Further covalent bonding

1. Further covalent bonding Chapter 56 - 57

2. Further covalent bonding • Describe dative covalent (coordinate) bonding. • Use dot-and-cross diagrams to represent covalent bonding.

3. Dative covalent bonds • Also known as a coordinate bond • One of the atoms supplies both of the shared electrons • A dative covalent bond can be written as A  B • The direction of the arrow shows the direction in which the electron pair donated. • An example is the ammonium ion, NH4+

4. In the diagram the ammonium ion has one covalent bond and one dative bond • One of the electron pairs, is a lone pair. It provides both the bonding electrons when bonding with the H+ ion, and the resulting NH4+ ion has a positive charge of 1+.

5. An ammonium ion. Notice that the dative covalent bond is shown with an arrow. The arrow indicates the origin of the bonded pair. • Once formed, the dative covalent bond is equivalent to all other covalent bonds

6. The oxonium ion, H3O+ • Can you try and draw the oxonium ion? • When an acid is added to water, water molecules form oxonium ions. • E.g. If hydrogen chloride gas is added to water, oxonium ions form. These are responsible for the reactions of acids. Often, they are simplified to H+

7. Complete qu.1 • Page 57 of your book

8. How many covalent bonds? • Octet rule • This is not always possible: • There may not be enough electrons to reach an octet • More than four electrons may pair up in bonding (expansion of the octet) • Period two elements boron and beryllium both form compounds with covalent bonds • However, Be and B don’t have enough unpaired electrons to reach noble gas configuration • They can pair up any unpaired electrons

9. Example – boron trifluoride Each fluorine has 7 outer shell electrons Three covalent bonds can be formed Each of boron’s 3 electrons is paired How many electrons surround B? How many electrons does each fluorine atom have?

10. Expansion of the octet • For elements in group 5-7 something odd happens from period 3. • More of the outer shell electrons are able to take part in bonding. • Some molecules therefore end up with more than 8 electrons in their outer shell. • This breaks the octet rule and is known as expansion of the octet.

11. Rules • Atoms of non-metals in Group 5 can form 3 or 5 covalent bonds • Atoms of non-metals in Group 6 can form 2, 4 and 6 covalent bonds • Atoms of non-metals in Group 7 can form 1, 3, 5 or 7 covalent bonds • Elements that typically expand their octet are;

12. Example • Can you draw the structure of the compound sulfur hexafluoride? • Label on; • The number of covalent bonds formed. • How many sulfur electrons are paired. • How many electrons surround the sulfur. • How many electrons each fluorine atoms has.

13. Check your answer Six covalent bonds can be formed Each of sulfur’s 6 electrons are paired Each of the 6 fluorine’s has 8 electrons in its outer shell, attaining the octet 12 electrons surround sulfur

14. Complete qu. 2 • Page 57 of your text book

15. A better rule • Can you come up with a better rule than the octet rule?