Bonding
The Key to this section is the Noble Gases configuration. These gases are the most stable pure substances and the reason is that they have a complete outer shell of electrons. This is a very stable arrangement to have,
Imagine the landlord of an apartment block
For every floor they have they have to pay electricity, carpet the floor, provide security etc.
They would prefer if the floors were full, that way they maximise their profits.
Atoms want to maximise their stability and so prefer when all their shells are full.
When we put 2 or more atoms together we say they are bonded into molecules*
OC41 understand how atoms of elements combine to form compounds
Why do elements react?
The reason the Nobel gases went so long without being discovered comes mainly from their unwillingness to react with other elements.
The reason for this unwillingness to react with other elements is because all their shells are full.
This means they do not want to lose or gain electrons.
All other elements react with each other in order to obtain a full outer shell.
These elements are said to be 'bonded' to the other element, and these atoms now form a compound
All atoms want to have a full outer shell of electrons. They achieve this by bonding.
There are 3 different types of bonding,
- Ionic Bonding, caused by electrostatic attraction between charged ions
- Metal + nonmetal = ionic bond
- Covalent Bonding, where electrons are shared
- Nonmetal + nonmetal = covalent bond
- Metallic bonding, where only an overall net balance is maintained, electrons find it easy to pass
- Metal + metal = metallic bond
we will however only need to study the 1st two.
Ionic & Covalent
These two methods of bonding describe how different elements atoms interact with each other
OC42 recall that ionic bonding is an attraction between positive and negative ions;
describe the bonding in NaCl and MgO as examples
Ionic Bonding
Ionic bonding is the bonding between 2 or more ions. An ion is an atom which has lost or gained electrons.
An Ion is an atom that does not have the same number of electrons as it has protons.
If an atom has more electrons than protons it will be a Negative Ion
If an atom has less electrons than protons it will be a Positive Ion
The two atoms are held close to each other by electostatic force between the Negative Ion and the Positive Ion
Just like in magnetism
Opposites Attract
click on image to see a bigger version
Electrons are swapped from one atom to another so that each atom involved has a full outer shell.
Ionic bonds are held together because oppositly charged atoms are attracted to each other.
Ionic bonds only Occur between atoms of either side of the periodic table
Ionic Bonding: one atom loses electrons and a different atom gains electrons,
the ions stick together because opposite charges attract !
Elements lose or gain electrons in order to become ions.
They do this to obtain a full outer shell.
This is what all atoms strive to have.
thanks to Alex C for the Image
lets take NaCl
Sodium needs to gain __ electrons or lose __ to have a full outer shell.
Its easier to ______________. This makes Sodium ___________
Chlorine needs to gain __ electrons or lose __ to have a full outer shell.
Its easier to ______________. This makes Chlorine ___________
The positive ________ ion and the negative ________ ion attract each other to form an ionic bond.
Oxygen needs to gain __ electrons or lose __ to have a full outer shell.
Its easier to ______________. This makes Oxygen ___________
Calcium needs to gain __ electrons or lose __ to have a full outer shell.
Its easier to ______________. This makes Calcium ___________
The positive ________ ion and the negative ________ ion attract each other to form an ionic bond.
Valency
Every element can have a Valency number assigned to it. The number is the number of electrons that the atom has either surplus (extra) or is short of a full outer shell.
We write the Valency as being the number of electrons that it is easier to gain or lose ....
So write down the valency for Sodium , Chlorine Oxygen Calcium
Elements lose or gain electrons in order to become ions.
They do this to obtain a full outer shell.
This is what all atoms strive to have.
For some elements it is easier to lose an electron than to gain loads of them.
OC43 state what a molecule is, understand that covalent bonds involve the sharing of pairs of electrons, and describe the bonding in H2, O2, H2O, CH4 as examples of covalent bonding
A Molecule is a group of atoms (2 or more) bonded together.
This is the smallest unit of any substance that can exist, while retaining the properties of that substance.
A compound is a molecule that has 2 or more different elements in it.
examples of covalent compounds (methane and water)
Water H2O
Methane CH4 .
Covalent Bonding: atoms share electrons in their outer shell.
The Covalent Bonding of Carbon: A Special Investigation
The way that carbon atoms bond with each other and with other elements deserves special treatment. This is not only because it is fundamental to life on Earth, but also because by understanding the behaviour of Carbon we can gain great insight into the chemistry of the other elements involved in the successful operation of solar energy cells.
We have already seen that the element Carbon has four outer shell electrons and needs to have a total of eight to be stable. Since it can neither gain nor lose these electrons, it needs to share them with other elements.
This can be clearly seen in the formation of the chemical Methane, where Carbon combines with four Hydrogen atoms in order to gain 8 electrons in its outer shell. This is also true of Carbon in its interactions with any other elements, or indeed in its interactions with other Carbon atoms. It is this property that allows the many allotropes of pure Carbon to exist.
Chemistry Definitions: Building Carbon Chains
We can use the electron dot diagram of Carbon to construct molecules that are chains of Carbon atoms bonded to each other. The alkane series are a group of molecules made entirely from Carbon and Hydrogen. They are very simple molecules that allow us to master the basics of molecule model building. Once we have mastered the single bonded Carbon atoms, we can consider the alkene series which contain slightly more complex bonding. Once we understand this series, being able to construct the alkyne series is simple.
OC44 investigate the ability of ionic and covalent substances to conduct electricity
properties of ionic and covalent substances
Properties of Ionic and Covalent Compounds
The differences between ionic and covalent compounds is commonly asked.
A tip is to remember that table salt (NaCl) is an ionic compound and O2 is a covalent compound.
Properties of Ionic and Covalent Compounds
Property Ionic Compounds Covalent Compounds
State
Melting pt & boiling pt
Solubility in water
Conduct electricicty
Speed of Reactions
Examples
The differences between ionic and covalent compounds is commonly asked.
A tip is to remember that table salt (NaCl) is an ionic compound and O2 is a covalent compound.
Being able to draw and explain the bonding between Ionic and Covalent compounds is important to get a good grade.
Test your knowledge of bonding
http://www.sciencequiz.net/jcscience/jcchemistry/bonding/bonding1a.htm
There is really a third type of bonding but it is not needed to be studied for this course.
METALLIC BONDING.
Metal and metal:
Metal atoms surrender their outer shell electrons to form stable positively charged ions. The positive ions are locked into layers by the presence of the free electrons, which are in turn retained by the charge of the ion layers.
Amazing page on this site with some very excellent ideas on bonding ....
especially the metallic bonding idea
http://www.uwgb.edu/dutchs/EarthSC202Notes/minerals.htm
I like his O2 idea of sharing .... always been a problem for me because there are 4 electrons to be shared