Electrophilic addition mechanism

In the first step of the mechanism for addition reactions to alkenes the carbon-carbon double bond is the nucleophile.  The more electropositive atom in the reagent is the electrophile. 

This step involves simultaneous

• formation of a bond between one of the doubly bonded carbons and the more electropositive portion of the reagent
• breaking a bond in the reagent.

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Because charge is conserved, electron movement in the first step results in two charged intermediates.
 

• 
  The carbon-containing intermediate is referred to as a carbocation due to the positive charge at carbon.
• The charge changes are consistent with electron movement from the double bond toward the electrophile 
   
  The carbon is positive in the intermediate due to "owning" one fewer electrons than it did in the reactant.  The N is negative due to owning one more electron in the intermediate than in the reactant.

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sIn the second step of the mechanism a bond forms between the two intermediates.