Appearance
9.6 — The Elements That Matter
Ninety-four elements occur naturally and a handful account for almost everything. This chapter takes six of them and works through why each one behaves as it does, starting from the electron configuration and ending with why it is used where it is used.
A note on abundance first, because it explains a great deal.
| Element | Universe | Earth's crust | Human body |
|---|---|---|---|
| Hydrogen | 74 % | 0.14 % | 10 % (63 % of atoms) |
| Helium | 24 % | trace | trace |
| Oxygen | 1 % | 46 % | 65 % |
| Carbon | 0.5 % | 0.02 % | 18 % |
| Silicon | 0.07 % | 28 % | trace |
| Iron | 0.1 % | 5.6 % | 0.006 % |
Three completely different lists, and each difference has a cause.
The universe is hydrogen and helium because those are what the Big Bang made (Chapter 12.6), and everything heavier was made later in stars.
The crust is oxygen and silicon because Earth's gravity could not hold hydrogen and helium, and because oxygen and silicon form the silicate minerals that make up rock. The iron sank to the core during the planet's molten phase, which is why the crust has 5.6 % and the whole Earth has 32 %.
The body is oxygen, carbon and hydrogen because it is mostly water and organic molecules.
Hydrogen
Configuration: 1s^1. One proton, one electron. The simplest atom there is.
Why it is unique. It sits in group 1 and is not an alkali metal. It has one electron like lithium, and one electron short of a filled shell like fluorine. It genuinely belongs to neither group, which is why the periodic table usually floats it alone at the top.
Three ways it bonds:
Lose the electron to give H⁺ — which is a bare proton, with no electrons at all. No other cation is like this, and its radius is 10^{-15} m rather than 10^{-10} m, a hundred thousand times smaller than any other ion. It never exists free in solution; it always attaches to something, usually water, giving H₃O⁺.
Gain an electron to give H⁻, the hydride ion, in compounds like sodium hydride.
Share the electron covalently, which is what it does in almost everything organic.
Isotopes with names. Hydrogen is the only element whose isotopes have separate names, because the mass differences are proportionally enormous.
| Isotope | Name | Neutrons | Abundance |
|---|---|---|---|
| ¹H | Protium | 0 | 99.98 % |
| ²H | Deuterium | 1 | 0.02 % |
| ³H | Tritium | 2 | trace, radioactive |
Deuterium is twice the mass of protium, so isotope effects are far larger than for any other element (Chapters 7.4 and 7.P). Heavy water freezes at 3.8 °C, boils at 101.4 °C, and is toxic in quantity because it slows enzyme reactions.
Where it matters:
Water. Two hydrogen atoms per molecule, and the hydrogen bonding that Chapter 10.3 develops is the reason water behaves unlike any comparable liquid.
Acids. An acid is a proton donor, and pH measures the concentration of hydrogen ions.
Fusion. Hydrogen fusing to helium powers every star (Chapter 12.1).
Ammonia synthesis. The Haber process combines nitrogen and hydrogen to make fertiliser, and it sustains roughly half the human population's food supply. It consumes about 1 % of world energy.
As a fuel. 142 MJ/kg, three times petrol's — and its density is so low that a kilogram of hydrogen gas at atmospheric pressure occupies 11 cubic metres. Storage is the whole problem, and it is a consequence of hydrogen having the smallest possible molecule.
Carbon
Configuration: 1s^22s^22p^2. Four valence electrons, and it is exactly halfway to a filled shell.
Carbon forms more compounds than every other element combined — over 10 million known, against a few hundred thousand for everything else. Four properties conspire, and no other element has all four.
1. It makes exactly four bonds. With four valence electrons and four spaces, sharing four gives a filled shell. Four is the maximum for a second-row element and it allows three-dimensional branching structures, which two or three bonds cannot.
2. It bonds strongly to itself. The C–C bond is 348 kJ/mol, and carbon chains of any length are stable. Compare silicon, directly below: Si–Si is 226 kJ/mol, and silicon chains longer than a few atoms fall apart. This one number is why life is carbon-based and not silicon-based.
3. It forms multiple bonds. C=C (614 kJ/mol) and C≡C (839) are strong and common. Silicon does not form stable double bonds, because its larger atoms cannot get their p orbitals close enough to overlap sideways.
4. Its electronegativity is 2.55 — almost exactly in the middle. So it bonds comfortably with metals and non-metals alike, and C–H bonds are barely polar, which makes hydrocarbons stable and unreactive.
The allotropes are a striking demonstration that structure matters as much as composition.
| Form | Bonding | Property |
|---|---|---|
| Diamond | sp^3, 3D network | Hardest known, insulator, k = 2000 W/m/K |
| Graphite | sp^2 sheets | Soft, conducts in-plane, lubricant |
| Graphene | Single sp^2 sheet | Strongest material known, 130 GPa |
| Fullerene C₆₀ | Closed cage | Molecular, soluble |
| Nanotube | Rolled sheet | Conducts or semiconducts by geometry |
Diamond and graphite are the same element and differ in every property. Diamond conducts heat five times better than copper while being an electrical insulator — because the heat is carried by lattice vibrations in an exceptionally stiff, light lattice (Chapter 3.7), not by electrons.
Graphite conducts electricity along its sheets and not between them, because each carbon uses three electrons for in-plane bonds and the fourth is delocalised across the sheet. The sheets slide over each other easily, which is why it works as a pencil and a lubricant.
Graphene was isolated in 2004 with sticky tape, by Geim and Novoselov at Manchester, and won the 2010 Nobel Prize.
And diamond is metastable at room conditions. Graphite is the thermodynamically stable form, so every diamond is slowly turning into graphite — with an activation energy so large that the process takes far longer than the age of the universe.
Carbon dating was covered in Chapter 9.2. The carbon cycle moves about 800 gigatonnes of carbon through the atmosphere, oceans and biosphere annually, and the imbalance from fossil fuel burning is what drives the greenhouse mechanism of Chapter 3.7.
Nitrogen
Configuration: 1s^22s^22p^3. Half-filled p subshell, three unpaired electrons by Hund's rule.
N₂ is 78 % of the atmosphere and almost completely unreactive, and the reason is one number.
The N≡N triple bond has a strength of 945 kJ/mol — the strongest bond between two atoms of the same element, and among the strongest of any bond. Breaking it costs more than almost any reaction can supply.
This is why nitrogen is inert despite being surrounded by things it would happily react with. Air is 78 % nitrogen and 21 % oxygen sitting together indefinitely, and thermodynamically they should form nitrogen oxides. Kinetics forbids it (Chapter 10.6).
The nitrogen problem for life. Every protein and every strand of DNA needs nitrogen, and no plant or animal can use N₂ directly.
Two routes exist:
Biological fixation. Certain bacteria, notably those living in legume root nodules, carry the enzyme nitrogenase, which breaks N≡N at ordinary temperature and pressure. Nobody knows exactly how, and reproducing it industrially is a long-standing goal. The enzyme contains an iron–molybdenum cofactor and consumes 16 ATP per nitrogen molecule.
The Haber–Bosch process, 1909–1913:
\text{N}_2+3\text{H}_2 \rightleftharpoons 2\text{NH}_3
450 °C, 200 atmospheres, iron catalyst. The conditions are severe precisely because the triple bond is so strong.
Its significance is hard to overstate. Roughly half of the nitrogen atoms in your body passed through a Haber–Bosch reactor. The process supports an estimated 3.5 to 4 billion people who could not otherwise be fed. Fritz Haber received the Nobel Prize in 1918 — and also directed Germany's chemical weapons programme in the First World War, personally supervising the first chlorine gas attack at Ypres. His wife, herself a chemist, killed herself with his service pistol shortly afterwards.
Explosives. Almost every explosive is nitrogen-based — TNT, nitroglycerine, ammonium nitrate, RDX. The reason is that forming N₂ releases the enormous triple bond energy, so any reaction that produces nitrogen gas from a nitrogen compound is violently exothermic and generates a large volume of gas at once.
Oxygen
Configuration: 1s^22s^22p^4. Two electrons short of a filled shell, electronegativity 3.44 — second only to fluorine.
Oxygen is 46 % of the Earth's crust and 21 % of the atmosphere, and the atmospheric part is entirely biological in origin.
The Great Oxidation Event. For the first two billion years of Earth's history there was no free oxygen. Cyanobacteria began producing it about 2.4 billion years ago, and it first oxidised dissolved iron in the oceans — laying down the banded iron formations that supply most of the world's iron ore today — and only then accumulated in the air.
It was a mass extinction. Oxygen was toxic to the anaerobic life that dominated, and most of it died. The atmosphere you breathe is pollution produced by bacteria, which killed almost everything alive at the time.
Why oxygen is so reactive. It needs two electrons and pulls hard on them. Combustion, corrosion and respiration are the same chemistry at different rates — all are oxidation, and the energy released comes from forming strong bonds to oxygen.
And O₂ is a genuine puzzle. Drawn as O=O with all electrons paired, it should be diamagnetic. Liquid oxygen sticks to a magnet, so it has unpaired electrons. Simple Lewis structures get this wrong, and Chapter 10.2 shows that molecular orbital theory predicts exactly two unpaired electrons — the single clearest demonstration that the simple picture is inadequate.
Ozone, O₃. A bent molecule, unstable, and it absorbs ultraviolet between 200 and 315 nm, which is exactly the range that damages DNA. The stratospheric ozone layer is what makes land life possible.
The hole and its repair is the clearest example of a global environmental problem being identified and fixed. Molina and Rowland showed in 1974 that chlorofluorocarbons would destroy ozone catalytically — one chlorine atom destroys about 100,000 ozone molecules before being removed — the Antarctic hole was found in 1985, the Montreal Protocol was signed in 1987, and the ozone layer is now recovering and is projected to return to 1980 levels around 2066.
Water is oxygen's most important compound and Chapter 10.3 devotes a section to why it behaves so strangely.
Silicon
Configuration: [\text{Ne}]3s^23p^2. Directly below carbon, four valence electrons, and it behaves completely differently.
Why silicon is not carbon. Three differences, all traceable to its being one period lower and therefore larger.
Weaker bonds to itself. Si–Si is 226 kJ/mol against C–C's 348, so long silicon chains are unstable.
Stronger bonds to oxygen. Si–O is 452 kJ/mol against C–O's 358. So silicon in nature is always found as an oxide, and any silicon–silicon chain in an oxygen atmosphere converts rapidly to silicate.
No stable double bonds. Silicon atoms are too large for effective sideways p-orbital overlap.
The consequence: carbon chemistry is chains and rings of carbon; silicon chemistry is networks of Si–O–Si. Rock rather than life.
Silicates are built from the SiO₄ tetrahedron, and how those tetrahedra connect determines the mineral:
| Sharing | Structure | Example |
|---|---|---|
| Isolated | Separate tetrahedra | Olivine |
| Chains | Single or double | Pyroxenes, amphiboles |
| Sheets | Two dimensions | Mica, clay |
| Framework | All four corners | Quartz, feldspar |
Mica splits into sheets and clay is slippery when wet because their bonding is two-dimensional, with weak forces between layers — structurally the same reason graphite is a lubricant.
Semiconductors. Silicon's band gap is 1.12 eV, which is close to ideal: large enough that a device is not swamped by thermally excited carriers at room temperature, small enough to be controllable.
But the real reason silicon won is not the band gap. Germanium has a better gap for some purposes and lost anyway, because silicon has SiO₂.
Silicon dioxide is a superb insulator, forms naturally on a silicon surface by exposure to oxygen, adheres perfectly, blocks dopant diffusion, and can be etched selectively. Germanium's oxide is water-soluble and useless. The entire semiconductor industry exists because of the properties of a native oxide layer, and Volume III, Chapter 2 develops what is built on it.
Glass, silicones, cement and ceramics are all silicon–oxygen chemistry.
Iron
Configuration: [\text{Ar}]3d^64s^2. A transition metal with four unpaired d electrons.
Iron is the most abundant element in the Earth by mass — 32 % of the whole planet, concentrated in the core — and Chapter 9.2 explained why it is so abundant cosmically: it sits at the peak of the binding energy curve, so it is where both fusion and fission stop.
Ferromagnetism. Chapter 4.8 covered the mechanism. Iron's four unpaired 3d electrons, combined with a lattice spacing that gives the right exchange interaction, produce spontaneous alignment up to 1043 K.
The Earth's magnetic field comes from convecting liquid iron in the outer core — a self-sustaining dynamo, since the core is far above iron's Curie temperature and cannot be a permanent magnet. That field deflects the solar wind and is why Earth kept its atmosphere while Mars did not (Chapter 4.5).
Steel. Pure iron is soft. Adding 0.2–2 % carbon transforms it, because carbon atoms sit in the interstices of the iron lattice and block the movement of dislocations, which is how metals deform.
The heat treatments matter as much as the composition. Quenching from red heat traps carbon in a distorted structure called martensite, giving extreme hardness and brittleness; tempering — reheating gently — trades some hardness for toughness. This was known empirically for three thousand years before anyone understood why.
Alloys: stainless steel adds 11 %+ chromium, which forms a self-healing oxide layer a few nanometres thick. Tool steels add tungsten and vanadium for hardness at high temperature.
Rust. Iron oxidises to Fe₂O₃·nH₂O, and unlike aluminium's or chromium's oxide, rust flakes off rather than protecting the metal beneath. The oxide has a larger volume than the metal it replaced, so it cracks and exposes fresh surface. Corrosion costs an estimated 2–4 % of global GDP annually, essentially because iron's oxide has the wrong molar volume.
In biology. Haemoglobin carries one iron atom in each of four haem groups, and each binds one O₂ molecule reversibly. The iron must be Fe(II); oxidised to Fe(III) it cannot bind oxygen, which is what carbon monoxide poisoning and methaemoglobinaemia do.
About 4 grams of iron in an adult, mostly in haemoglobin, and iron deficiency is the most common nutritional deficiency in the world.
The technology elements
A brief note on the others that matter, and why.
Lithium. Lightest metal, lowest electrode potential, so lithium-ion cells achieve the highest energy density of any commercial battery. Its position at the top of group 1 is the reason — small, light, and gives up its single electron readily.
Neodymium. Its 4f electrons give a large magnetic moment, and NdFeB magnets have the highest energy product of any material. In every hard drive, wind turbine and electric motor.
Copper. Second-best electrical conductor after silver and a hundred times cheaper. The entire electrical grid.
Aluminium. Third most abundant crustal element, and it was more valuable than gold until 1886, because reducing Al₂O₃ requires enormous energy. The Hall–Héroult electrolysis process made it cheap, and aluminium production still consumes about 3 % of world electricity.
Titanium. The best strength-to-weight ratio of any metal, and it forms a passivating oxide like aluminium's. Biocompatible, because that oxide is inert, which is why implants are titanium.
Tungsten. Melting point 3422 °C, the highest of any metal. Light bulb filaments and cutting tools.
Helium. Second lightest element, chemically inert, and the only substance that stays liquid to absolute zero (Chapter 7.9). Irreplaceable for cooling superconducting magnets, and a genuinely finite resource — it is produced by alpha decay underground, and once released it escapes the planet.
Where this shows up in your life
Every material choice ever made traces to a configuration. Copper for wires, silicon for chips, titanium for implants, tungsten for filaments, lithium for batteries, neodymium for magnets.
Your body is oxygen, carbon, hydrogen and nitrogen by mass, in that order, with a few grams of iron doing the most important single job.
The food you eat exists because of Haber–Bosch, which exists because of nitrogen's triple bond.
The air you breathe was produced by bacteria and is protected by ozone.
And the phone in your hand contains about 60 different elements, drawn from every block of the periodic table.
What Part 9 established
Part 9 went from the discovery of the atom to the properties of the elements.
Rutherford's scattering showed the atom is 99.99999999999 % empty, with a nucleus 10^{-15} times its volume, and the argument was quantitative: Thomson's model predicted deflections of half a degree and 1 in 8000 alphas came straight back.
The nucleus is held by the residual strong force, and its binding energy curve — rising to iron, falling beyond — contains the whole of nuclear energy. Fusion below iron, fission above, and nothing from iron itself.
Radioactivity comes in three modes, obeys an exponential decay law derived from a constant per-nucleus probability, and gives dating methods spanning from decades to billions of years.
Many-electron atoms cannot be solved exactly, and the screening approximation plus penetration gives the filling order — which produces the periodic table's shape as pure arithmetic: 2, 8, 8, 18, 18, 32, 32.
Mendeleev built the table without knowing any of this, left gaps, and predicted germanium's density to two significant figures fifteen years before it was found.
And the trends — size, ionisation energy, electronegativity — all follow from two competing quantities, Z_{\text{eff}} and n, with the interesting parts being exactly the places where the smooth trend breaks and reveals subshell structure.
What the next Part fixes
Atoms have been described in isolation. Almost nothing exists as isolated atoms — the noble gases and nothing else. Part 10 takes the configurations derived here and asks what happens when atoms meet: why they bond at all, what shapes the resulting molecules take and why, what happens between molecules that makes water behave so strangely, how reactions reach equilibrium and how fast they get there, what acids and batteries have in common, and finally how a few kinds of carbon compound became alive.