Dear ma'am/Sir , actually while writing names i was getting stuck ... when we take C3H7 and then aad CHO to it so the name is given by looking at 3 carbons or 4 .. pls help me by providing some more examples regarding this problem .. thanks

Since an aldehyde carbonyl group must always lie at the end of a carbon chain, it is by default position #1, and therefore defines the numbering direction and this number is not included in the name.

 

Aldehydes take their name from their parent alkane chains. The -e is removed from the end and is replaced with -al. 

 

So, the IUPAC name of C₃H₇CHO is butanal.

 

All the carbon atoms are included in the naming and numbering.