The number of protons in the nucleus of an atom determines what element it is, so the number of protons is fixed. But, the number of neutrons can vary. Nuclei with the same number of protons but different numbers of neutrons are called isotopes. For example, carbon, with six protons, can have five, six, seven, or eight neutrons. The isotopes are labeled carbon-11, carbon-12, carbon-13, and carbon-14. A more compact notation is ¹¹C, ¹²C, ¹³C, and ¹⁴C. Chemical properties in general do not depend on the isotope. There are at least 3,100 isotopes of all the elements presently known.