[deutdeut]

Nucleophiles are usually molecules with lone pairs of electrons. But for the molecule HBr, it contains lone pair of electrons, but it's classified as electrophiles? Why? How can I know which molecule are electrophiles and which are not?

[english]

It depends on the reaction mechanism.   

HBr acts as an electrophile in addition reactions with alkenes, such that the pi electrons of the alkene "attack" the H⁺ of HBr.  This is because Br in HBr is partially negative and withdraws electrons away from hydrogen.  Hydrogen is therefore electron deficient, or electrophilic. 

But remember HBr is a strong acid, so in water it exists as H₃O⁺ and Br^- ions.
However, an addition reaction of an alkene with HBr does not use water as a solvent, but something organic, such as diethyl ether.  HBr will not behave the same way in an ether solvent.


Generally, it depends on the reaction and what must occur for the mechanism to work, to know whether a species will be electrophilic or nucleophilic.  Do not necessarily assume that a molecule cannot act as an electrophile just because one constituent atom has one or more lone pairs.