Atoms
“Nothing exists except atoms and empty space. Everything else is opinion.” ~ Democritus
All matter is made of atoms. An atom is a conglomeration of a nuclear core (nucleus) covered by a swirling cloud of electrons at a distant remove.
With its single proton and solitary electron, monatomic hydrogen (hydrogen-1) is the progenitor of all other matter. Unsurprisingly, hydrogen is by far the most abundant chemical element, constituting ~75% of material mass in the universe.
That atoms are spherical is a crude approximation. Only the simplest atoms come close to being round. Heavier atoms have more complex shapes.
There are 92 distinct configurations of atoms (species). Men with ungodly machines have created new heavy elements which rapidly decay for lack of natural stability – a gamey alchemy at the atomic level.
The maximum size of an atomic nucleus is determined by its tendency to decay. Obese nuclei decay by shucking off excess neutrons – particulate radioactivity.
“There is no evil in the atom; only in men’s souls.” ~ American politician Adlai Stevenson II
Protons
The proton is such a complicated system. ~ German physicists Jan Bernauer and Randolf Pohl
Every atom has a nucleus that carries a positive charge by virtue of possessing at least 1 proton. The atomic number of an atom, its nuclide, refers to the number of protons in the nucleus.
◊ ◊ ◊
Like the base of a pyramid, the physics of protons provide the foundation of what is understood about matter. It is a tenuous base. Researchers cannot agree about the radius of the proton; somewhere around 0.88 femtometers, ±4%. 4% sounds small, but the discrepancy cannot ostensibly be explained by either experimental method or error. It may be that the uncertainty principle is exercising itself in a grandiose way, in that how big protons are depends upon how they are observed. But that is less uncertain than it is unlikely.
The situation is a bit tight inside a proton. Pressure in the center of a proton is 10 times greater than in the heart of a neutron star, which is as packed as atomic matter may be.
The quark-trio picture by which protons are painted is simplistic. In addition to these ever-present constituents, a swarm of transient particles churn within a proton. Meantime, gluons – the bosonic glue that holds protons together – ceaselessly careen between quarks.
The upshot of this bustle is that the properties of protons, and their neutral cousins, neutrons, are hard to get a handle on. Spin exemplifies the problem. Physicists have studied subatomic spin for decades, but it’s still not sorted out.
Like the Earth rotating on its axis, quantum particles act as if they are whirling at blistering speed. Because a rotating charge creates a magnetic field, this spin makes protons behave like tiny magnets. This property is key to the medical imaging procedure called magnetic resonance imaging, popularly known by its acronym: MRI.
But there’s no actual spin going on. Because fundamental particles like quarks don’t have a finite physical size, they can’t twirl. They just give the appearance that they do, yielding a proton spin of 1/2.
In 1987, physicists discovered that only a small fraction of the spin owed to the quarks inside. They then suspected gluons as being spin-meisters. No such luck.
The current tally is that gluons are responsible for only ~35% a proton’s spin. Quarks make up ~25%, leaving 40% unaccounted for.
We have absolutely no idea how the entire spin is made up. We maybe have understood a small fraction of it. ~ American nuclear physicist Elke-Caroline Aschenauer
Experimental physicists get little help from their theoretical counterparts when trying to unravel the proton’s perplexities. Quantum chromodynamics – the theory of the strong force transmitted by gluons – is a mathematical marvel of such complexity that its equations cannot be solved. Instead, theorists rely upon an approximate technique that roughly quantizes the quantum actors and their actions. Results only coarsely correspond with experimental measurements, which indicates that rough is not good enough.
The proton is not something you can calculate from first principles. ~ Elke-Caroline Aschenauer
◊ ◊ ◊
We have a lot of circumstantial evidence that something like unification must be happening. ~ Indian nuclear physicist Kaladi Babu
Protons seemingly live forever: a fact that physicists are reluctant to accept. The rub is that all of the various physical models which unify electromagnetism with the nuclear forces demand that protons give up the ghost some time.
Experiments show that a proton has a life expectancy of at least 1.6 x 1034 years. That may understate the situation, as proton decay has never been seen.
The proton is the most fundamental building block of everything, and until we understand that, we can’t say we understand anything else. ~ Scottish nuclear physicist Evangeline Downie
◊ ◊ ◊
Hydrogen-1 is the only instance of an atom comprising only a proton and an electron, as protons will not bind with each other: their electromagnetic repulsion is stronger than their nuclear strong force attraction.
Enter neutrons, the strong-force glue in atoms, holding protons together in a nucleus. The number of neutrons in an atom determines the isotope of an element.
Besides having selfsame spin, protons and neutrons have equivalent girth: 10–14 meters; and about the same mass: 1.7 x 10–24 grams.
(A 2010 measurement of the proton, using a muon racing around it as a metric, put the proton 4% smaller than previously found. The measurement was supposedly more accurate than those used before. The different result put quantum electrodynamics (QED), which describes how light and matter interact, in deep trouble, as it predicts a fatter proton. At of 2018, whether QED is off-base, or using a muon somehow mucked the result, or the experiments themselves were faulty, remains unknown.)
Neutrons
Experimenters have shown that, to very high precision, the neutron has no electric dipole moment. ~ American physicist Adrian Cho
Neutrons bound into an atomic nucleus are homey hadrons, but when set free they quickly become unstable, radiating away, undergoing beta decay with an average lifetime of less than 15 minutes. A decaying neutron turns into a proton, letting fly an electron (the beta particle) and a ghostlike antineutrino.
Decay rate can be affected by magnetic fields, a phenomenon without explanation in known physics. Extra-dimensional dynamics explain this oddity: symmetry via ed mirror particles.
A neutron is made up of 2 charged quarks: 2 down quarks, each with a negative charge 1/3rd that of an electron, and an up quark that carries a 2/3rds positive charge. The arrangement leaves a neutron electrically neutral. Hence the particle’s name.
Nuclear Clusters
“The internal structure of a nucleon – a proton or a neutron – depends on its environment. The structure of a nucleon in empty space is different from its structure when it is embedded inside an atomic nucleus. There is a variation in the momentum distribution of quarks inside the nucleons embedded in nuclei. Specific nucelons are altered by interacting in pairs over brief time periods. Similar nucleons are less likely to pair up than are dissimilar nucleons.” ~ American nuclear physicist Gerald Feldman
The textbook view of an atomic nucleus is that of a largely homogenous collection of protons and neutrons as a spherical dollop of nuclear matter. But nucleons are predisposed to cluster into certain configurations according to relative stabilities.
Neutrons bring the bosonic strong force to atoms, binding with protons that are otherwise ill-disposed to being in close quarters with other protons. The bosonic character of a nucleus, based upon quantum spin, determines cluster stability. A helium-4 nucleus, with 2 protons and 2 neutrons, is exceedingly stable. This owes to its clustering quality.
Such nucleon quartets – with even and equal number of protons and neutrons – make the most stable atomic nuclei. This includes beryllium-8, carbon-12, oxygen-16 and neon-20.
Nuclear clustering has cosmological significance. Supposedly, within a few minutes after the Big Bang, a soup of free protons and neutrons had formed in a 6 to 1 ratio.
When the cosmos had cooled enough to permit nuclear binding, almost all the earliest nuclei were helium-4. Hence, the early universe had a hydrogen/helium ratio of 3 to 1, like today, with nearly all the neutrons in the universe trapped in helium-4.
Carbon-12 is formed by triple fusion of helium-4; a process termed triple-alpha. A star feeds itself during its red-giant phase through the triple-α process, fusing helium into carbon.
Without nuclear clustering, there would be little carbon in the universe, and no organic life.
Magic Nuclei
Nucleons are always antsy: racing about each other in an intricate orbital dance orchestrated by the strong force. They fill orbital slots in a sequential manner according to energy levels.
Spin-orbit interaction is the interplay between a proton’s orbital momentum and its angular momentum (spin). It is critical in keeping nuclei steadfast. Radioactive beta decay is a product of instability in spin-orbit interaction.
Most atomic nuclei have a fairly constant density, regardless of the number of nucleons they contain. But some exhibit orbital oddities.
Silicon-34, a radioactive isotope with a life expectancy of less than 3 seconds, is one such heretic. It has a largely empty orbital, creating a bubble in its nucleus.
The orbital shells of protons and neutrons are independent of each other. The number of nucleons (either protons or neutrons) forming a complete nuclear shell, and so promoting maximal stability, is its magic number. Silicon-34 is doubly magic, in that both its protons and neutrons are at a magic level.
A magic number means that the energy needed to boost a nucleon into the next orbital is particularly high. This explains a nuclear bubble’s origin.
“‘Doubly magic’ nuclei have fully occupied shells of protons and neutrons. Such nuclei are therefore more strongly bound together and more difficult to excite than their neighbors.” ~ American nuclear physicists Gaute Hagen & Thomas Papenbrock
For a proton in silicon-34 to jump into the unfilled central orbital, it needs an energetic boost. Hence, its center proton shell remains sparsely populated.
○○○
Eternal Element
Named after a villain in Greek mythology, tantalum is an uncommon heavy metal found in Earth’s crust. By weight, 1.5 parts per million of Earth’s crust is tantalum. Most tantalum is in the form tantalum-181. But 0.01% is in the isomer form of tantalum-180m.
An isomer is a molecule with the same molecular formula as another, but with a different chemical structure. Tantalum-180m is an isomer with a naturally agitated nucleus.
Normally, excited nuclei quickly calm down, dropping to a lower energy state and emitting a photon in the process. Somehow tantalum-180m is stuck in its frenzied state.
After extensive observation, physicists decided that tantalum-180m could not have a half-life shorter than 45 million billion years. No other known element has anywhere near such a determined buzz.
(Half-life is the duration required for a material to decay to half of its initial mass. The term is commonly used in nuclear physics to state the radioactive decay rate of atoms. Because of the uncertainty principle, it is impossible to predict when radioactive decay may occur. So, decay rate is expressed in terms of probability, represented as a half-life.)
The oddest thing is that tantalum-180m even exists. The element-forging processes that transpire in stars and supernovas seem to bypass this nuclide of 73 protons.
“We don’t understand how it is created.” ~ American nuclear physicist Eric Norman
○○○
The fusion of lighter elements into heavier ones is termed nucleosynthesis. Heavy elements are created via nucleosynthesis during supernova explosions. But there have been too few exploding stars to account for all the corpulent chemicals that exist.
Some forms of nucleosynthesis, such as the fusion of hydrogen into helium, provide the energy that keeps a star from collapsing and generate its luminosity. Others do not, such as the transformation of gold into mercury by adding a neutron. Some elements, once fused, remain locked in the dead cores of stars and are not released into the surrounding galaxy. ~ Jennifer Johnson
Supernovas enrich the universe with matter in 3 ways. 1st, they eject the products of nucleosynthesis built up over the star’s lifetime. Most oxygen, carbon, and magnesium are made during star time. The explosion simply hurls these elements into space.
2nd, the extreme densities and temperatures caused by the shock wave of a supernova drives additional nucleosynthesis. The iron ejected by core-collapse supernovae does not come from the core but from explosive fusion of material in the silicon shell during the supernova.
3rd, the explosive shock of ejected material plowing into ambient gas generates cosmic rays by accelerating some particles to near light speed. Cosmic rays are energetic enough to break apart heavier nuclei, producing lighter elements through fission. Cosmic ray disassembly is responsible for large fractions of the lithium, beryllium, and boron made. Cosmic ray fission also renders elements such as carbon and oxygen, though the abundance of those elements is dominated by other modes of production.
Black holes occasionally collide with neutron stars. The black hole chews on the star and spits out neutron-rich material which forms heavy elements once swept up in another star.
Periodic Table of Elements
In the late 5th century bce, Empedocles originated the cosmogenic theory of there being 4 basic substances: earth, water, wind, and fire. He also proposed that love and strife could mix and separate these substances, which Plato termed elements. (Ancient traditions throughout the world had also found these 4 basic elements.) Aristotle furthered Empedocles’ elementalism with the idea that all matter was made from a mixture of 1 or more elements (which he called roots).
The periodic table is intertwined with the quest to discover new elements. The 1st element discovered since ancient times was furtively made by German merchant and alchemist Hennig Brand, who in 1669 distilled human urine to reveal a glowing white substance which he named phosphorous. Brand’s reeking distillation was part of his continuing quest to find the philosopher’s stone.
Brand kept his discovery of phosphorus secret until its published rediscovery by Anglo Irish chemist Robert Boyle in 1680. That same year, Boyle put a phosphorous paste with a sulfur tip on wooden sticks and invented the forerunner of modern matches. (Phosphorus’ fiery reactivity earned it the distinction of being “the Devil’s element.”)
Boyle defined a chemical element as “a substance that cannot be broken down into a simpler substance by a chemical reaction.” This definition served for 3 centuries, until subatomic particles were discovered, whereupon the definition shifted to characterizing an element by proton count.
Lavoisier’s 1789 Elementary Treatise of Chemistry, the first modern chemistry textbook, listed the “simple substances” that Plato and Boyle had termed elements, bifurcating them into metals and nonmetals. Lavoisier also listed light and caloric as elements, which were believed at the time to be material substances.
German chemist Johann Wolfgang Döbereiner began classifying elements into groups by atomic weight in 1817. In arranging the chemical elements by atomic weight, French geologist and mineralogist Alexandre-Emile Béguyer de Chancourtois first noticed their periodicity. de Chancourtois failed to publish the irregularly arranged table that he had constructed. It did not matter. His 1862 article was ignored by chemists, having been written in terms of geology.
Based upon accumulated understanding of chemical properties, Russian chemist Dmitry Mendeleyev published the modern form of the periodic table in 1869, having noticed the regularities that form the basis of the table, and having the foresight to complete the table by predicting the characteristics of elements not yet discovered.
The periodic table is a tabular arrangement of the 118 chemical elements (94 of which occur naturally on Earth), organized on the basis of their atomic numbers, electron configurations, and recurring chemical properties. Elements are ordered by atomic number, which is a product of protonic girth.
The table is typically a grid of elements, with rows called periods and columns called groups. Element groups have the same number of electrons in their outer shell. A new row (period) begins when an element sprouts a new electron shell.
The distinction between metals and nonmetals is one of the most fundamental in chemistry. Good conductors of heat and electricity, most elements are metal. They may be pulled into wires because they are ductile; hammered into sheets because they are malleable; and most are lustrous (shiny).
(Hydrogen (H) is the only nonmetal on the left side of the table (at least at ambient temperature and pressure). The chemical properties of elements 109–111 and 113–118 are unknown.)
Toward the right of the table in the figure above, metalloids (semimetals) are shown darkly shaded. To the right of the semimetals are nonmetals. The lightly shaded, far-right group (18) comprise the noble (inert) gases. (The term noble refers to low reactivity. It was first used in 1898 by German chemist Hugo Erdmann, as an analogy to “noble metals,” which also have hesitant reactivity.)
