Homework Private study work bring notes to show

Homework Private study work (bring notes to show me next lesson); • Look at the following websites • http: //www. chemguide. co. uk/atoms/bondi ngmenu. html#top Covalent bonding and dative covalent • http: //www. chemnotes. org. uk/f 321. html • Topic 5, concentrate on covalent bonding

1. Draw a dot and cross diagram to show the bonding in methane (CH 4) 2. Draw a dot and cross diagram to show the bonding in lithium fluoride (Li. F)

Covalent Bonding 01 December 2020

Lesson Objectives 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 Al 2 Cl 6 and NH 4+

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.

Covalent bonding

Activity Draw dot and cross diagrams for the following covalently bonded molecules: • H 2 O • Br 2 • NH 3

Single Bonds • The three molecules we just looked at are single covalent bonds. • A single bond only involves one pair of electrons. • Now draw dot and cross diagrams for the bonding in: – O 2 – N 2 – CO 2

Multiple Bonds • If two electron pairs are shared between atoms then this is called a double covalent bond. • E. g. – O 2, written O=O – CO 2, 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. N 2, written NΞN

Exam questions 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, CH 4

Exam questions Draw the dot and cross diagrams for (1 mark each): Ethene, CH 2 = CH 2

Exam questions Draw the dot and cross diagrams for (1 mark each): Nitrogen, N 2

Exam questions

Lesson Objectives 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 Al 2 Cl 6 and NH 4+

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)

Lesson Objectives 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 Al 2 Cl 6 and NH 4+

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 1. Water 2. Ammonia 3. Methane

Dative Bonding • Now draw the bonding in the ammonium ion, NH 4+. • Imagine that it is made from the combinations of NH 3 and H+. • Draw those two species first and then try to draw them bonded together.

Also written:

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 coordinate bonds.

Co-ordinate bonding

Al 2 Cl 6 is another example Draw a dot and cross diagram for this

Examples of co-ordinate bonds

Lesson Objectives 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 Al 2 Cl 6 and NH 4+

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. BF 3; three unpaired electrons 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 SF 6. 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, NH 4+

Exam questions Nitrogen forms an oxide called nitrous oxide, N 2 O. The bonding in nitrous oxide can be represented as: Complete the diagram below for the N 2 O molecule using dots or crosses to represent electrons. Just show all of the outer shell electrons. (3 marks)

Lesson Objectives 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 Al 2 Cl 6 and NH 4+