Intermolecular Forces and Intramolecular forces for Graphite

Note for graphite, both covalent and intermolecular force of attraction applies. (Pure)
When explaining the high temperature in which graphite melts, we say that they are breaking strong covalent bonds. (Pure)
When explaining why graphite act like a lubricant, we say that the graphite molecules are arranged in layers which are held by weak IMF therefore can slide over each other easily. GET IT? (Pure)

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Json Lim
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Bonding

Bonding and Structures
Chemical bond is a force of attraction holding atoms together in a molecule or crystal
Three types of chemical bonds
• Ionic bonding
• Covalent bonding
• Metallic bonding

Ionic bonding – involving metal and non-metal elements. Force of attraction between oppositely charged ions. Only exception is compounds like ammonium salts. Although ammonium is made up entirely of NON_METAL, it is still an Ionic compound because it contains charges.

Covalent bond – formed between 2 combining atoms of non-metallic by mutual sharing of one or more electrons

Metallic bonding – type of chemical bond that holds the atoms together in a solid metal. It’s the attraction between the sea of electrons and the positive metal ions. (more important for pure chem.)



Covalent compounds
- simple molecular compounds like hydrogen molecule, water molecule etc.
- giant molecular structure like diamonds and silicon dioxide (pure)
- graphite
Ionic compounds – self explanatory
Metallic substances – pure metals and alloys

Types of substances
Simple molecular
Types of bonds broken during change of state: Intermolecular forces of attraction.
Strength of the bond broken:Weak
Melting point and Boiling point:Low

Giant molecular or Giant Covalent
Types of bonds broken during change of state:Covalent
Strength of the bond broken:Strong
Melting point and Boiling point:High

Ionic Compounds
Types of bonds broken during change of state:Ionic Bonds
Strength of the bond broken:Strong
Melting point and Boiling point:High

Metals Types of bonds broken during change of state:Metallic Bonds
Strength of the bond broken:Strong
Melting point and Boiling point:High

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Json Lim
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Comparing the MP/BP of Ionic and Covalent Compouds

This question is by far my favourite and it seems like most students have difficulty giving a decent answer for this straight forward question. Lets look at how a sample question would look like.

Question normally ask you to state which substance has a higher MP or BP and explain why.

For our answer, it will be definitely be the ionic compound of magnesium oxide. For those who are lost, this is because ionic compounds have high MP and BP while simple covalent compounds have low MP and BP. If you have no clue how to differentiate between ionic and covalent compounds refer to this post.

Now the next step is to explain why does magnesium oxide has high MP and BP.

This is the suggested template.

a)Start by identify the type of compounds for the substances.

For our question magnesium oxide is the ionic compound and the oxide of phophorous is a simple covalent molecule.

b) Next, tell the marker the kinds of bonds broken when the substances undergo change of state.

This is the general rule of selection when it comes to identifying the kinds of bond broken.

Ionic Compounds --- break strong ionic bonds during change of state (aka melting and boiling)
Giant Covalent Compounds --- break strong covalent bonds during change of state (aka melting and boiling) Simple Covalent Molecules --- break weak intermolecular force of attraction or van da waal's forces of attraction during change of state (aka melting and boiling)

c)Last part sum up your answers by telling the marker the amount of energy needed to carry out the change of state.

That's all. I am gonna end this post by writing a sample answer for this type of question.

Qn: Which of the 2 substances, magnesium oxide and phophorous oxide has a higher melting point. Explain.

Ans: Magnesium oxide is an ionic compound and phosphorous oxide is a simple covalent compound. During change of state, magenesium oxide breaks strong ionic bonds while phosphorous oxide breaks weak intermolecular force of attraction between the molecules. Hence a lot of energy is needed to break the strong ionic bonds therefore MP is high, while lesser energy is needed to break the weak intermolecular force of attraction hence MP of simple covalent molescules is low.

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Json Lim

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Formulae of Ions Part 1 - First Twenty Elements

How do we find decide the ionic charges of the various ions? For O level examinations, you are expected to know how to derive the ionic charges for certain elements.

Technique.
Lets select an element, e.g. Magnesium.
Magnesium has the following information based on the periodic table.
Number of protons: 12
Number of neutrons: 12
Number of electrons: 12

The electronic configuration of magnesium is 2,8,2.
The next thing is to ask yourself, in order to obtain a stable electronic configuration (full outer electron shell), the atom needs to do something.
a) Lose 2 electrons so that the configuration becomes 2,8 (full outer electron shell)
b) Gain 6 electrons so that the configuration becomes 2,8,8 (full outer electron shell again)

From all the atoms' point of view, it is easier to lose 2 electrons than to gain 6. So in order to form a stable electronic configuration, magnesium will choose to lose 2 electrons and the updated sub atomic particles looks like this.
Number of protons: 12
Number of neutrons: 12
Number of electrons: 10
Since electrons are negatively charged and protons are positively charged, there will be a total of 12 positive charges and 10 negative charges and hence the overall charge is +2.
Therefore the charge on magnesium is +2

Lets take another example, oxygen.
Subatomic particles of oxygen atom looks like this
Number of protons: 8
Number of neutrons: 8
Number of electrons: 8
Electronic configuration: 2,6
In order to be stable oxygen needs to gain 2 electrons (instead of losing 6 electrons) to form a stable electronic configuration.

After gaining 2 electrons, the new numbner of subatomic particles looks like this.
Number of protons: 8
Number of neutrons: 8
Number of electrons: 10
In this ion, there are 8 positive charges and 10 negative charges, hence it has an overall charge of -2. Therefore the ion of oxygen has a charge of -2.

This is how you find the ionic charge of the first 20 elements. Cheers.

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Json Lim
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Differentiating Ionic and Covalent Compounds

A common question asked by my students all the time, how do you know if a substance is an ionic or covalent compound?

The general rule is basically very simple and i am sure your teachers have mentioned many times.

Ionic compounds - formed by substances that is made up with positive and negative ions.
The EASIEST method to decide if a substance is ionic basically is to look at the formula of the compound. For example, magnesium chloride, basically made up of magnesium ion (metallic) and chloride ion (non-metallic). So to recognize an ionic compound, look out for a metal and a non-metal in formula. This however is just a general guide as there are exception cases.

For example, a compound like ammonium chloride, NH4Cl. Notice there are no metals, in ammonium chloride but it is still considered an ionic compound. Reason is because it is made up of the positive ammonium ion and negative chloride ion. However, it is still quite safe to to identify ionic by the metal - non metal rule.

Finally for covalent substances, its the easiest to identify. As long as the substance does not contain any metals it can be taken to be a covalent substance. That of course taking into account the annoying ammonium based substances which defy this rule.

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Json Lim
http://www.oleveltuition.com/