Dot and cross diagrams for ionic compounds.
Do I show the actual electron transfer process?
If you have the space, do so, because usually CXC does not ask for it but want it anyway.
Do I show the inner electrons of ions?
No.
For a cation, do I show the 8 outer electrons or do I show none?
Any one is fine, but CXC likes the version with no outer cation electrons.
How do I represent multiple cations or anions, e.g. in MgCl2?
Either with a number in front of the ion or by drawing the ion multiple times.
Dot and cross diagrams for covalent compounds.
These diagrams require that:
1. Outer electrons only must be shown.
2. Shared electrons are in the region of overlap.
3. Circles to represent outer shells are to be used.
Note that in many textbooks, the circles are not shown. Although omission of the circles to represent outer shells will not result in lost marks, please include them in all of your covalent dot and cross diagrams.
Deciding what type of bonding occurs in different substances.
When given different named elements:
Metal and non metal : ionic bonding e.g. Na and Cl to form NaCl .
Non metal and non metal : covalent bonding e.g. H and Cl to give HCl.
If elements are the same:
Metal : metallic bonding e.g. Na.
Non metal : covalent bonding e.g. Cl and Cl to give Cl2 and C to give
diamond or graphite.
Note that if we have two different metals they do not undergo metallic bonding e.g. Na and Mg. This question should not even arise at the Form 4 to Form 5 level.
If we are given unnamed elements, e.g. ‘Element X’ or A the first step is to deduce whether they are metals or non-metals. This is achieved by writing the electronic configuration of that element from either the number of electrons, or protons (which are both the same for an atom not yet bonded). Remember that metals will have 1 to 3 outer electrons. Non metals have 4 and more outer electrons.
After this has been done, we can proceed with the method above to determine type of bonding, and even formulae. While working on scrap, it is usually possible to figure out what actual element is represented by the unknown. For example, element A, having a proton number of 11 or an electronic configuration of 2.8.1 is sodium. This knowledge simplifies the rest of the question.
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