How can I know how many hydrogens are there for each Carbon in bond line notations?

Line notation implies that you should well-know the octet rule and can mentally interconvert between full structures and line notations. Just mentally add a hydrogen until the central carbon has an octet (or 4 single bonds, two single bonds and one double bond, two double bonds, or one single and one triple bond).

i.e. propane is #CH_3CH_2CH_3#, and drawing its line notation implies that the center carbon has two hydrogens attached and two carbons attached. It just takes practice.

In bond line notation,

• The carbon atoms and the hydrogen atoms attached to them are not shown.
• Only the bonds between the carbon atoms are shown as lines.
• The vertices and end of lines represent the carbon atoms.
• Any unfilled valences on carbon are assumed to be filled by hydrogen atoms.
• All atoms other than carbon, plus any hydrogen atoms attached to them, are shown.

For example, the bond line notation for propane is

In the above diagram, the vertex and the ends of the lines represent the carbon atoms.

The two end #"C"# atoms each have one bond to middle #"C"# atom, so the number of hydrogen atoms on them is #4-n = 4-3 = 3#.

The middle #"C"# atom has two bonds going to the other carbons, so it has #4-n = 4-2 = 2# hydrogen atoms.

In the above molecule, the three #"C"# atoms on the right hand side each have one bond to a central carbon. Since #4-n =4-1=3#, they each have 3 #"H"# atoms (#"CH"_3# groups).

The central #"C"# atom is bonded to the three methyl groups and to the ring — a total of 4 bonds. #4-n =4-4 =0,# so this carbon has no #"H"# atoms.

The ring carbon that is attached to the side chain has 3 bonds, so it has one #"H"# atom.

The remaining four carbons each have two bonds, so they have two #"H"# atoms (#"CH"_2#) groups.