How to find neutrons on the periodic table?

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To find neutrons on the periodic table , subtract the atomic number from the mass number: neutrons = mass number − atomic number. The atomic number (protons) is the whole number on the tile, and the mass number is the atomic mass rounded to the nearest whole number. Carbon has atomic mass 12.011 and atomic number 6, so the common isotope has 12 − 6 = 6 neutrons. This works for the most common isotope when the atomic mass is close to a whole number. For elements whose mass is an average of several isotopes, such as chlorine (35.45), rounding gives 35 and 18 neutrons for chlorine-35, but chlorine-37 has 20, so a specific isotope's mass number is needed. For radioactive elements with no stable isotope, the mass in brackets, like [98] for technetium, is the mass number of the longest-lived isotope, so 98 − 43 = 55 neutrons. In a neutral atom the number of protons and electrons both equal the atomic number. Neutrons are not printed on the table, so you work them out. Take sodium: atomic number 11, atomic mass 22.990 and mass number 23, so 23 − 11 = 12 neutrons. Take iron: atomic number 26 and atomic mass 55.845, so about 56 − 26 = 30 neutrons in the most common isotope, iron-56. Take gold, atomic number 79 and atomic mass 196.967, which gives 197 − 79 = 118 neutrons, and gold has a single stable isotope, so this is exact. Isotopes of the same element have the same atomic number but different neutron numbers, such as hydrogen with 0, 1 or 2 neutrons, so the answer depends on which isotope you mean. An isotope name or symbol with the mass number, such as uranium-235, lets you work out the neutrons directly: 235 − 92 = 143. Atoms with too many or too few neutrons for their protons can be radioactive. That is how you determine the neutron number of an isotope from the table.