We have said that seven protons will combine stably with seven or eight neutrons. What happens if seven protons are combined with six or nine neutrons (to make N¹³ or N¹⁶)? Our rule does not prevent them from sticking together; it says only that these combinations would be _more_ stable if a proton could be converted into a neutron (in the case of six) or a neutron into a proton (in the case of nine). Actually seven protons and nine neutrons _do_ stick together, but such a nucleus is not stable and does not continue to exist indefinitely. The reason is quite simple and a little surprising: The conversion of a neutron into a proton is actually a physically realizable process, and furthermore it releases some energy. Similarly a nucleus containing seven protons and six neutrons will have an existence of only finite duration because the conversion of a proton into a neutron can also occur. Of course the proton is charged and the neutron is not. What happens to the charge during these transformations? Actually the neutron is transformed, not into a proton, but into a proton plus an electron. The proton is transformed likewise into a neutron plus something else. This something else is called a positron and is identical with the electron in every respect except in having a positive instead of a negative charge. The changes just described occur spontaneously. They are examples of radioactivity. More specifically they are called “beta decay” processes because an electron (or a positron) when emitted by a nucleus is called a beta ray. Such beta-radioactive substances are produced whenever nuclear energy is used in an explosion or in a power plant. Many of the difficulties and worries concerning nuclear energy are connected with these beta activities. We shall be concerned with them often as harmful, sometimes as helpful agents. When a neutron is converted into a proton and an electron inside a nucleus, the electron escapes immediately, but the proton remains in the nucleus. Similarly, when a proton is converted into a neutron and a positron, the positron escapes and the neutron remains in the nucleus. Since the electron and the positron have a negligible weight compared to a proton or a neutron, the process of beta decay leaves the weight of the nucleus nearly unchanged. Since the electron and the positron are charged, the process of beta decay increases or decreases the charge of the nucleus by one unit. After beta decay a nitrogen nucleus with seven protons and six neutrons (N¹³) becomes a nucleus with six protons and seven neutrons—carbon with weight 13 (C¹³), which is a stable combination. Similarly a nitrogen nucleus with seven protons and nine neutrons (N¹⁶) becomes a nucleus with eight protons and eight neutrons, oxygen with weight 16 (O¹⁶), which is ordinary stable oxygen.