Google have not specified any optimum word count that an article must have. Google suggests that the article must be unique, well written, it should be original and informative. Your social media response to the article is also very important than the length of the article, article with most FB likes, G+ and Tweets would be considered to be of good quality by Google.
Kindly refer to this article by Google about quality website. It gives you important insights on what a good article must constitute.
http://googlewebmastercentral.blogspot.com/2011/05/more-guidance-on-building-high-quality.html