Renyijc
When do we use AlCl3 for mainly ionic with covalent character vs mainly covalent with ionic character? 
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BedokFunlandJC
AlCl3 (monomer) and Al2Cl6 (dimer) are both simple molecular, ie. it's intramolecular bonds are primarily covalent but with significant ionic character (ie. it's covalent bonds are significantly polar).

If there is an exam question (eg. TYS or Prelims) that you're basing your question on, do specify the exact exam question, eg. state "2018 RJC P2 Q3c". If no source is given (eg. your school tutorial), take a photo (of both question and answer, if provided) using your handphone, and upload the image file to this forum in your post. You'll learn better in context, rather than general theory. 
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Renyijc
15653638305946287874065918202526.jpg it's actually YJC prelim p2 2018 q2ai.  Though the question never ask but when are the cases we consider AlCl3 to be mainly covalent and Al2O3 mainly ionic with covalent character and how it affects? Thank you. 
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BedokFunlandJC
The question shows you why you need to consider covalent vs ionic character in a compound : to explain the difference in melting and/or boiling points.

Al2O3 is mainly ionic with some covalent character, due to the larger magnitude of electronegativity difference between Al and O.
AlCl3 and Al2Cl6 is mainly covalent with some ionic character, due to the smaller magnitude of electronegativity difference between Al and O.

Hence, with AlCl3 and Al2Cl6 being simple molecular while Al2O3 being ionic, AlCl3 and Al2Cl6 have a much lower melting and boiling point, compared to Al2O3.

On a related note,
[500px-Aluminium-trichloride-3D-structures] 
AlCl3 adopts three different structures, depending on the temperature and the state (solid, liquid, gas). Solid AlCl3 is a sheet-like layered cubic close packed layers. In this framework, the Al centres exhibit octahedral coordination geometry.[7] When aluminium trichloride is in its melted state, it exists as the dimer Al2Cl6, with tetracoordinate aluminium. This change in structure is related to the lower density of the liquid phase (1.78 g/cm3) versus solid aluminium trichloride (2.48 g/cm3). Al2Cl6 dimers are also found in the vapour phase. At higher temperatures, the Al2Cl6 dimers dissociate into trigonal planar AlCl3, which is structurally analogous to BF3. The melt conducts electricity poorly,[8] unlike more-ionic halides such as sodium chloride.

https://en.wikipedia.org/wiki/Aluminium_chloride
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Renyijc
The monomer in the gas phase is just AlCl3 right? And is it necessary for me to write about the electronegativity difference between Al and O and Al and CL? 
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BedokFunlandJC
Yes that's right. Cambridge will give the molar mass for you to calculate whether a certain species is a monomer or dimer. Eg. AlCl3 vs Al2Cl6, ethanoic acid monomer vs ethanoic acid dimer (via hydrogen bonding), etc.
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