Objectives of the guide
1. To introduce the covalent bonding concept
2. To help understand why atoms need to go through covalent bonding
3. To help learn how to draw a Lewis Dot-Cross diagram of covalently bonded atoms
4. To help understand van der Waal’s forces between the small discrete molecules
5. To help describe how van der Waal’s forces affect the melting points and boiling points
6. To help understand polar covalent bonds and non-polar covalent bonds
7. To help describe what macromolecules are and their structure
8. To help relate how structure of macromolecules affect their physical properties
Friday, May 29, 2009
Unit 2
An introduction to Covalent Bonds
Why do covalent bonds occur?
Because atoms want to achieve a stable octet structure like that of noble gases.
However, bonding can be easily occurred between a metal and a non-metal, since a metal easily loses a electron and a non-metal is electronegative so it attracts electrons to itself more easily.
However, what happens when a non-metal wants to achieve a stable octet structure like that of a noble gas?
Ans: It SHARES electrons. It is also known as covalent bonding. Each electron contributes electrons to be shared.
A preview into covalent bonding and why they occur
Why do covalent bonds occur?
Because atoms want to achieve a stable octet structure like that of noble gases.
However, bonding can be easily occurred between a metal and a non-metal, since a metal easily loses a electron and a non-metal is electronegative so it attracts electrons to itself more easily.
However, what happens when a non-metal wants to achieve a stable octet structure like that of a noble gas?
Ans: It SHARES electrons. It is also known as covalent bonding. Each electron contributes electrons to be shared.
A preview into covalent bonding and why they occur
Unit 3
An extension to Covalent Bonding Concept
A covalent bond is thus formed by an overlap of two atomic orbitals. The atomic orbitals is essentially the "home" of the electrons. So when the orbitals overlap, the electrons are shared.
Covalent bonding usually occurs between non-metals that would prefer not to lose electrons but would still like to gain a stable octet structure. This is because non-metals usually belong to group 6 or group 7 which are usually a -2 or - charge so it is harder for the atom to lose electrons in order to gain a stable octet structure.
So each atom shares the amount of electrons it needs to achieve a noble gas electronic structure.
Covalent Bonding may result in simple covalent molecules or macromolecules like diamond and graphite.
Illustration of how covalent bonding occurs
A covalent bond is thus formed by an overlap of two atomic orbitals. The atomic orbitals is essentially the "home" of the electrons. So when the orbitals overlap, the electrons are shared.
Covalent bonding usually occurs between non-metals that would prefer not to lose electrons but would still like to gain a stable octet structure. This is because non-metals usually belong to group 6 or group 7 which are usually a -2 or - charge so it is harder for the atom to lose electrons in order to gain a stable octet structure.
So each atom shares the amount of electrons it needs to achieve a noble gas electronic structure.
Covalent Bonding may result in simple covalent molecules or macromolecules like diamond and graphite.
Illustration of how covalent bonding occurs
Unit 4
Lets get hands-on and practical with Lewis Dot-Cross Diagrams!
Example: Hydrogen has one valence electron. Its electronic configuration is 1s1. Oxygen is a Group 6 atom so it has six valence electrons. Hydrogen needs 1 more electron to achieve stable octet structure so it offers to share one electron while oxygen needs two more electrons to achieve stable octet structure so it offers to share two electrons with 4 lone pairs.
Your Lewis Dot Cross Diagram should look something like this.
Example two: Methane
Carbon is a Group 4 atom while hydrogen is a Group 1 electron. Carbon has 4 valence electrons and it needs 4 more electrons to achieve a stable electronic configuration so it shares 4 electrons. Hydrogen needs 1 more electron to achieve a stable electronic configuration so it shares 1 electron.
Your Lewis Dot-Cross Diagram should look something like this.
How to draw Lewis Dot-Cross diagrams of water and ammonia
And REMEMBER, ALWAYS have your "legend" after you complete your Lewis Dot-Cross Diagram.
Example: Hydrogen has one valence electron. Its electronic configuration is 1s1. Oxygen is a Group 6 atom so it has six valence electrons. Hydrogen needs 1 more electron to achieve stable octet structure so it offers to share one electron while oxygen needs two more electrons to achieve stable octet structure so it offers to share two electrons with 4 lone pairs.
Your Lewis Dot Cross Diagram should look something like this.
Example two: Methane
Carbon is a Group 4 atom while hydrogen is a Group 1 electron. Carbon has 4 valence electrons and it needs 4 more electrons to achieve a stable electronic configuration so it shares 4 electrons. Hydrogen needs 1 more electron to achieve a stable electronic configuration so it shares 1 electron.
Your Lewis Dot-Cross Diagram should look something like this.
How to draw Lewis Dot-Cross diagrams of water and ammonia
And REMEMBER, ALWAYS have your "legend" after you complete your Lewis Dot-Cross Diagram.
Unit 5
An idea of van der Waal's forces and how they affect physical properties such as melting and boiling points
During melting and boiling, the force that is to be overcome is the weak forces of attraction between the molecules which are the van der Waal's forces.
Note: The covalent bond only exists between the atoms that make up the compound. van der Waal's forces only exists between the molecules.
van der Waal's forces is an extremely small amount of attraction that exists between the molecules.These forces of attraction are very weak so naturally the melting points of simple covalent molecules would be extremely low.
Notes:Simple covalent compounds are made up of small dicrete molecules.
During melting and boiling, the force that is to be overcome is the weak forces of attraction between the molecules which are the van der Waal's forces.
Note: The covalent bond only exists between the atoms that make up the compound. van der Waal's forces only exists between the molecules.
van der Waal's forces is an extremely small amount of attraction that exists between the molecules.These forces of attraction are very weak so naturally the melting points of simple covalent molecules would be extremely low.
Notes:Simple covalent compounds are made up of small dicrete molecules.
Unit 6
Introduction to polar and non-polar covalent bonding
Even though we say sharing occurs in covalent bonding, the sharing is not always 50-50. Sharing would only be equal in identical atoms such as H-H or 0=0.
When identical atoms are sharing electrons, sometimes there will be a temporary dipole-dipole attraction in the non-polar bond( The electronegativity of atoms is the same).
Example of a temporary dipole-dipole attraction: H-H
However, when an atom in a covalent bond is more electronegative than the other atom, permanent dipole-dipole attraction occurs such that the electrons will be "pulled" towards the atom that is more electronegative than the other.
Example of a permanent dipole-dipole attraction : H-Cl
Since Cl is a Group 7 atom, it will be highly electronegative, "pulling" the electrons to its side such that it will be more negative on the side of the atom Cl and more positive on the side of atom H.
This results in an unequal distribution of charge on the whole molecule.
Video summary on polar and non-polar covalent bonding and provides an insight into the next topic, hydrogen bonding
Even though we say sharing occurs in covalent bonding, the sharing is not always 50-50. Sharing would only be equal in identical atoms such as H-H or 0=0.
When identical atoms are sharing electrons, sometimes there will be a temporary dipole-dipole attraction in the non-polar bond( The electronegativity of atoms is the same).
Example of a temporary dipole-dipole attraction: H-H
However, when an atom in a covalent bond is more electronegative than the other atom, permanent dipole-dipole attraction occurs such that the electrons will be "pulled" towards the atom that is more electronegative than the other.
Example of a permanent dipole-dipole attraction : H-Cl
Since Cl is a Group 7 atom, it will be highly electronegative, "pulling" the electrons to its side such that it will be more negative on the side of the atom Cl and more positive on the side of atom H.
This results in an unequal distribution of charge on the whole molecule.
Video summary on polar and non-polar covalent bonding and provides an insight into the next topic, hydrogen bonding
Unit 7
Introduction to Hydrogen Bonding, a special case of permanent dipole-dipole attraction
Hydrogen Bond is an extreme case of polar covalent bonds. When hydrogen bonds with a highly electronegative atom like N,F OR O, the only electron that hydrogen has is "pulled" all the way to the other atom.
This literally leaves hydrogen's nucleus "bare and unshielded". So the hydrogen atom, void of electrons will be attracted to the electrons of the electronegative atom on an adjacent molecule.
A quick idea of how hydrogen bonds form
For a better idea of how to draw hydrogen bonds, watch this video.
Hydrogen Bond is an extreme case of polar covalent bonds. When hydrogen bonds with a highly electronegative atom like N,F OR O, the only electron that hydrogen has is "pulled" all the way to the other atom.
This literally leaves hydrogen's nucleus "bare and unshielded". So the hydrogen atom, void of electrons will be attracted to the electrons of the electronegative atom on an adjacent molecule.
A quick idea of how hydrogen bonds form
For a better idea of how to draw hydrogen bonds, watch this video.
Unit 8
An introduction to covalently bonded macromolecules
Covalent Bonds does not always result in simple covalent molecules. It can also result in giant covalent macromolecules.
Example: Diamond
Structure: Diamond exist as a giant covalent macromolecule where every carbon atom is covalently bonded to six other carbon atoms.
Example: Graphite
Structure: Graphite exists as a giant covalent macromolecule made up of layers of carbon covalently bonded in a hexagonal lattice stacked together and between the layers is a 'sea' of electrons.
Covalent Bonds does not always result in simple covalent molecules. It can also result in giant covalent macromolecules.
Example: Diamond
Structure: Diamond exist as a giant covalent macromolecule where every carbon atom is covalently bonded to six other carbon atoms.
Example: Graphite
Structure: Graphite exists as a giant covalent macromolecule made up of layers of carbon covalently bonded in a hexagonal lattice stacked together and between the layers is a 'sea' of electrons.
Unit 9
Properties of macromolecules
Macromolecules have very high melting and boiling points as a lot of energy is required to break the strong covalent bonds between the atoms in a macromolecule covalent lattice in order for the compound to melt or boil unlike simple covalent molecules which have low melting and boiling points.
Example: Diamond has a melting point of 3550 degrees Celcius and a boiling point of up to 4827 degrees Celcius.
Most macromolecules do not have mobile particles to conduct electricity. However, graphite is able to as there are delocalised electrons between the layers.
(the electrons are in between the layers)
Diamond is one of the hardest substances known as its structure consists of a giand 3D covalent lattice of tetrahedrally bonded Carbon atoms where each carbon atom is covalently bonded to 4 other Carbon atoms. The strong lattice system of covalent bonds is thus hard to break making diamond a extremely hard substance often used for cutting.
The forces between the layers of hexxagonal lattices in graphite is very weak hence layers can slip over each other easily making graphite a good lubricant.
Macromolecules have very high melting and boiling points as a lot of energy is required to break the strong covalent bonds between the atoms in a macromolecule covalent lattice in order for the compound to melt or boil unlike simple covalent molecules which have low melting and boiling points.
Example: Diamond has a melting point of 3550 degrees Celcius and a boiling point of up to 4827 degrees Celcius.
Most macromolecules do not have mobile particles to conduct electricity. However, graphite is able to as there are delocalised electrons between the layers.
(the electrons are in between the layers)Diamond is one of the hardest substances known as its structure consists of a giand 3D covalent lattice of tetrahedrally bonded Carbon atoms where each carbon atom is covalently bonded to 4 other Carbon atoms. The strong lattice system of covalent bonds is thus hard to break making diamond a extremely hard substance often used for cutting.
The forces between the layers of hexxagonal lattices in graphite is very weak hence layers can slip over each other easily making graphite a good lubricant.
Quiz
Now to test what you have learnt!
Apply what you have learnt in this quiz!
http://www.mystudiyo.com/ch/a88949/go
Apply what you have learnt in this quiz!
http://www.mystudiyo.com/ch/a88949/go
Credits
Thank You!
Special Thanks to:
Mrs Chua for her teachings
Mr Li JX for providing the notes as reference
www.youtube.com for providing the educational videos
images.google.com for providing the pictures as illustration
By
Samuel Wong Wen Jun (3P222)
Khor Guan Koi (3P210)
Special Thanks to:
Mrs Chua for her teachings
Mr Li JX for providing the notes as reference
www.youtube.com for providing the educational videos
images.google.com for providing the pictures as illustration
By
Samuel Wong Wen Jun (3P222)
Khor Guan Koi (3P210)
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