Question

How to tell the largest number of neutrons in an element?

a) Subtract the atomic number from the atomic mass.
b) The largest number of neutrons is always 2² .
c) Refer to the periodic table for neutron information.
d) Neutron count is irrelevant to element identification.

Answer 1

To determine the largest number of neutrons in an element, subtract the atomic number from the mass number of the element's most neutron-rich isotope as found in the periodic table.

Step-by-step explanation:

To find the largest number of neutrons in an element, one must subtract the atomic number (number of protons) from the mass number (the sum of protons and neutrons). This is because the atomic mass, which is found in the periodic table, is often the weighted average of all naturally occurring isotopes, and may include a decimal.

For example, the atomic mass of chlorine is 35.45, with most common isotopes having mass numbers of 35 (17 protons and 18 neutrons) and 37 (17 protons and 20 neutrons). Hence, for an element's most neutron-rich isotope, you would look for the isotope with the highest mass number and perform the subtraction: mass number - atomic number = number of neutrons.