Doubt from coordinate geometry

I have two doubt
Doubt 1
How we know the coordination number of Cr is 6? Do we have to remember these values? Can anyone tell which other element to remember coordination number?

Doubt 2
In the possible isomers, why there is no isomer in which there are 3 chlorine inside?

For Doubt1) You dont really have to memorize the values for elements just write electronic configuration of the element, and pick out the number of electrons available for coordinate bond formation.

For Doubt2) In hydrate isomer of any compound H_2O molecule acting as ligand is replaced by anions present outside the coordination sphere. In the compound [CrCl_3.3H_2O].3H_2O there are no chloride ions outside which can be replaces by water molecule.
Hope it helps.

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Write electronic configuration of free atom and check according to it?
Because if we write configuration when it has formed complex, each time it's configuration will be different for different isomers, we can see it in this example also

Im not able to understand your query. Can you please elaborate what you wanna ask?

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I understood now, thanks for answering

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