The elimination mechanism is shown with three curly arrows. The hydroxide ion acts as a base and removes a proton from the carbon atom next to the carbon bonded to the halogen.The first curly arrow goes from a lone pair on the hydroxide ion to a hydrogen atom on the adjacent carbon. The hydroxide ion becomes a water molecule.The second curly arrow goes from the C-H bond to the bond between the two carbon atoms, forming the C=C double bond.The third curly arrow goes from the C-X bond to the halogen atom, which leaves as a halide ion.For bromoethane the products are ethene, water and a bromide ion. All three arrows must start at a lone pair or a bond.