graphite) HClWhich types of chemical bonding are present within the substance in the solid, liquid and gaseous states.The name given to the type of substance.The nature of the bonding present.The arrangement of particles in space within the solid, liquid and gaseous states.How strongly the particles are held together.What happens to the particles and bonds present as the substance is gradually warmed from below its melting point to above its boiling point.Points to consider:
46Aluminium oxideWhich types of chemical bonding are present within the substance in the solid, liquid and gaseous states.Ionic bonds are present in the solid and liquid statesNo bonds are present in the gaseous stateThe name given to the type of substance.Aluminium oxide is an ionic compound
47Aluminium oxide The nature of the bonding present. Ionic bonds are the electrostatic attraction between oppositely charged ionsThe arrangement of particles in space within the solid, liquid and gaseous states.In the solid the ions are fixed in place within a crystal latticeIn the liquid the ions can move through the substance but are still held close togetherIn the gas the ions are completely separate from each other and move completely freely
48Aluminium oxide How strongly the particles are held together. Ionic bonds are strong attractionsWhat happens to the particles and bonds present as the substance is gradually warmed from below its melting point to above its boiling point.In the solid the ions vibrate about fixed pointsAt the melting point some attractions between ions breakThe melting point is relatively high because ionic bonds are strong
49Aluminium oxideWhat happens to the particles and bonds present as the substance is gradually warmed from below its melting point to above its boiling point.In the liquid the bonds between ions are constantly breaking and reforming so that they can move through the substanceAt the boiling point all ionic attractions breakThe boiling point is relatively high because ionic bonds are strongIn the gas the ions move completely freely of each otherIn all three states the ions move faster when heated