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Learn about covalent bonding, including its definition, drawing dot and cross diagrams, and understanding the relationship between bond lengths and strengths. Explore dative covalent (coordinate) bonding with examples like Al2Cl6 and NH4+.
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Homework Private study work (bring notes to show me next lesson); • Look at the following websites • http://www.chemguide.co.uk/atoms/bondingmenu.html#top Covalent bonding and dative covalent • http://www.chemnotes.org.uk/f321.html • Topic 5, concentrate on covalent bonding
Draw a dot and cross diagram to show the bonding in methane (CH4) • Draw a dot and cross diagram to show the bonding in lithium fluoride (LiF)
Covalent Bonding Saturday, 21 December 2019
Covalent Bonding and Dative Covalent (co-ordinate) Bonding Describe what a covalent bond is Draw dot and cross diagrams for covalent substances Describe the relationship between bong lengths and bond strengths Explain dative covalent (coordinate) bonding Draw dot and cross digarams showing dative covalent (coordinate) bonding including Al2Cl6 and NH4+ Lesson Objectives
Definition • Write a definition of a covalent bond. • “Strong electrostatic attraction between two nuclei and the shared pair of electrons between them” • A pair of electrons shared between two non-metal atoms. The attraction of the nuclei to the electron pair effectively holds them together. • The attraction is directional, it only acts on the sharing atoms. Remember that an ionic bond attracts in all directions.
Activity Draw dot and cross diagrams for the following covalently bonded molecules: • H2O • Br2 • NH3
Single Bonds • The three molecules we just looked at are single covalent bonds. • A singlebond only involves one pair of electrons. • Now draw dot and cross diagrams for the bonding in: • O2 • N2 • CO2
Multiple Bonds • If two electron pairs are shared between atoms then this is called a double covalent bond. • E.g. • O2, written O=O • CO2, written O=C=O (two double bonds) • If three electron pairs are shared between atoms then this is called a triple covalent bond. • E.g. N2, written NΞN
Define the term covalent bond (2 marks) Exam questions
Explain how the atoms are held together by the covalent bond in a molecule of hydrogen (1 mark) Exam questions
Exam questions Draw the dot and cross diagrams for (1 mark each): Methane, CH4
Exam questions Draw the dot and cross diagrams for (1 mark each): Ethene, CH2 = CH2
Exam questions Draw the dot and cross diagrams for (1 mark each): Nitrogen, N2
Covalent Bonding and Dative Covalent (co-ordinate) Bonding Describe what a covalent bond is Draw dot and cross diagrams for covalent substances Describe the relationship between bong lengths and bond strengths Explain dative covalent (coordinate) bonding Draw dot and cross digarams showing dative covalent (coordinate) bonding including Al2Cl6 and NH4+ Lesson Objectives
Strength of covalent bonds The stronger a bond the harder it is to break it The shorter the covalent the bond the stronger it is
Limitations to models of covalent bonding • Dot and cross model only shows how atoms share electrons pairs. Doesn’t explain anything about bond lengths • Most bonds aren’t purely ionic or covalent • Some compounds that use covalent bonding end up with more than 8 electrons in their outer shell (use d orbitals to expand their octet)
Covalent Bonding and Dative Covalent (co-ordinate) Bonding Describe what a covalent bond is Draw dot and cross diagrams for covalent substances Describe the relationship between bong lengths and bond strengths Explain dative covalent (coordinate) bonding Draw dot and cross digarams showing dative covalent (coordinate) bonding including Al2Cl6 and NH4+ Lesson Objectives
Lone Pair • What do you notice about the electrons in these diagrams?
Lone Pair • Once covalent bonding has happened, all electrons are paired. • Some of these electrons are shared pairs which are called bonding pairs (or bonding electrons). • The pairs of electrons which are not involved in bonding are called lone pairs. • How many lone pairs in • Water • Ammonia • Methane
Dative Bonding • Now draw the bonding in the ammonium ion, NH4+. • Imagine that it is made from the combinations of NH3 and H+. • Draw those two species first and then try to draw them bonded together.
Dative Bonding • A dative covalent bond is a shared pair of electrons which have both been provided by the same atom. • Dative covalent bonds are also called co-ordinate bonds.
Al2Cl6 is another example Draw a dot and cross diagram for this
Covalent Bonding and Dative Covalent (co-ordinate) Bonding Describe what a covalent bond is Draw dot and cross diagrams for covalent substances Describe the relationship between bong lengths and bond strengths Explain dative covalent (coordinate) bonding Draw dot and cross digarams showing dative covalent (coordinate) bonding including Al2Cl6 and NH4+ Lesson Objectives
How many covalent bonds? When covalent bonds form, unpaired electrons often pair up so that the bonded atoms obtain a noble gas electron configuration (obeying the octet rule) This is not always possible: -there may not be enough electrons to react an octet -more than four electrons may pair up in bonding (expansion of the octet)
Not enough electrons to reach an octet • In period 2 beryllium, Be and boron, B form covalent compounds • However they do not have enough unpaired electrons to reach a noble gas configuration • But they can pair up any unpaired electrons • E.g. BF3; three unpairedelectrons from B pair up
Expansion of the octet As you move down the periodic table more of the outer – shell electrons are able to take part in bonding In the resulting molecules one of the bonding atoms may finish up with more than eight electrons in its outer shell This breaks the octet rule and is called the expansion of the octet rule
Expansion of the octet Elements that can expand their octet; P, As, S, Se, Te, Cl, Br, I, At E.g. Sulfur hexafluoride SF6. Six covalent bonds can form. Each of sulfurs six electrons is paired meaning that 12 electrons surround sulfur.
Exam questions Draw the dot and cross diagrams for (1 mark each): The ammonium ion, NH4+
Exam questions Nitrogen forms an oxide called nitrous oxide, N2O. The bonding in nitrous oxide can be represented as: Complete the diagram below for the N2O molecule using dots or crosses to represent electrons. Just show all of the outer shell electrons. (3 marks)
Covalent Bonding and Dative Covalent (co-ordinate) Bonding Describe what a covalent bond is Draw dot and cross diagrams for covalent substances Describe the relationship between bong lengths and bond strengths Explain dative covalent (coordinate) bonding Draw dot and cross digarams showing dative covalent (coordinate) bonding including Al2Cl6 and NH4+ Lesson Objectives