Atomic physics
Atomic physics is the scientific discipline focused on understanding the structure and interactions of atoms, which are the fundamental building blocks of matter. An atom comprises a nucleus, containing positively charged protons and neutral neutrons, surrounded by negatively charged electrons. The concept of atoms dates back to ancient Greek philosophy, with significant advancements in understanding occurring in the 19th century, notably through the work of scientists like John Dalton and J.J. Thomson.
Atomic physics distinguishes itself from nuclear physics, which specifically studies atomic nuclei. The number of protons in an atom, referred to as its atomic number, defines its elemental identity, influencing its chemical properties. Atoms can become ionized by gaining or losing electrons, which significantly alters their reactivity. Isotopes are variations of elements that contain the same number of protons but differ in neutron count, affecting their atomic mass and stability. Understanding atomic physics is crucial for applications such as nuclear energy and medical technologies, reflecting its importance across various scientific fields.
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Atomic physics
Atomic physics is the field of scientific study concerning the structure of atoms and the interactions and properties of the particles that make up atoms. An atom is the fundamental building block of all matter and consists of a nucleus surrounded by a system of particles that give each atom its specific characteristics. The existence of atoms was first theorized by the ancient Greeks, yet atoms were not conclusively discovered until the nineteenth century. Scientists consider the field of atomic physics to refer to the system of the atom as a whole. They differentiate it from nuclear physics, which refers solely to properties of the atomic nucleus.
Background
Atoms are the smallest units of matter that make up chemical elements and still maintain the characteristics of that element. While they may vary in size, even the largest atoms are extremely small. They are measured in angstroms, a unit corresponding to about one-hundred millionth of a centimeter. The diameter of most atoms is about two to three angstroms. Each atom consists of a combination of three principle subatomic particles: protons, neutrons, and electrons. The nucleus, or center, of an atom is made up of protons, which have a positive electrical charge, and neutrons, which have no charge. The protons and neutrons are surrounded by a cloud of negatively charged electrons, which circle the nucleus like planets orbiting the sun.
The idea that matter was made up of extremely small particles was first suggested by the Greek philosopher Democritus in the fifth century BCE. Democritus argued that if a piece of matter was to be cut in half, and that half further divided, it would eventually reach a point where it could not become any smaller. He referred to these smallest possible pieces of matter as atoms, from the Greek term for "unable to be cut." His theories were rejected a century later by the Greek philosopher Aristotle, who argued that matter was composed of only five elements—earth, fire, water, air, and aether, a mysterious substance that made up the heavens.
In the early nineteenth century, the work of English physicist John Dalton confirmed that chemical elements were made of small particles called atoms. Dalton also found that the atoms of a given element were identical, but that atomic properties differed between various elements. In 1897, another English physicist, J.J. Thomson, discovered electrons, referring to them as negatively charged "plums" suspended in a "pudding" of positive matter. The atomic nucleus was discovered in 1911 by Ernest Rutherford, a physicist from New Zealand. Rutherford noticed that most charged particles shot at a piece of gold foil passed through unhindered, while a very small amount bounced back. This suggested that atoms were mostly made of empty space, but something dense existed at their cores. He was the first to discover that the nucleus contained positively charged protons.
Danish physicist Niels Bohr worked with Rutherford to refine the model of the atom and discovered that electrons circled the positively charged nucleus at high speeds. The atom was held together by electrical forces exerted by the positive and negative charged particles. While this model predicted the existence of neutrons, they were not discovered until 1932. Scientists built upon the early work in atomic physics to determine that atoms can be further broken down into their respective particles. The splitting of atomic nuclei, part of the field of nuclear physics, led to the creation of nuclear weapons in the 1940s. The study of nuclear decay, or the loss of radiation from the nucleus, was instrumental in creating nuclear power. Further branches of physics also determined that protons and neutrons are themselves made up of smaller elementary particles, such as quarks and gluons.
Overview
Despite being only a small fraction of the total size of that atom, the nucleus contains almost all the atom's mass. A proton and neutron are each about 1,800 times heavier than an electron. The number of protons in an atom determines the properties of the atom and defines what element it is. For example, hydrogen is the simplest element consisting of one proton. Oxygen atoms have eight protons, iron atoms have twenty-six protons, and uranium atoms have ninety-two. The number of protons in an atom is referred to as its atomic number.
In a stable atom, the number of protons is equal to the number of electrons. The positive charge of the proton cancels out the negative charge of the electron, and the atom is considered neutral with a very weak electrical charge. If an atom gains or loses an electron, it becomes electrically charged, or ionized. This can occur after contact with other atoms or high-energy charged particles such as radiation. An ionized atom, unlike a neutral atom, is strongly attracted to other atoms and can be very chemically reactive. An atom can also be ionized through the loss or gain of a proton, but that process requires a nuclear reaction.
The combined number of protons and neutrons in an atom is referred to as its atomic mass. Atoms of the same element that have a different number of neutrons are called isotopes. Isotopes have the same atomic number but different atomic masses. The basic form of hydrogen, for example, consists of one proton, one electron, and no neutrons. This hydrogen isotope is called protium. A hydrogen atom with one proton, one electron, and one neutron is called deuterium. An atom with one proton, one electron, and two neutrons creates the hydrogen isotope tritium. Protium and deuterium are stable elements, meaning that internal forces within their nuclei are strong enough to hold them together. Tritium is unstable and will lose neutrons in an attempt to become stable. The energy released in this manner makes the tritium isotope radioactive.
Bibliography
"Atomic Physics." LibreTexts, 20 Feb. 2022, phys.libretexts.org/Bookshelves/College‗Physics/College‗Physics‗1e‗(OpenStax)/30%3A‗Atomic‗Physics. Accessed 26 Nov. 2024.
"Basics of Nuclear Physics and Fission." Institute for Energy and Environmental Research, May 2012, ieer.org/resource/factsheets/basics-nuclear-physics-fission/. Accessed 26 Nov. 2024.
Budker, Dmitry, et al. Atomic Physics: An Exploration through Problems and Solutions. Oxford UP, 2008.
Dobrijevic, Daisy, and Tim Sharp. "What Is an Atom? Facts About the Building Blocks of the Universe." Live Science, 15 Dec. 2021, www.livescience.com/37206-atom-definition.html. Accessed 26 Nov. 2024.
"Introduction to Atomic Physics." AtomicArchive.com, www.atomicarchive.com/Physics/Physics1.shtml. Accessed 26 Nov. 2024.
Pullman, Bernard. The Atom in the History of Human Thought. Translated by Axel R. Reisinger, Oxford UP, 1998.
Sagan, Carl. Cosmos. Ballantine Books, 2013, pp. 171–206.
Waldek, Stefanie. "Alpha Particles and Alpha Radiation: Explained." Space.com, 13 May 2022, www.space.com/alpha-particles-alpha-radiation. Accessed 26 Nov. 2024.
Yang, Fujia, and Joseph H. Hamilton. Modern Atomic and Nuclear Physics. World Scientific, 2010.