According to page 6 of Pub 501 under the heading, "marital status", if you are legally separated from your spouse under a divorce or separate maintenance decree as of December 31, 2016, you are considered unmarried for the whole year. It goes on to say that "state law governs whether you are married or legally separated under a divorce or separate maintenance decree".
https://www.irs.gov/pub/irs-pdf/p501.pdf
So if you are considered legally separated under your state law, then you can file as either single or head of household. There are specific requirements that must be met in order to file as head of household. Please refer to the following FAQ to learn more about these requirements. https://ttlc.intuit.com/replies/3288637
