What is the number between brackets of the angle in X-ray diffraction such as 170.57(6)?
That number is the e.s.d. or estimated standard deviation for that measurement - you see it in bond lengths as well. When comparing two angles or distances, you shouldn't consider them different unless they are at least 3 esds different (very rough statement - the mathematics of the system is much more complex.)
Dear Joseph S Merola many thanks for you. Could you please give me example to explain how can I compare between two angles if they are different or no?
Before I do that, as I mentioned above, the "error" is more complex than the e.s.d.s might indicate and I suggest the following as a reading: http://www.uh.edu/~chembi/ChemSocRev_Jones_critical.pdf
For a simple example, suppose you are comparing two angles, one with the number you give in your question: 170.57(6). In a similar structure you find the same angle to be 171.12(5) degrees. Assuming you take 3 sigma as the determiner for difference at the 97% confidence level. The range for the first angle of +/- 0.18 degrees gives you 170.39-170.75 degrees for that angle. For the second, it would be +/- 0.15 degrees or 170.97-171.27 degrees. There is no overlap of those ranges so they would be significantly different. But, as I said, please read the Jones article .
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