Gchem Lecture 5: Nuclear Structure

Protons and neutrons in a nucleus are held together by the strong nuclear force. It's the strongest of the four fundamental forces because it must overcome the electrical repulsion between the protons.

Unstable nuclei are said to be radioactive, and they undergo a transformation to make them more stable--they do this by altering the number and ratio of protons and neutrons. This is called radioactive decay. There's 3 types: alpha, beta, and gamma. The nucleus that undergoes radioactive decay is called the parent, and the resulting (more stable nucleus) is called the daughter.

Alpha: When a large nucleus wants to become more stable by reducing the number of protons and electrons it emits an alpha particle--it contains 2 protons and 2 neutrons. This reduces the parent's atomic number by 2 and the mass number by 4.

Beta: there are 3 types: Beta (-), Beta (+), and electron capture. Each type involves the transmutation of a neutron into a proton (and vice versa) through the action of the weak nuclear force; beta particles are less massive than alpha particles and therefore less dangerous

Beta (-): Unstable nucleus contains too many neutrons--> it converts a neutron into a proton and an electron (Beta (-) particle that is ejected; the resulting atomic number is increased by 1 but the mass number remains the same. This is the most common type of beta decay so when the MCAT mentions it, it means this.

Beta (+): Unstable nucleus contains too few neutrons--> it converts a proton into a neutron and a positron (ejected). The positron is like an electron, only positive. The resulting atomic mass is 1 less than the parent but the mass number remains the same.

Electron Capture: unstable nucleus capture an electron from the closest electron shell (n=1) and uses it to convert a proton into a neutron--> causes the atomic number to be reduced by 1 while the mass number remains the same

Gamma Decay: is simply an expulsion of energy; a nucleus in an excited energy state (which is usually the case after a nucleus has undergone alpha or any type of beta decay) can "relax" to its ground state by emitting energy in the form of one or more photons. These photons are called gamma photons. They have neither mass nor charge. Their ejection from a radioactive atom changes neither the atomic number nor the mass number of the nucleus (i.e. does not change the identity of the nucleus like alpha or beta decay).

Quick note on nuclear binding energy: every nucleus that contains protons and neutrons has this. It is the energy that was released when the individuals nucleons were bound together by the strong force to form the nucleus. It's also equal to the energy that would be required to break up the intact nucleus into its individual nucleons. In short, the greater the binding energy per nucleon, the more stable the nucleus.

Mass defect: when nucleons bind together to form a nucleus, some mass is converted to energy, so the mass of the combined nucleus is less than the sum of the masses of all its nucleons individually. The difference, deltaM, is the mass defect and will always be positive.
DeltaM=total mass of separate nucleons - mass of nucleus