Volcanic eruptions release magma, which solidifies to form igneous rocks.  Gases within the magma, such as argon, can readily escape before the magma solidifies.  However, gases formed after the magma solidifies are trapped within the newly formed igneous rocks.  For this reason, the ages of igneous rocks are often determined by potassium-argon dating.Potassium-40 (40K) is a radioactive isotope that decays through the two modes outlined in Figure 1.  In approximately 90% of cases, it undergoes β− decay to become calcium-40 (40Ca).  In the remaining 10% of cases, 40K undergoes electron capture to become argon-40 (40Ar).  Collectively, these two processes have a half-life of approximately 1.25 billion years.  After 1.25 billion years, 50% of the original amount of 40K remains unchanged, 45% has become 40Ca, and 5% has become 40Ar.Figure 1  Schematic of the two modes of 40K decay with potassium in the form of a K+ ionBased on the known decay rate of 40K, the time that has elapsed since an igneous rock solidified can be determined by measuring the ratio of 40K to its daughter nuclides.  However, 40Ca is the most abundant calcium isotope, and igneous rocks commonly contain 40Ca from sources other than the decay of 40K.  Consequently, geologists typically only consider the 40K : 40Ar ratio when determining the age of an igneous rock. Question 58Suppose a 40K+ ion undergoes β− decay.  Which of the following changes to the electron configuration of the resulting isotopes is most likely to occur after the decay?A.It will quickly lose an electron from the 4s shell to the environment to restore a full octet in the valence shell.B.An electron from the environment will quickly enter the 4s shell to restore a full octet in the valence shell.C.It will quickly lose two electrons from the 4s shell to the environment to restore a full octet in the valence shell.D.The electron configuration will not change because the valence shell contains a full octet both before and immediately after the decay.