Presentation on theme: "Covalent Bonding 5 May, 2015 By the end of this lesson, you should be able to…. Describe and define a covalent bond Describe single and multiple covalent."— Presentation transcript:
Covalent Bonding 5 May, 2015 By the end of this lesson, you should be able to…. Describe and define a covalent bond Describe single and multiple covalent bonding Use dot and cross diagrams to represent covalent bonding
Covalent Bonds Only occurs between non-metals Formed by a shared pair of electrons
Why is there an attraction? A covalent bond is directional, it only acts on the atoms involved, unlike an ionic bond
Hydrogen and Hydrogen H H2H2 H Click for another example Click for animation
Nitrogen and Hydrogen (Ammonia) Click for animation Click for another example H H H N NH 3
Hydrogen and Oxygen Click for animation H2OH2O H H O Click here to return to bonding options
Task This is the dot-and-cross diagram for a water molecule. Draw dot-and-cross diagrams for the following: 1.F 2 2.H 2 3.HCl 4.CH 4 5.SCl 2
Lone pairs When 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.
Multiple covalent bonds Some atoms can share more than one pair of electrons to form a multiple bond. O O
Carbon Dioxide OO C
Task Draw dot and cross diagrams for the following: a)C 2 H 4 b)HCΞN c) H 2 C=O
Naming covalent compounds Two nonmetals – Name each element – End the last element in -ide – Add prefixes to show more than 1 atom Prefixes mon1 penta5 di2 hexa6 tri3 tetra4
Naming Covalent Compounds Fill in the blanks to complete the following names of covalent compounds. CO carbon ______oxide CO 2 carbon _______________ PCl 3 phosphorus _______chloride CCl 4 carbon ________chloride
Answers CO carbon monoxide CO 2 carbon dioxide PCl 3 phosphorus trichloride CCl 4 carbon tetrachloride
Dative Bonding A 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
Dative Bonding You can represent a dative covalent bond with an arrow, A B The direction of the arrow, shows the direction that the electron pair has been donated.
Dative Bonding When an acid is added to water, oxonium ions can form (H 3 O + )
Dative Bonding Draw dot-cross diagrams for: a)PCl 4 + b)H 3 O + c)H 2 F +
Complications……(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 Octet there may be more than four electrons that pair up during bonding – this is called ‘Expansion of the Octet’
Not 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……
Expansion 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 Rule e.g. phosphorous can form two chlorides, PCl 3 and PCl 5 it just depends on how much chlorine is available.
Task Draw dot-cross diagrams for the following compounds: 1.BF 3 2.PF 5 3.SO 2 4.SO 3 Extension: 5. BF 3 NH 3 (This compound is formed when BF 3 and NH 3 react together)
Shapes of Molecules Now 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: 1.Draw the dot and cross diagram 2.Count the number of electrons surrounding the central atom 3.Divide 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
Use the molymods to make the following 3D structures CH 4 PCl 3 NH 3 CO 2 SF 6 NO 2 +
Describing shapes of molecules Bond length is the distance between the nuclei of two bonded atoms. Bond angle is the angle between two covalent bonds. bond angle bond length Counting electrons enables the basic shape of the molecule and its approximate bond angles to be predicted. The shape of a molecule can be described in terms of its bond lengths and bond angles.
How to draw 3D molecules
For the following covalent compounds do the following: 1.Draw the dot-cross diagram 2.Predict and draw the shape of the molecule 3.Predict the bond angle CH 4 BeCl 2 BF 3 NH 4 + SF 6 SiF 4
Bonding and lone pairs A 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. Electron 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. lone pair bonding pair
Effect 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-shaped replacing one bonding pair with a lone pair replacing another bonding pair with a lone pair
Effect of lone pairs on bond angles
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 shape SiCl 4 PCl 6 - H2SH2S SiCl 6 2- PCl 4 + XeF 4
Nitrogen and Boron can form the chlorides NCl 3 and BCl 3. a)Draw dot cross diagrams to show the bonding in both NCl 3 and BCl 3. b)Draw the shapes of both of these molecules. Show the approximate values of the bond angles and the name of the shape. c)Explain why the shape of both of these molecules is different.
Effect of lone pairs on bond angles
Strength of metallic bonding: ion charge The strength of metallic bonding depends on two factors: 1. the charge on the metal ions 1. The charge on the metal ions The 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. 2. the size of the metal ions. NaMg Al Element Charge on ion Melting point (K)
Strength of metallic bonding: ion size Element Ionic radius (nm) Melting point (K) LiNaKRbCs The size of the metal ions The 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.
Types of bonding
substances: sodium chloride aluminium water iodine diamond carbon graphite carbon hydrogen chloride Discuss the bonding, structure and properties of the following substances. Points to consider and an example follow.
NaCl Al H 2 O I 2 C (diamond) C(graphite) HCl Which 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:
Aluminium oxide Which 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 states No bonds are present in the gaseous state The name given to the type of substance. Aluminium oxide is an ionic compound
Aluminium oxide The nature of the bonding present. Ionic bonds are the electrostatic attraction between oppositely charged ions The arrangement of particles in space within the solid, liquid and gaseous states. In the solid the ions are fixed in place within a crystal lattice In the liquid the ions can move through the substance but are still held close together In the gas the ions are completely separate from each other and move completely freely
Aluminium oxide How strongly the particles are held together. Ionic bonds are strong attractions What 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 points At the melting point some attractions between ions break The melting point is relatively high because ionic bonds are strong
Aluminium oxide What 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 substance The boiling point is relatively high because ionic bonds are strong In all three states the ions move faster when heated At the boiling point all ionic attractions break In the gas the ions move completely freely of each other