Alkenes have the general formula CnH2n
So an alkene always has twice as many hydrogen atoms as carbon atoms.
4:26 understand how to draw the structural and displayed formulae for alkenes with up to four carbon atoms in the molecule, and name the unbranched-chain isomers. Knowledge of cis/trans or E/Z notation is not required
The displayed formulae show all the atoms and bonds drawn out.
The molecular formulae just show the number of each type of atom in the molecule.