A nucleus is written as a symbol with two numbers. The mass number goes at the top and is the total number of protons and neutrons. The atomic number goes at the bottom and is the number of protons. For example, 3517Cl is chlorine with mass number 35 and atomic number 17.
The atomic number is also the charge of the nucleus in units of the proton charge, so this nucleus has a charge of +17. To find the number of neutrons, subtract the atomic number from the mass number. Chlorine-35 has 35 - 17 = 18 neutrons.
Isotopes of one element have the same atomic number but different mass numbers. Chlorine-37, written 3717Cl, also has 17 protons, but it has 37 - 17 = 20 neutrons, so it is heavier. Carbon-12 and carbon-14, 126C and 146C, are another pair: 6 protons each, with 6 and 8 neutrons.