Vladimir Vasiley'evitch Markovnikov (don't argue about spelling - it can only be done properly in Cyrillic) gave his empirical rule for addition of HBr to unsymmetrical alkenes in 1888. The reason for its happening had to await mechanistic studies.

Give the structural formula of the compound formed when HBr reacts with propene, and explain your choice.

The product is 2-bromopropane; the hydrogen goes to that carbon in the double bond which already carries more hydrogens. This is the 'rule' though like most rules in chemistry there are plenty of exceptions because the subject is complex and chemical structures are so varied. That partly is the reason why only a restricted range of substances is studied. (Indeed I set a question on the addition of HBr to acrolein, CH₂=CHCHO; the answer expected was CH₃CH(Br)CHO, although in fact the presence of the -CHO group changes the orientation of addition. That is why the original question said 'assume normal behaviour....’; to those who wrote letters - yes, I do know!)

Citing M's rule is not an explanation; to explain the orientation you have to consider the 'stability', and thus the likelihood of formation, of the intermediate carbocations. The secondary carbocation formed when 2-bromopropane is the product is more 'stable' than the primary one which is the intermediate when 1-bromopropane is the product. This means that the (free) energy of activation for the formation of the secondary ion is less than that for the primary ion. Both are formed; the 2-isomer is the major product, not the only one.

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