100 Fun Facts About Atomic Structure

The word 'atom' comes from the ancient Greek 'atomos', which described matter as being what?

Uncuttable is what 'atomos' meant, and the name stuck even after 1897, when the discovery of the electron showed that atoms have smaller parts after all.

A proton is roughly how many times heavier than an electron, which is why the nucleus holds nearly all an atom's mass?

About 1,800: the measured ratio is 1,836 to one. Yet the two particles carry exactly equal and opposite charges, which is why one proton and one electron make a neutral hydrogen atom.

Which radioactive substance with a 12-year half-life makes exit signs and watch hands glow with no power source?

Tritium is hydrogen with two neutrons. Sealed in glass tubes, it fires off beta particles that strike a phosphor coating, which glows for years without batteries or sunlight.

On average, how long does a neutron last outside an atomic nucleus before it decays into a proton?

About 15 minutes: a free neutron is slightly heavier than a proton, so it can decay into one, shedding an electron and an antineutrino. Locked inside a stable nucleus, it can last indefinitely.

The name for the atom's dense centre, adopted by Rutherford in 1912, is a Latin diminutive of the word for what?

Nut is nux in Latin, and 'nucleus' means little nut, or kernel. Biology got there first: botanist Robert Brown named the cell's nucleus in 1831, some eighty years before Rutherford's atom had one.

The Aufbau principle, by which electrons fill the lowest-energy subshells first, takes its name from German for what?

Building up is what 'Aufbau' means, but atoms don't always obey: chromium and copper each shift one electron out of turn, because a half-filled or completely filled subshell is more stable.

J. J. Thomson's Nobel Prize showed the electron is a particle; his son George's Nobel showed it is also what?

A wave is what George Paget Thomson saw electrons behave as, diffracting through crystals, for which he shared the 1937 Nobel with Clinton Davisson.

Which English chemist's 'A New System of Chemical Philosophy' held that all atoms of one element are identical?

John Dalton was colour-blind and gave the condition its first scientific description, which is why several languages still call it Daltonism.

A hydrogen atom is roughly how many times wider than the lone proton at its centre?

60,000 times: the proton is under two femtometres across while the atom spans about 106 picometres, yet that speck of a nucleus holds more than 99.9% of the atom's mass.

An atom that loses electrons becomes which kind of ion, its name taken from the Greek for 'down'?

A cation is an atom left with a positive charge after shedding electrons. In table salt, sodium cations cling to chloride anions, the opposite kind, whose name comes from the Greek for 'up'.

In the isotope label cobalt-60, what does the number 60 count?

Protons plus neutrons: every cobalt atom has 27 protons, so cobalt-60 carries 33 neutrons, one more than cobalt-59, the element's only stable form, and that is enough to make it radioactive.

Which isotope, discovered by Harold Urey in 1931, gives heavy water its extra weight?

Deuterium is hydrogen with a neutron added. Urey's team found it by distilling liquid hydrogen until only the last drops were left, rich enough in the heavier isotope to show in the spectrum.

Unlike the proton, the electron has no known inner parts, so physicists class it as what kind of particle?

Elementary means no known inner parts. The proton lost that status in 1968, when electrons fired at it in a Stanford accelerator revealed hard points inside, later identified as quarks.

Which type of subshell adds the ten electrons that lift an electron shell's limit from 8 to 18?

The d subshell has five orbitals, each holding two electrons. It first appears in the third shell but starts to fill only after the fourth shell's s subshell, so the periodic table's third row stops at eight elements.

Whose equation of 1926 replaced Bohr's electron orbits with the probability clouds now called orbitals?

Erwin Schrödinger disliked the way others read his equation as mere probabilities, and in 1935 he invented a cat that is at once alive and dead to show how absurd he found the idea.

The word 'isotope', coined in 1913, is built from two Greek roots that together mean what?

Same place is 'isos' plus 'topos': isotopes of an element share one square of the periodic table despite their different masses.

The electron belongs to which family of particles, ones that never feel the strong force binding the nucleus?

Leptons take their name from the Greek for 'small', coined when only light ones were known. The tau, found in the 1970s, spoiled it: this lepton weighs nearly twice as much as a proton.

Which metal, a main ingredient of solder, has ten stable isotopes, more than any other element?

Tin holds the record because its 50 protons complete a shell inside the nucleus, an arrangement so sturdy that ten different neutron counts can sit alongside it without the nucleus breaking down.

What name did J. J. Thomson give the particles he found in cathode rays in 1897, before 'electron' won out?

Corpuscles came out the same whichever kind of atom they were torn from, so Thomson argued that they were a component of every atom, the first sign that atoms have parts.

The orbital letters s, p, d and f came from old spectroscopy jargon; what does the 's' stand for?

Sharp described how one series of spectral lines looked, a label coined before anyone had pictured an orbital. That s orbitals turned out to be spheres is a coincidence the letter never meant.

Borrowed from X-ray notation, the label for the electron shell nearest the nucleus is which letter?

K is followed outward by L, M, N, O, P and Q; Charles Barkla, whose X-ray scattering work supplied the letters, had originally called his two X-ray types 'A' and 'B'.

Which quantum number, written n, fixes an electron's shell and therefore most of its energy?

The principal quantum number sets the energy step an electron stands on: when one drops from n = 3 to n = 2 in hydrogen, the atom gives off the deep-red light astronomers call H-alpha.

The symbol Z for an element's proton count comes from which German word, meaning 'number'?

Zahl at first meant only an element's position in the periodic table. X-ray experiments in 1913 then showed that the position was a physical quantity: the charge on the atom's nucleus.

The unified atomic mass unit is defined as a fixed fraction of the mass of one atom of which isotope?

Carbon-12 ended a quarrel: until 1961 chemists and physicists both set the unit by oxygen, but by two slightly different oxygens, so their tables of atomic weights never quite matched.

James Chadwick discovered the neutron in 1932 by firing polonium's alpha particles at which light metal?

Beryllium under alpha bombardment gives off neutral radiation that the Joliot-Curies had taken for gamma rays; Chadwick showed it was a neutral particle with nearly a proton's mass.

Which combination of three quarks makes up every proton in an atom's nucleus?

Two up, one down adds up: up quarks carry +2/3 each and the down quark −1/3, giving the proton its charge of exactly +1.

Which American chemist's 1916 paper introduced the shared electron pair and the 'rule of eight'?

Gilbert N. Lewis had sketched 'cubical atoms' in lecture notes since about 1902, with electrons at the corners, and Lewis dot diagrams still carry his name.

Which German physicist's rule has electrons fill a subshell's orbitals singly, with parallel spins, before any pair up?

Friedrich Hund's rule works like a bus filling up: each passenger takes an empty double seat before anyone sits beside a stranger, because an electron with an orbital to itself costs the atom less energy.

J. J. Thomson pictured the atom as electrons dotted through a ball of positive charge, a model nicknamed after which dessert?

Plum pudding stuck as the popular name because the electrons sat in the positive ball like fruit in the pudding. The model was dropped in 1911, once Rutherford showed the charge is packed into a tiny nucleus.

Which Cambridge polymath, who also coined the word 'scientist', suggested the term 'ion' to Michael Faraday?

William Whewell coined 'scientist' in 1833 to answer a challenge from the poet Coleridge; until then English had only 'natural philosopher' and 'man of science'. Faraday's 'anode' and 'cathode' were his too.

Which Irish physicist coined the word 'electron' in 1891, six years before the particle itself was detected?

George Johnstone Stoney meant 'electron' as the name of a fixed unit of electric charge, not of a particle; physicists later borrowed his word for the thing found in cathode rays.

Francis Aston's Nobel-winning mass spectrograph resolved which noble gas into isotopes of mass about 20 and 22?

Neon's two components came out near whole numbers, the pattern Aston enshrined as the 'whole number rule' for isotope masses.

The evacuated glass tubes used to study cathode rays from the 1870s are named after which British physicist?

William Crookes's tubes made their own glass glow where the rays struck. In 1895 Wilhelm Röntgen was experimenting with one when he stumbled on a new kind of ray, the X-ray.

Which English physicist, killed at Gallipoli in 1915, used X-rays to show each element's atomic number is its nuclear charge?

Henry Moseley found that an element's X-ray frequencies climb in step with its atomic number, which turned the periodic table's running order into physics and exposed gaps for elements still unfound. He died at 27.

Nuclei with 2, 8, 20, 28, 50, 82 or 126 protons or neutrons are unusually stable; what are these counts called?

Magic numbers mark filled nuclear shells, much as full electron shells make noble gases inert. Lead-208, with 82 protons and 126 neutrons, is doubly magic and the heaviest stable nucleus known.

Which Greek thinker, known in antiquity as the 'laughing philosopher', called the smallest bodies of matter 'atoms'?

Democritus is said to have earned the nickname for his cheerfulness. His teacher Leucippus left so little trace that Epicurus reportedly denied he had ever existed.

Which English chemist introduced the word 'isotope' in 1913, on the suggestion of his novelist friend Margaret Todd?

Frederick Soddy had shown that atoms of one element can differ in weight, upending a century-old belief that they are all alike. Todd, a doctor who also wrote novels, supplied the Greek name.

Hantaro Nagaoka's 1904 model of the atom, with electrons circling a massive positive centre, was inspired by which planet?

Saturn's rings hold steady because the planet's pull is so strong, and Nagaoka argued electrons could ring a heavy centre the same way. Rutherford cited the idea in 1911 when he unveiled the atomic nucleus.

Which Japanese physicist predicted the meson, a particle carrying the force that binds protons and neutrons?

Hideki Yukawa's particle seemed to turn up in 1936, but that find proved an impostor, the muon. The true meson, the pion, was caught in 1947, and his Nobel Prize followed two years later.

Which nucleus has the highest binding energy per nucleon, narrowly beating the iron-56 usually credited?

Nickel-62 is the most tightly bound nucleus of all. Iron-56 gets the credit because it has the lowest mass per nucleon, helped by its larger share of protons, which are lighter than neutrons.

Which German physicist refined Bohr's 1913 model by letting electrons travel in elliptical rather than circular orbits?

Arnold Sommerfeld added the ellipses because Bohr's circles could not explain why hydrogen's spectral lines split into finer ones, a pattern his elliptical orbits were built to reproduce.

Which particle carries the strong force between quarks, binding them into protons and neutrons?

Gluons take their name from glue, since they stick quarks together. Unlike photons, gluons also pull on each other, because they carry the very charge that the strong force acts on.

Which American chemist introduced the word 'orbital' in 1932 as shorthand for a much longer technical phrase?

Robert Mulliken needed the word for his molecular orbital method, which treats electrons as moving through the whole molecule instead of belonging to single atoms; it won him the 1966 Nobel Prize in Chemistry.

Which two Dutch physicists proposed electron spin in 1925, yet never won a Nobel Prize for it?

Uhlenbeck and Goudsmit nearly backed out: after talking to Lorentz, Uhlenbeck feared no electron could spin so fast and reportedly asked their professor to hold the paper, only to hear it had already been sent.

Which physicist, the second woman to win the physics Nobel, shared the 1963 prize for nuclear shell structure?

Maria Goeppert Mayer worked out the shell model as a 'volunteer' professor at Chicago: anti-nepotism rules meant the universities that hired her husband would not give her a salaried post.

Canal rays, found in 1886 streaming through holes in the back of a cathode, turned out to be made of what?

Positive ions made up Eugen Goldstein's rays: atoms stripped of electrons, pulled towards the negative cathode and coasting on through its holes. Sorting such rays by mass later grew into mass spectrometry.

Which French physicist, later a Nobel laureate for Brownian motion, proposed a 'positive sun' atom in 1901?

Jean Perrin pictured strongly charged 'positive suns' circled by small negative planets a decade before Rutherford, but never developed the idea further.

In Bohr's 1913 model, an electron's angular momentum is restricted to whole-number multiples of what quantity?

The reduced Planck constant, h/2π, fixed which orbits were allowed, and in those orbits Bohr simply decreed that the accelerating electron does not radiate.

Which word for one species of nucleus, fixed by its proton and neutron counts, did Truman Kohman coin in 1947?

Nuclide is Truman Kohman's coinage, and he went on to help discover one himself: aluminium-26, which scientists now use to date meteorites.

Which name is given to ordinary hydrogen, the isotope with no neutrons that makes up over 99.98% of the natural element?

Protium comes from the Greek for 'first'. Harold Urey coined it in the 1930s to complete a set with the names he gave hydrogen's two heavier isotopes, the 'second' and the 'third'.

Rutherford's team found the atomic nucleus in an experiment nicknamed for the thin foil of which metal?

Gold foil let almost every alpha particle fly straight through, yet a rare few bounced back. That told Rutherford an atom's positive charge is packed into a tiny core.

J. J. Thomson's atom, electrons dotted through a ball of positive charge, is nicknamed after which British dessert?

Plum pudding was a journalist's comparison, not Thomson's. A British pharmacy magazine likened his electrons to raisins in a pudding in 1906, and the name stuck.

Most alpha particles flew straight through Rutherford's metal foil, so he concluded that an atom is mostly what?

Empty space makes up nearly all of an atom. Blow one up to the size of a football stadium and its nucleus would be no bigger than a blueberry.

Which element was spotted as a yellow line in the Sun's spectrum decades before anyone found it on Earth?

Helium is named after helios, the Greek word for the Sun. It showed up during an 1868 eclipse and was not isolated on Earth until 1895.

Beams of electrons like the cathode rays J. J. Thomson studied went on to light the screens of which devices?

Television sets were built around the cathode ray tube, invented in 1897, which steers a beam of electrons across a glowing screen to draw the picture.

Whose 1905 paper on pollen grains jiggling in water gave convincing evidence that atoms really exist?

Albert Einstein showed that the jitter comes from unseen particles of water pounding each grain from all sides, a prediction that experiments confirmed three years later.

Which scientist coined the word 'radioactivity' and is the only person to win Nobel Prizes in two different sciences?

Marie Curie's papers from the 1890s are still so contaminated that they are kept in lead-lined boxes, and anyone consulting them must wear protective clothing.

The word 'electron' traces back to the Greek name for which honey-coloured gem, which attracts dust when rubbed?

Amber is 'elektron' in Greek, and the ancient Greeks noticed that it pulled in small objects after being rubbed with fur. 'Electric' and 'electricity' share the root.

Strip the electrons from a gas's atoms and you get which state of matter, the stuff of lightning and stars?

Plasma is known as the fourth state of matter. It is rare on Earth, yet an estimated 99.9% of the ordinary matter in the universe is in this form.

Which Greek philosopher, tutor to Alexander the Great, rejected atoms and taught that matter is continuous?

Aristotle argued that earth, air, fire and water were continuous rather than grainy, and that the void atoms would need to move through could not exist.

Which element, the most abundant in Earth's crust, has atoms that each hold exactly eight protons?

Oxygen makes up almost half of the crust by mass, locked into rock as oxides. Any atom with eight protons is this element, however many neutrons it carries.

Which planet-named metal has 92 protons, the most of any element found in quantity in nature?

Uranium was named after Uranus, the planet discovered eight years earlier. Elements with more protons turn up in nature only in traces, or have to be made in reactors.

Which metal's atoms give older street lamps their yellow-orange glow, the same colour table salt turns a flame?

Sodium atoms glow when energised electrons fall back to a lower level and release the difference as light, nearly all of it at a single yellow wavelength.

Which Russian chemist's periodic table sorted elements by atomic weight and left gaps for ones not yet discovered?

Dmitri Mendeleev used his table to predict the properties of three undiscovered elements, now called gallium, scandium and germanium, and all three were later found.

Ernest Rutherford, who discovered the atomic nucleus, was born in which country, which later put him on a banknote?

New Zealand put his portrait on its hundred-dollar note in the 1990s. He was born in Brightwater, the fourth of twelve children of a farmer.

Which German physicist's uncertainty principle says an electron's exact position and momentum can never both be known?

Werner Heisenberg worked it out in Copenhagen in 1927. The limit is a property of nature itself, not a shortcoming of measuring instruments.

Which of nature's four fundamental forces holds an atom's electrons in place around its positively charged nucleus?

Electromagnetism attracts each electron to the protons, yet the same force makes those protons repel one another, so a stronger short-range force is needed to hold the nucleus together.

An atom's outermost electrons, the ones that form its chemical bonds, are described by which word?

Valence electrons decide how an atom bonds, which is why elements in the same column of the periodic table, sharing the same outer count, behave so alike.

Two or more atoms held together by chemical bonds, like the H2O unit of water, make up what?

A molecule made of more than one element, such as water's, is also a compound, but two identical atoms can pair up into one just as well.

Which German chemist, best known for a laboratory gas burner, found two new elements by the colours their atoms emit?

Robert Bunsen's burner gave a very hot, clean flame, ideal for studying the light of heated elements. With Gustav Kirchhoff he found rubidium and one other metal from unfamiliar spectral lines.

Rutherford said alpha particles bouncing back off his foil were as incredible as a 15-inch shell rebounding off what?

Tissue paper was his image for the atom he had expected: soft and spread out. 'It was quite the most incredible event that has ever happened to me in my life,' he said.

Which German physicist ran Rutherford's foil experiment with Ernest Marsden, counting alpha-particle flashes by eye?

Hans Geiger spent hours on end in a darkened lab, counting the faint flashes through a microscope. He later invented the electrical radiation counter that bears his name.

Which American physicist measured the electron's charge by watching charged oil droplets drift between two metal plates?

Robert Millikan found every droplet's charge was a whole-number multiple of one tiny value, the charge of a single electron. The work won him the 1923 Nobel Prize in Physics.

Rutherford caused the first artificial nuclear reaction by firing alpha particles at which gas, knocking out protons?

Nitrogen nuclei hit by alpha particles threw out hydrogen nuclei, showing Rutherford that these were components of heavier atoms. He later named them protons.

Which French chemist, guillotined in the Revolution, showed that mass is neither gained nor lost in a reaction?

Antoine Lavoisier proved it by weighing more precisely than anyone before him. Atomic theory later supplied the reason: a reaction only rearranges atoms, never makes or destroys them.

Which Austrian physicist's exclusion principle says no two electrons in an atom can share all four quantum numbers?

Wolfgang Pauli stated the rule in 1925. It is why electrons stack up in shells instead of all crowding into the lowest one, and so why the elements differ chemically.

Which antimatter twin of the electron did Carl Anderson discover in cosmic-ray tracks, winning a Nobel Prize at 31?

The positron was the first antimatter ever detected. Paul Dirac had predicted such a particle a year earlier, one that would vanish in a burst of energy on meeting an electron.

An electron dropping to a lower energy level sheds the difference as which particle, giving each element its colours?

A photon carries off exactly the energy gap between the two levels. Because every element has its own ladder of levels, each one's glowing gas shows a unique pattern of spectral lines.

An atom whose electron has absorbed energy and jumped to a higher level is said to be in which state?

Excited states are usually short-lived. The electron soon falls back towards its lowest available level, giving out the extra energy as it goes.

The official length of one second is counted out by the vibrations of which metal's atoms inside atomic clocks?

Caesium atoms switch between two energy states at exactly 9,192,631,770 cycles per second, and counting those cycles is how the second is officially defined.

Which letter labels the atomic orbitals shaped like a dumbbell, with two lobes on opposite sides of the nucleus?

Each p orbital points along a different axis, so they come in sets of three, and they first appear in an atom's second electron shell.

Which halogen's electrons are arranged 2, 8, 7, leaving the atom one short of a full outer shell?

Chlorine's hunger for that one missing electron makes it fiercely reactive, the property behind household bleach and swimming-pool disinfectant.

Which family of soft, reactive elements, including potassium and rubidium, have atoms with a single outer electron?

Alkali metals shed that lone electron so readily that all of them react with water, and in nature they turn up only in salts, never as the pure metal.

IBM scientists once wrote the letters 'IBM' with 35 individually placed atoms of which noble gas?

Xenon atoms were nudged one at a time across a chilled metal crystal with the tip of a microscope. It was the first time atoms had been precisely positioned on a flat surface.

Atomic sizes are often quoted in which unit, one ten-billionth of a metre, named after a Swedish physicist?

The angstrom also suits light: the wavelengths we can see run from about 4,000 to 7,000 of them. It honours Anders Ångström, who charted the spectrum of sunlight in 1868.

Which French physicist discovered radioactivity when a mineral salt fogged photographic plates stored in a dark drawer?

Henri Becquerel had put the plates away because the weather was too overcast for his sunlight experiment. The SI unit of radioactivity is named after him.

Ernest Marsden, who first saw alpha particles bounce back off Rutherford's foil, held what position in the laboratory?

An undergraduate of just 20, he was asked to look for particles bouncing backwards even though none were expected. The surprise result rewrote the model of the atom.

Which American physicist said the one sentence to save if science were lost is that all things are made of atoms?

Richard Feynman opened his Caltech lectures with it, describing atoms as little particles in perpetual motion that attract when slightly apart but repel when squeezed together.

Which element, number 43 in the periodic table, was the first to be made artificially?

Technetium is the lightest element with no stable form: every one of its isotopes is radioactive. It was first produced in 1937.

Which English physicist used X-rays to rank the elements by nuclear charge before being killed at Gallipoli, aged 27?

Henry Moseley's measurements also exposed gaps in the sequence, at numbers 43, 61, 72 and 75, where elements were still waiting to be discovered.

Which Swiss schoolteacher found the formula that predicts the wavelengths of hydrogen's visible spectral lines?

Johann Balmer was sixty and teaching at a girls' school in Basel when he published it in 1885. Nobody could say why it worked until Bohr's model of the atom, 28 years later.

Cockcroft and Walton first split an atom with a particle accelerator by firing protons at which light metal?

Lithium nuclei hit by the protons broke into pairs of alpha particles. The experiment brought the two men the 1951 Nobel Prize in Physics.

Which Dutch physicist discovered that a strong magnetic field splits the spectral lines of glowing atoms into several?

Pieter Zeeman saw the effect in 1896, before the electron had even been identified. Part of the splitting stayed a puzzle for decades, until electron spin explained it.

Which Scottish physicist's cloud chamber made the tracks of single subatomic particles visible to the naked eye?

C. T. R. Wilson began by studying how clouds form at the observatory on Ben Nevis. His chamber earned him a share of the 1927 Nobel Prize in Physics.

Which metal was long thought to be the heaviest stable element, until its incredibly slow decay was detected in 2003?

Bismuth's half-life turned out to be more than a billion times the age of the universe, so for every practical purpose it still behaves as if it were stable.

Which Roman poet's 'On the Nature of Things' carried the ancient Greek idea of atoms to Renaissance readers?

Lucretius set out in verse the idea that everything is made of tiny particles moving through a void. The poem was rediscovered in 1417 by the Italian book hunter Poggio Bracciolini.

Which Swedish chemist devised the one- and two-letter symbols for the elements, such as Fe and Pb, still used today?

Jacob Berzelius abbreviated each element's Latin name, which is why iron is Fe, from ferrum. He wrote the atom counts as superscripts, where today's formulas use subscripts.

The electron and the neutron were both discovered, 35 years apart, at which Cambridge physics laboratory?

Cavendish researchers also worked out the structure of DNA there. Thomson found the electron in 1897 and Chadwick the neutron in 1932.

The first atoms made entirely of antimatter, a handful of antihydrogen atoms, were created at which laboratory?

CERN's team made the first antihydrogen in 1995. By 2004 millions of the atoms were being produced, and in 2010 some were trapped long enough to be studied.

An atom with equal numbers of protons and electrons carries what overall electric charge?

Zero is the total because each proton's positive charge exactly cancels one electron's negative charge. Add or remove electrons and the balance tips, turning the atom into an ion.