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.
Masses of Subatomic Particles
|Particle||Symbol||Mass (amu, atomic mass unit)|
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 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 . For example, nitrogen-14 is represented by and nitrogen-15 is represented by . 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.
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 . 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).
Stability of Nuclei
Two opposing forces act within an atomic nucleus. The strong nuclear force is a attractive, short-range force that acts between all nucleons—the charged protons as well as the neutral neutrons. The electromagnetic force is a repulsive force that acts between positively charged protons. Adding protons to a small nucleus increases both the attractive strong nuclear force and the repulsive electromagnetic force. Adding neutrons to a small nucleus increases only the attractive strong nuclear force.
The short range of the strong nuclear force means nucleons can form a limited number of strong nuclear attractions. As the nucleon count increases, the strong nuclear force increases linearly. Electromagnetic force is not limited by range. Every proton added will repulse every other proton. As proton count increases, electromagnetic force increases exponentially. Because of this difference, nuclei above 270 nucleons are very unstable. The largest nuclei that has been observed has 294 nucleons.
The ratio N:Z (where N is the neutron number and Z is the number of protons or atomic number) is significant with respect to nuclear stability. A graph of neutron number versus proton number for all known isotopes shows certain patterns:
- For small atoms, with a proton number up to 20, the N:Z ratio of the most stable isotope is about 1:1. As the nucleus gets larger, the number of neutrons in the most stable isotope increases faster than the number of protons. The most stable isotope of gold, for example is , giving a N:Z ratio of 1.49.
- The zone of stability, or band of stability, is the region that represents stable, nonradioactive isotopes on a graph of the neutron number versus the proton number for all known isotopes.
- Nuclei above the zone of stability are rich in neutrons and are unstable.
- Nuclei below the zone of stability are rich in protons and are unstable.
Zone of Stability
These patterns can be partially explained by the shell model of the nucleus, a model that defines the locations of protons and neutrons in shells that are partially analogous to electron shells. According to the shell model, pairs of neutrons or pairs of protons represent a more stable arrangement, similarly to what is seen with pairs of electrons.
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 () is a positron (), 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, .
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: or . Gamma rays do not change the proton or neutron numbers and are commonly not written in nuclear reactions.