List different classes of pentose + HEXOSE

A pentose is a monosaccharide with five carbon atoms having the chemical formula C5H10O5.
They either have an aldehyde functional group in position 1 (aldopentoses), or a ketone functional group in position 2 (ketopentoses).

The aldopentoses have three chiral centres ("asymmetric carbon atoms") and so 8 different stereoisomers are possible.

The ketopentoses have 2 chiral centres and therefore 4 possible stereoisomers — ribulose (L- and D-form) and xylulose (L- and D-form).

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A hexose is a monosaccharide with six carbon atoms having the chemical formula C6H12O6.
Hexoses are classified by functional group, with aldohexoses having an aldehyde at position 1, and ketohexoses having a ketone at position 2.

The aldohexoses have four chiral centres for a total of 16 possible aldohexose stereoisomers. The D/L configuration is based on the orientation of the hydroxyl at position 5, and does not refer to the direction of optical activity.

The ketohexoses have 3 chiral centres and therefore eight possible stereoisomers.

I hope this can help your understanding. =)