Let
V be a vector space.Then,
1.
V is closed under vector addition.(The sum of two vectors is again a vector.)
2.For any two vectors

and

in
V we have by definition,
(Commutativity)3.For any three

,

and

in
V,we again have by definition
(Associativity)4.There exists a null vector such that,

for all

in
V.
(Existence of identity)5.There exists a 'negative' vector

for any

in
V such that,
(Existence of inverse)Thus,
V satisfies all the requirements of an abelian group.