Reactive Nonmetals

This family is not a column and not a block. It is a corner — the top-right region of the table, bounded below by the metalloid staircase and on the right by the noble gases — and it is the only category on this site whose shape is a triangle. Twelve elements sit inside it, and what they have in common is that they are the elements that take, share and hold electrons rather than surrendering them.

The practical consequence is that this corner supplies the other half of almost everything. Nearly every mineral in the crust is a metal joined to oxygen, sulfur or a halide; nearly every molecule in a living cell is carbon joined to hydrogen, oxygen, nitrogen, phosphorus and sulfur. The metals are the more numerous group by a factor of about six, but they spend most of their existence bonded to these twelve.

Why the corner is where it is

Two gradients cross here. Crossing a period from left to right, each step adds a proton to the nucleus and an electron to the same shell — and an electron in the same shell shields its neighbours poorly, so the effective nuclear charge felt by the valence electrons rises steeply while the shell does not move outward. Atoms get smaller and hold their electrons harder from left to right, exactly opposite to the intuition that heavier means bigger.

Descending a column has the opposite effect, adding a full shell of shielding each time. High electron-attracting power therefore accumulates in one corner: high in the table and far to the right, stopping short of the noble gases only because those have nothing left to gain.

It is worth knowing that electronegativity is a derived quantity rather than something anyone measures directly. Pauling constructed his scale in 1932 from the extra stability of bonds between unlike atoms; Mulliken derived his from the average of ionisation energy and electron affinity; Allred and Rochow computed theirs from the effective nuclear charge acting at the covalent radius. The three scales agree on the ranking and disagree on the spacing, which is a reasonable reminder that the number is a model of a tendency rather than a property of an atom.

No other region of the table changes shape like this

Allotropy — the same element existing as structurally different substances — is scattered thinly through the periodic table and concentrated heavily here. The two elements with the biggest tallies of forms, sulfur and carbon, are both in this corner, and which of them leads turns on an accounting question rather than a chemical one — whether fullerene cages of different sizes count separately.

Phosphorus offers the starkest contrast between forms. White phosphorus is built from discrete P₄ tetrahedra with punishing bond angles of 60°, which is why it is so strained and so reactive; red phosphorus is an amorphous polymer of broken-open tetrahedra and is far less so; black phosphorus is a layered semiconductor whose single layers are the material now studied as phosphorene. Same element, three substances with almost nothing in common.

Carbon is the celebrated case, and the discoveries kept arriving: diamond and graphite were the whole list until 1985, when Harold Kroto, Robert Curl and Richard Smalley identified C₆₀ in the soot of a laser-vaporised graphite target and named it buckminsterfullerene. Nanotubes followed, and in 2004 Andre Geim and Konstantin Novoselov isolated single-layer graphene, a form that theory had suggested could not be thermodynamically stable in isolation.

The reason allotropy clusters in this corner is structural. These elements bond covalently and directionally, with a limited number of bonds per atom, and there is usually more than one geometry that satisfies the valence requirement at similar energy. A metal has no such constraint — its bonding is non-directional, so atoms simply pack as densely as they can and there is normally one clear winner.

The crust is sorted by which of these an element prefers

In 1923 Victor Goldschmidt classified the elements by where they concentrated during the Earth's differentiation, and the categories are named for the nonmetal each group binds to. Lithophile elements went with oxygen into the silicate crust and mantle. Chalcophile elements went with sulfur into sulfide minerals. Siderophile elements dissolved in metallic iron and sank to the core.

That single distinction still organises extractive metallurgy. Aluminum, titanium, magnesium, calcium and the lanthanides are won from oxides and silicates, and their oxides are so stable that electrolysis or a very aggressive reductant is required. Copper, zinc, lead, nickel, silver, cadmium and mercury are won from sulfides, which roast to the oxide with far less trouble — and which is why smelter towns historically had a sulfur dioxide problem and aluminum smelters had a fluoride one. Whether a metal is mined as an oxide or a sulfide is not a fact about the metal alone; it is a fact about which element in this corner it bonded to four billion years ago.

The halogens form the third mineral class, and because they occur as columns rather than as regions they have their own treatment at group 17.

Almost all of a body, by mass and by count

Oxygen, carbon, hydrogen and nitrogen account for roughly 96 per cent of human body mass. Add phosphorus, sulfur and chlorine and the figure passes 99, leaving the metals — calcium included — to share what remains.

The reason is a coincidence of bond energies. Covalent bonds between these light atoms fall into a narrow useful range: strong enough that a molecule does not fall apart at body temperature, weak enough that an enzyme can break one at ordinary metabolic cost. Carbon anchors it because it forms four bonds of nearly equal strength to itself and to hydrogen, oxygen and nitrogen, so long chains and rings can be built without any position being a weak point. Selenium joins the list as the twenty-first amino acid's business end — selenocysteine, incorporated by a dedicated codon reassignment — which is a remarkable amount of biological machinery devoted to swapping one sulfur for the element directly below it.

Fixing nitrogen, and running out of phosphorus

Two of the twelve dominate industrial chemistry in ways that are not obvious from their positions.

Nitrogen makes up most of the atmosphere and almost none of it is available to life, because breaking the triple bond in N₂ costs 945 kJ/mol — more than an organism can pay by ordinary enzymatic means, which is why biological nitrogen fixation is confined to a handful of bacteria running a specialised metalloenzyme. Fritz Haber found conditions under which iron would catalyse the reaction with hydrogen and Carl Bosch built the plant that made it work at scale, from 1913 onwards. Estimates put its share of global energy use in the low single figures of a percentage point, and much of the nitrogen held in the tissue of anyone alive today entered the biosphere by that route. It is hard to name another single chemical reaction that has moved the size of the human population as directly.

Sulfur runs the causation the other way. Nitrogen's supply had to be invented to meet a demand; sulfur's demand has to be found for a supply that arrives whether anybody wants it or not, because fuel regulations oblige refiners to strip it out. Sulfuric acid is the largest-tonnage manufactured chemical there is, and essentially every tonne of it now begins life as a contaminant somebody was required to remove. What that arrangement does to the market as refining contracts is worked through on the sulfur page.

Phosphorus is the one with no substitute and no reservoir. Nitrogen can be pulled from the air and carbon cycles through the atmosphere, but phosphorus has no volatile form in the natural cycle: it comes from phosphate rock, it washes to the sea, and it returns on a geological timescale. Every crop grown depends on it, the deposits are concentrated in a handful of countries, and there is no chemistry that makes more of it.

Oxygen is the quiet third case. Almost none of it is manufactured in the chemical sense — it is separated from air by cryogenic distillation — but the tonnage is enormous, and the largest consumer is not medicine or chemistry but steel. The basic oxygen process blows pure oxygen through molten iron to burn out excess carbon, and it replaced the older air-blown converters precisely because nitrogen in the blast wasted heat and embrittled the product.

Where the corner is misread

• Nonmetal does not mean gas. Carbon, phosphorus, sulfur, selenium and iodine are all solids at room temperature, and one of them — carbon, in either of its network forms — melts higher than every metal in the table.
• These elements are not "the opposite of metals" in a simple way. Hydrogen is placed at the far left of the table by its electron configuration and belongs to this corner by every aspect of its behaviour, which is why some tables float it above the table entirely rather than committing.
• Reactivity here is not one scale. Nitrogen is the least reactive element in the region despite sitting near the middle of it, because of that triple bond; white and red phosphorus differ enormously from each other while being the same element. Reactivity belongs to a substance and a situation, not to a position in the table.
• The category boundary is a colour decision. Selenium and iodine are drawn here; astatine is drawn here on the strength of its column, though almost nothing about it has been observed. Where a claim rests on position rather than measurement, this site says so on the element's own page.