For the first 20 elements on the periodic table, atoms must have close to a ______ ratio of protons to neutrons in order to be stable.

For the first 20 elements on the periodic table, atoms must have close to a ______ ratio of protons to neutrons in order to be stable.

• A. 2:1
• B. 1:2
• C. 1:1
• D. 3:2

Answer: (C)

Stability of a nucleus comes from balancing the short-range attractive nuclear force with the repulsive electric force between protons. Neutrons add binding without adding charge, so the proton-to-neutron ratio is a key factor in whether a nucleus is stable. For the light elements up through about calcium, stable isotopes tend to have roughly equal numbers of protons and neutrons, so the ratio is near 1:1. This near-equality arises because the strong force binds nucleons effectively when there are enough neutrons to offset the increasing electrostatic repulsion as protons accumulate, but before you reach heavier elements, you don’t yet need a large excess of neutrons. Therefore, a proton-to-neutron ratio close to 1:1 best describes the stable nuclei among the first twenty elements.

Stability of a nucleus comes from balancing the short-range attractive nuclear force with the repulsive electric force between protons. Neutrons add binding without adding charge, so the proton-to-neutron ratio is a key factor in whether a nucleus is stable. For the light elements up through about calcium, stable isotopes tend to have roughly equal numbers of protons and neutrons, so the ratio is near 1:1. This near-equality arises because the strong force binds nucleons effectively when there are enough neutrons to offset the increasing electrostatic repulsion as protons accumulate, but before you reach heavier elements, you don’t yet need a large excess of neutrons. Therefore, a proton-to-neutron ratio close to 1:1 best describes the stable nuclei among the first twenty elements.