No Subject

As long as we are talking about John 1:1, I'd like to ask a question.  
PLease forgive me if this sounds simple, but I am a first year grk 
student.  At the end of the verse, it says KAI THEOS HN HO LOGOS.  In most 
translations I've seen (NIV, NRSV, etc.)  it reads "...and the Word was 
God."  My question: Why does THEOS not have an article? Could not this be 
translated "a God?" Why not?

Leroy Huizenga
Jamestown College, ND