Upload
traci
View
211
Download
4
Tags:
Embed Size (px)
DESCRIPTION
20 October, 2014. Covalent Bonding. 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 - PowerPoint PPT Presentation
Citation preview
Covalent Bonding
20 April 2023
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
HH2
H
Click for another exampleClick for animation
Nitrogen and Hydrogen (Ammonia)
Click for animationClick for another example
H
H
HN
NH3
Hydrogen and Oxygen
Click for animation
H2O
H H
O
Click here to return to bonding options
TaskThis is the dot-and-cross diagram for a water molecule. Draw dot-and-cross diagrams for the following:
1.F2
2.H2
3.HCl
4.CH4
5.SCl2
Lone pairsWhen 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
O OC
TaskDraw dot and cross diagrams for the
following:
a) C2H4
b) HCΞN
c) H2C=O
Naming covalent compounds
Two nonmetals– Name each element – End the last element in -ide– Add prefixes to show more
than 1 atomPrefixes
mon 1 penta 5di 2 hexa 6tri 3tetra 4
Naming Covalent CompoundsFill in the blanks to complete the following
names of covalent compounds.
CO carbon ______oxide
CO2 carbon _______________
PCl3 phosphorus _______chloride
CCl4 carbon ________chloride
Answers
CO carbon monoxide
CO2 carbon dioxide
PCl3 phosphorus
trichloride
CCl4 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 BondingYou can represent a dative covalent bond with an arrow, AB
The direction of the arrow, shows the direction that the electron pair has been donated.
Dative BondingWhen an acid is added to water, oxonium ions can form (H3O+)
Dative Bonding
Draw dot-cross diagrams for:
a)PCl4+
b)H3O+
c)H2F+
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 octetIn 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, PCl3 and PCl5 it just depends on how much chlorine is available.
Task
Draw dot-cross diagrams for the following compounds:
1. BF3
2. PF5
3. SO2
4. SO3
Extension:5. BF3NH3 (This compound is formed when BF3
and NH3 react together)
Shapes of MoleculesNow 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 diagram2. Count the number of electrons surrounding the
central atom3. 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
CH4
PCl3
NH3
CO2
SF6
NO2+
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
CH4
BeCl2
BF3
NH4+
SF6
SiF4
Bonding and lone pairsA 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.
lonepair
bondingpair
Effect of lone pairs on shapeThe 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.
tetrahedral pyramidal V-shaped
replacing one bonding pair
with a lone pair
replacing another bonding pair with
a lone pair
Effect of lone pairs on bond angles
QUESTIONSQUESTIONS
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
SiCl4
PCl6-
H2S
SiCl62-
PCl4+
XeF4
Nitrogen and Boron can form the chlorides NCl3 and BCl3.
a) Draw dot cross diagrams to show the bonding in both NCl3 and BCl3.
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 chargeThe strength of metallic bonding depends on two factors:
1. the charge on the metal ions
1. The charge on the metal ionsThe 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.
Na Mg Al
1+ 2+ 3+
371 923 933
Element
Charge on ion
Melting point (K)
Strength of metallic bonding: ion size
Element
Ionic radius
(nm)Melting point (K)
Li Na K Rb Cs
0.076 0.102 0.138 0.152 0.167
454 371 337 312 302
2. The size of the metal ionsThe 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 H2O I2 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 oxideWhat 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