Video Transcript
The ratio between neutrons and protons in an atom of vanadium-51, 51 23 V, is approximately 1.22 to one. As the number of protons in an atom increases, how does the ratio between neutrons and protons change? (A) The ratio increases. (B) The ratio increases until it reaches that in uranium and then decreases. (C) The ratio increases until it reaches that in uranium and then flattens out. (D) The ratio decreases. Or (E) the ratio decreases until it reaches that in uranium and then increases.
The stability of atoms depends on the ratio of protons to neutrons. We can see how this ratio changes by observing the belt of stability. But let’s first clear some space. For light elements, they are stable when the number of protons is approximately equal to the number of neutrons. In other words, very light elements with a proton number of around 20 or less are stable with a one-to-one neutron-to-proton number ratio.
For medium mass elements, we see that they are stable when the number of neutrons is a little greater than the number of protons. For example, vanadium-51 is stable when it has 28 neutrons and 23 protons with a neutron-to-proton ratio of approximately 1.22 to one. For heavier elements, we see that the ratio increases even more. Consider a very heavy nucleus, such as lead-208, for example, which is the heaviest stable nucleus. That is, it is the heaviest nucleus that does not undergo spontaneous radioactive decay.
Lead-208 contains 82 protons and 126 neutrons. This means that the ratio of neutrons to protons in lead-208 is around 1.53. In the belt of stability, we see that the ratio of neutrons to protons steadily increases over the course of the periodic table. Even after uranium, it continues to increase. Let’s now bring back the original answer choices. So the answer to the question “As the number of protons in an atom increases, how does the ratio between neutrons and protons change?” is (A): the ratio increases.
