Carbon has an atomic number of 6. You have found an isotope that has a mass of 14. What is different about this isotope than a carbon atom with a mass of 12?
Carbon 12 has six protons and six neutrons in the nucleus where as carbon 14 has 8.
Protons define the atomic number due to their positive charge and six of them bound in an atomic nucleus is an atomic number of six. This makes the element carbon. It has six orbiting electons which cancel out the charge on the nucleus. There are neutrons which helps keep the nucleus together and add mass but no charge. Carbon 6 has six protons and six neutrons. Carbon 14 has 6 protons and eight neutrons in it's nucleus.