Arrow Electron Flow Ex from Course Reader

[Alexis Kupka 1H]

In the course reader example with CH3CH=CHCH3 and H-Br, why does the arrow flow show two electrons moving to the H? How can you tell how many electrons are being transferred?

[Andre Tsivis 1K]

2 electrons are being transferred if there is a double-headed arrow, and only one electron is being transferred if there is a single-headed arrow (pg. 86 course reader).

[Andre Tsivis 1K]

I am also assuming that when a BOND is broken, 2 e- will be transferred and when there is just a single dot drawn in the Lewis structure then only that one e- will be transferred.