As promised: Bonding review.

I hope that you have been all preparing for the test on Friday.  I have had a look over the first part of your tests and I think I will start by clearing up a common misconception.  Covalent lattices have some of the same properties as covalent molecules – except their melting point and hardness.  That is, they will not conduct electricity as a solid/liquid/in solution.

Also, please make sure that you read the question carefully.  Many of you are including information that is not required.  If the extra information that you have included is incorrect you will have marks deducted even though the information was not relevant.

I have created the following summary table for PRIMARY BONDING to help you with your revision.

Bonding

Structure

Properties

Metallic

(metals)

  • metal atoms arrange in lattice and achieve full valence shell by sharing delocalised electrons which move freely
  • the layers of atoms can slide over each other
  • held together by electrostatic attraction

 

  • strong – primary bonds are hard to break
  • shiny – light reflects off delocalised electrons
  • malleable – layers of atoms are able to slide over each other
  • good conductor of electricity – delocalised electrons are free to move and therefore conduct electricity
  • high melting point – primary bonds require a large amount of heat energy to separate
  • good conductor of heat – delocalised electrons carry heat throughout the metal

Ionic

(metals and non-metals)

 

  • lattice formed from electrostatic attraction between oppositely charged anions and cations
  • ions are formed by transfer of electrons
  • in the solid state the ions are held in place
  • once melted or dissolved they are free to move

 

  •  hard – strong primary bonds are hard to break
  • crystalline – ions arrange themselves in regular lattice structure
  • soluble in water – ions separate from lattice and form secondary bonds with water
  • high melting point – primary bonds require a large amount of heat energy to separate
  • conduct electricity once melted or dissolved – ions are free to move and therefore conduct electricity
  • brittle – when a force is applied to the lattice the ions with like charges come into contact with each other and repel each other causing the lattice to shatter

Covalent molecules

(non-metals and non-metals)

  • molecules with no charged formed when electrons are shared
  • strong bonding inside each molecule but weak bonding between molecules

 

  • soft – secondary bonds between molecules are weak
  • low melting point – secondary bonds between molecules don’t require very much heat energy to separate
  • don’t conduct electricity when solid, liquid or dissolved – no charged particles free to move

Covalent lattices

  • huge lattice with millions of covalent bonds
  • no weak secondary forces to break
  • strong – no weak secondary forces to break
  • hard – no weak secondary forces to break
  • high melting point – large amount of heat energy required to separate primary bonds

But more on covalent bonding…

The following video gives a good overview of classifying bonding types and naming compounds. (Beware, it’s a long one, and you don’t need to know about expanded octets at the end – not until next year at least).

You also need to know how to draw electron dot diagrams and determine molecule shapes. The following interactive tutorials 7.5 , 7.6 and 7.9 may help you out.

Then you need to know about bond polarity and molecule polarity.  A polar BOND occurs when an atom involved in a bond has a greater share of the electrons due to a difference in electronegativity.  This results in one atom having a very small positive charge and one atom having a very small negative charge (dipole).  In other words, one atom hogs the electrons more than another.  But how do you know which atoms hog the electrons the most?  You need to consider the electronegativity of the atom (general trend from the periodic table or table of values – both will be provided in the test).  The atom that has the bigger number/value will want the bigger share of the electrons.  This website gives an interesting representation if you want to check it out further.

But polar MOLECULES are different to polar BONDS.  For a molecule to be polar, you must consider 2 things: the shape of the molecule and if polar bonds are present.  This is why it is important to understand how to work out molecule shapes.  If polar bonds are present and there are multiple axes of symmetry, then the polar bonds will cancel each other out, and the molecule will be non-polar (eg CCl4).  If there are polar bonds present and the polar bonds don’t cancel each other out, then the molecule will have a slightly positive end and a slightly negative end.  This results in a polar molecule (eg H2O). You can investigate the concept further here.

Once you have a good understanding of polar molecules, you will be able to answer questions about secondary bonding.   Secondary bonds (also known as secondary forces, or secondary interactions, or intermolecular interactions) are much weaker in comparison to primary bonds (metallic, ionic, covalent).  This accounts for the much lower melting point of covalent compounds (with the exception of the covalent lattices). Secondary bonds are the forces acting between covalent molecules.  There are 3 types of secondary bonds that we need to worry about.  The first is dispersion forces.
Dispersion forces are the weakest of the secondary forces and occur between non-polar molecules.  Can’t remember how it works? Check it out here.


Dipole-dipole interactions are a little stronger than dispersion forces and occur between polar molecules.  The partial charges between neighbouring molecules attract each other and result in a slightly stronger force of attraction.

A special type of dipole-dipole interaction is hydrogen bonding.  Hydrogen bonding occurs when a O-H, N-H or F-H bond is present in a molecule.  Because O, N and F are very electronegative, this results in a larger difference in electronegativity than normal, therefore there is a lightly larger dipole present than normal.  This of course means that the attractions are stronger and a greater amount of energy is required to overcome these forces and separate the molecules.  This site shows the difference between intermolecular (between molecules) and intramolecular (within molecules) forces that occur between water molecules which exhibit hydrogen bonding.

I hope that covers it all. Please come see me before the test if you have any further questions.
Happy studying and good luck!

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1 Comment (+add yours?)

  1. mrselliott
    May 05, 2012 @ 16:29:44

    Reblogged this on mrs elliott.

    Reply

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