An iron needle slowly placed on the surface of water floats on it because

Due to the concave shape of the surface film, total surface tension force on the needle = sl+sl =2sl. Also water displaced by the needle produces a buoyant force B on the needle. Resultant of buoyant force and surface tension force, balances the weight of the needle. Since buoyant force B is very small, weight of the needle is balanced chiefly by surface tension force.