Nuclear notation and isotopes

A nucleus is written as the symbol of the element with two numbers beside it. The mass number is the top number and is the total number of protons and neutrons. The atomic number is the bottom number and is the number of protons. Carbon-14 is written ¹⁴₆C. The atomic number is also the charge of the nucleus, so this nucleus has a charge of +6.

To find the number of neutrons, subtract the atomic number from the mass number. For ¹⁴₆C this is 14 − 6 = 8 neutrons.

Isotopes of one element have the same atomic number but different mass numbers. ¹²₆C and ¹⁴₆C both have 6 protons, so the same charge, but they have 6 and 8 neutrons, so different masses. Chlorine has two common isotopes, ³⁵₁₇Cl with 18 neutrons and ³⁷₁₇Cl with 20 neutrons.