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Answer 1 · 2018-01-04T14:05:48

Here's my explanation.

The nucleophile is not a bromine atom, #"Br·"#. A bromine atom is electron deficient (it "wants" an electron to complete its electron shell).

The nucleophile is a bromide ion, #"Br:"^"-"#. It can use its electron pair to form a bond to another atom.

An example is the use of hydrobromic acid to convert an alcohol to an alkyl bromide.

#"CH"_3"CH"_2"OH" + "HBr" → "CH"_3"CH"_2"Br" + "H"_2"O"#

The final step in the mechanism involves the nucleophilic attack of a bromide on an alkyloxonium ion.

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The electrophile is not a bromine atom. It is a bromine molecule, #"Br"_2#.

An example is the bromination of an alkene.

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The #"Br"_2# molecule attracts the π electrons of the double bond toward itself to start the reaction.

#"Br·", "Br"^"-"#, and #"Br"_2# are different chemical species and they have different chemical properties.