In order to answer you question, I went straight to the horse's mouth (IRS) and copied the question below from their site. I have included a link in order for you to view other FAQs you may have.
www.irs.gov/faqs/faq2-3.html
My husband and I were separated the last 11 months of the year and our two children lived with me. My husband provided all the financial support. Who can claim the children as dependents on the tax return?
Since your children lived with you for the greater part of the year, you are considered the custodial parent and you are the parent who is eligible to claim them as dependents on your tax return if the other dependency tests are met. Your husband is the non-custodial parent and would only be able to claim the children if you release your claim to the exemption by completing Form 8332 (PDF), Release of Claim to Exemption for Child of Divorced or Separated Parents, or signing a substantially similar statement. Please be aware that if you release your claim to the dependency exemption for a child, you may not claim a Child Tax Credit for that child.
Refer to Publication 501, Exemptions, Standard Deduction, and Filing Information or Publication 504, Divorced or Separated Individuals, for more information on the special rule for children of divorced or separated parents.
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