Further covalent bonding

Chapter 56 - 57

Further covalent bonding• Describe dative covalent (coordinate)

bonding.

• Use dot-and-cross diagrams to represent covalent bonding.

Dative covalent bondsAlso known as a coordinate bondOne 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+

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+.

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

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+

Complete qu.1 Page 57 of your book

How many covalent bonds?Octet ruleThis is not always possible:

There may not be enough electrons to reach an octetMore 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

Example – boron trifluorideEach fluorine has 7 outer shell electronsThree 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?

Expansion of the octetFor 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.

RulesAtoms of non-metals in Group 5 can form 3

or 5 covalent bondsAtoms 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 bondsElements that typically expand their octet are;

Group 5 Group 6 Group 7P S ClAs Se Br

Te IAt

ExampleCan 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.

Check your answerSix covalent bonds can be formed

Each of sulfur’s 6 electrons are paired

12 electrons surround sulfur

Each of the 6 fluorine’s has 8 electrons in its outer shell, attaining the octet

Complete qu. 2 Page 57 of your text book

A better ruleCan you come up with a better rule than the

octet rule?