The Nucleus

Many fields of chemistry look at the interactions between atoms. In chemical reactions, bonds between atoms break, and new bonds form. Chemical reactions do not involve changes in the atomic nucleus. Nuclear chemistry is the field of chemistry that studies changes in atomic nuclei. The nucleus can gain or lose particles, or particles in the nucleus can change into other particles. Because elements are defined by the number of protons in the nucleus, which is the atomic number, any change in atomic number changes the element.

The nucleus of an atom is made up of positively charged protons and neutral neutrons. The protons and neutrons have comparable masses. Both protons and neutrons have a much greater mass than electrons. A nucleon is a proton or a neutron in an atomic nucleus.

Two fundamental forces of the universe play major roles in the nucleus. The strong nuclear force is the short-range force that acts between protons and neutrons, keeping the nucleus together. Its range is so short that it acts only between nucleons that are close together. The second fundamental force that acts on nucleons is the electromagnetic force, a repulsion between positively charged protons. This repulsion is weak compared to the strong nuclear force. However, the electromagnetic force can act at much greater distances. Each proton in the nucleus repels all other protons, not just the neighboring ones.

In nuclear chemistry, the number of neutrons in a nucleus is important. A nuclide is an atomic nucleus with a specific number of protons and neutrons. Nuclides are represented in the form of ${}_Z^A\rm {X}$ where A is the atomic mass, Z is the proton number (atomic number), and X is the element symbol. This representation allows for calculation of the number of neutrons, N, through the formula $N+Z=A$. For example, nitrogen-14 is represented by ${}_{\,\,7}^{14}\rm {N}$ and nitrogen-15 is represented by ${}_{\,\,7}^{15}\rm {N}$. The term isotope is often used interchangeably with the term nuclide. Isotopes are nuclei with the same number of protons but different numbers of neutrons.

Neutrons are stable in a nucleus. Outside of a nucleus, they are not stable and eventually decay into a proton, releasing an electron. The decay of a neutron into a proton and an electron can be written as an equation.

The difference between the total mass of the individual nucleons that make up a nucleus and the actual mass of the nucleus is called mass defect. Mass defect occurs because nucleons in a nucleus are more stable and have lower potential energy than nucleons outside of a nucleus. Consider a helium atom with two protons and two neutrons. Each proton has a mass of 1.007276 amu, so the total proton mass is 2.014552 amu. Each neutron has a mass of 1.008665 amu, so the total neutron mass is 2.017330. Altogether the mass of the helium atom is 4.031882 amu. This is different from the experimentally determined mass of the helium nucleus, which is 4.00151 amu. The mass difference is $4.031882-4.00151=0.03037\;{\rm{amu}}$. This difference in mass is not unique to helium. The total mass of the individual nucleons that make any nucleus is greater than the mass of the nucleus itself. The energy that binds nucleons manifests as a difference in mass. Energy and mass are directly proportional; when energy decreases, so does mass. It requires energy to break a nucleus apart. The energy required to break a nucleus into its component nucleons is the nuclear binding energy.

Mass and energy are related to each other. The equation that relates energy and mass, known as the mass-energy equivalence equation, was formulated by the German physicist Albert Einstein. It defines the relationship between energy E in joules (J), mass m in kilograms (kg), and the speed of light c in meters per second (m/s).
The nuclear binding energy and mass-energy equivalence can be used to calculate the nuclear binding energy. The sum of mass and energy is always conserved. For example, the nuclear binding energy for helium can be calculated using the mass difference. Mass was determined to 0.03038 amu by solving for the mass difference. Mass is converted to kg using dimensional analysis. The speed of light c is $2.9979\times{10}^8\;{\rm{m/s}}$, which is a constant. Using these values, energy in joules can be determined.

The process by which unstable nuclei break down into other, smaller nuclei over time, releasing particles and/or energy, is radioactivity, or radioactive decay. An isotope with an unstable nucleus that experiences radioactive decay is called a radioisotope. There are different types of radioactive decay, depending on the properties of the nuclei undergoing it.

Radioactive decay often involves antiparticles. An antiparticle is a particle with the same mass as an elementary particle, but with the opposite charge. The antiparticle of an electron (${}_{-1}^{\;\;\,0}{\rm{e}}$) is a positron (${{}_{+1}^{\;\;\,0}{\rm{e}}}$), which has the same mass as an electron and a positive charge equal in magnitude to the negative charge of an electron. The antiparticle of a proton is an antiproton. An antiproton has the same mass as a proton and has an equivalent-magnitude, but negative, charge. Matter consisting of antiparticles such as antiprotons, antineutrons, and positrons is called antimatter. Antimatter, and antiparticles, annihilate when they interact with matter and elementary particles. The energy released in such an annihilation can be calculated by mass-energy equivalence, $E=mc^2$.

Nuclei above the zone of stability have an overabundance of neutrons relative to the number of protons. In such a nucleus, one of the neutrons is likely to decay into a proton. A high-energy electron is released when a neutron decays into a proton in the nucleus, called the beta particle (${{}_{-1}^{\;\;\,0}\beta}$). This type of radioactivity is called beta decay. Carbon-14 decays into nitrogen through beta decay.
Note that this decay increases the proton number, or atomic number, of the nucleus, thus changing its identity from carbon to nitrogen. Beta decay reduces the neutron number and increases the proton number, making a nucleus approach the band of stability. In other words, emission of a beta particle makes an unstable nucleus become more stable. Nuclei below the band of stability have an overabundance of protons, and too few neutrons. In such a nucleus, a proton is likely to change into a neutron. In this case, an antiparticle, a positron (${}_{+1}^{\;\;\,0}{\rm{e}}$) is emitted. For example, neon-19 decays into fluorine by emitting a positron. Positron emission reduces the neutron number and increases the proton number, making a nucleus approach the zone of stability. Electron capture is another process that converts a proton in the nucleus to a neutron. In this nuclear change, the nucleus captures an electron (${}_{-1}^{\;\;\,0}{\rm{e}}$). For example, potassium-40 nuclei can capture an electron to become argon. Electron capture is the reverse process of beta decay. In electron capture, as in positron emission, the proton number increases and a nucleus approaches the zone of stability. A large nucleus may undergo alpha decay to lose nucleons. During alpha decay, an alpha particle is lost from the nucleus. An alpha particle ($\alpha$) is a particle identical to a helium ion (He²⁺) that is emitted during the decay of radioactive elements. It is made up of two protons and two neutrons. An alpha particle is represented in a nuclear equation by the symbol ${}_2^4{\rm{He}}$. Alpha particles do not contain any electrons. Uranium-238, for example, decays into thorium-234 through alpha decay. An alpha particle decreases both the proton and the neutron numbers. A nucleus that is above the zone of stability typically undergoes alpha decay to approach the zone of stability.

A gamma ray, or gamma radiation, is high-energy electromagnetic radiation. Gamma rays are energy emitted by the nucleus as it becomes more stable through radioactivity. Gamma rays are symbolized by the lowercase Greek letter gamma: $\gamma$ or ${}_0^0\rm\gamma$. Gamma rays do not change the proton or neutron numbers and are commonly not written in nuclear reactions.