The World of Metals and Non-metals

Sudarshan uncle is an ironsmith in a village in Rajasthan. He heats a block of iron in his furnace until it glows red hot, then hammers it hard, again and again, on an anvil. The block does not shatter, it flattens, slowly taking the shape of an axe. Try that same trick with a lump of coal, though, and hammering does not shape it at all, it simply shatters into smaller, jagged pieces. Something about iron lets it bend under force instead of breaking, and that same something is shared by copper, aluminium, gold and most of the metals an ironsmith, a jeweller or an electrician reaches for every day, while coal, sulfur and most other non-metals simply do not have it. This chapter is about exactly that: a small set of properties, shine, hardness, how something responds to a hammer, a stretch, a strike, heat or electricity, that quietly sorts nearly every element into one of two very different families.

Rub a dull, tarnished piece of copper with sandpaper and something changes almost instantly: underneath the dullness is a bright, reflective shine. This shine, seen on a freshly cleaned surface of metals like copper, iron and aluminium, is called metallic lustre, and it is one of the first clues used to sort materials into metals and non-metals. A piece of coal or a lump of sulfur, no matter how hard it is rubbed or scrubbed, never develops that shine; non-metals are non-lustrous. But lustre alone does not tell the whole story about metals, since not every metal shares every other metal property evenly. Are all metals hard? Not quite: sodium and potassium are soft enough to be sliced clean through with an ordinary knife. Are all metals solid? Not quite that either: mercury, the metal used in old-style thermometers, stays liquid at room temperature, the only metal that does. Lustre is a strong first clue, but it is only the beginning of the story.

Place a piece of copper, an iron nail, a lump of coal and a pea-sized piece of sulfur on a hard surface, one at a time, and strike each firmly with a hammer. The copper and the iron nail both flatten, spreading into a thinner shape without cracking. The coal and the sulfur do the opposite: they shatter into smaller pieces the moment the hammer lands. This property, the ability of a material to be beaten or pressed into thin sheets without breaking, is called malleability, and most metals possess it, which is exactly why beaten metal sheets, from a thin foil wrapped around food to a silver leaf decorating a sweet, are entirely ordinary sights, while a sheet of beaten coal has never existed. Gold and silver are the most malleable metals of all. Materials that shatter under the same hammer instead of flattening, like coal and sulfur, are called brittle. Wood is neither: it does not flatten into a sheet under a hammer, but it does not shatter into pieces either, so it counts as neither malleable nor brittle.

Look closely at an electrical cord, a pair of earrings, or the strings of a sitar or guitar, and a shared feature emerges: all of them rely on metal drawn out into long, thin wires. This property of being drawn into wire is called ductility, and like malleability, it belongs mainly to metals. Gold is the most ductile metal known; a single gram of it, barely more than a pinch, can be drawn into a wire two kilometres long. Coal and sulfur, by contrast, cannot be drawn into wire at all, there has never been a coal wire or a sulfur wire, because non-metals simply do not have this property. Ductility and malleability together explain something else worth noticing: steel, a mixture of the metal iron with the non-metal carbon, keeps enough of iron's ductile character to be drawn into strong wire ropes, and those very ropes are what suspension bridges and heavy-lifting cranes depend on to support enormous loads without snapping.

Drop a metal spoon and a wooden spoon onto a hard floor from the same height, and the difference is immediate and unmistakable: the metal spoon rings out with a clear, bright sound, while the wooden spoon lands with a dull, short thud. Metals that produce this ringing sound when struck are described as sonorous, and this single property explains a surprising number of everyday sounds: a dropped coin, a struck gong, ghungroo anklets jingling with each step, and a school bell calling students to class are all sonority at work. Coal, sulfur and wood share none of this; struck or dropped, they simply produce a dull, short sound with no ring to it at all. Nobody has ever built a bell out of coal, and this property is exactly why.

Fill a glass tumbler with hot water, then lower a metal spoon and a wooden spoon into it side by side, leaving both undisturbed for a few minutes. Touch the upper end of each spoon afterwards, well above the waterline, and the metal spoon is noticeably hotter to touch than the wooden one, even though both spent the same time in water of the exact same temperature. Heat has travelled up through the metal spoon far more effectively than through the wooden one. This movement of heat from one point of a material to another is called conduction, and materials that conduct heat well, like most metals, are called good conductors; materials that resist it, like wood, are called poor conductors. This single difference explains a design choice seen in almost every kitchen: cooking vessels are made of metal, so heat reaches the food efficiently, while their handles are deliberately made of wood or plastic, poor conductors that stay cool enough to hold safely even while the metal body of the pan is too hot to touch.

Build a simple tester circuit, a battery, a bulb, and two loose wire ends, and touch those two ends to different materials one at a time: a piece of aluminium foil, an iron nail, a lump of sulfur, a piece of dry wood, a stone. Only some of these materials complete the circuit and make the bulb glow, and every single one of them turns out to be a metal. Materials that allow electricity to pass through them easily, letting the bulb glow, are called good conductors of electricity; materials that block it, like sulfur, wood and stone, are called poor conductors, or insulators. This is precisely why an electrician's screwdriver has a plastic handle and why electricians wear rubber gloves and shoes: plastic and rubber are poor conductors, so electric current traveling through a metal wire cannot pass through the electrician's own body to reach the ground. Metals, in summary, share a striking bundle of properties: they are generally lustrous, malleable, ductile, sonorous, and good conductors of both heat and electricity. It is time to see how they behave when something more than a hammer or a wire touches them: air and water.

Leave an iron nail out in the open for a few days and a brown, flaky coating creeps across its surface. What exactly causes this? To find out, three identical iron nails were sealed into three separate bottles: one with dry air only, silica gel keeping any moisture away; one in water that had been boiled and cooled to remove dissolved air, sealed under a protective layer of oil; and one left open to both ordinary moist air and water together. After more than a week, only one of the three nails had changed: the one exposed to both air and water at once. Dry air alone caused nothing. Air-free water alone caused nothing. This brown coating, now identified, is called rust, and the process that forms it is called rusting, and it clearly needs both moisture and air working together, neither alone is enough. Other metals discolour in their own ways when left exposed too, copper develops a green coating, silver turns black, and this broader gradual damage to any metal's surface from air, water or other substances is called corrosion. Yet one famous iron structure appears to defy all of this entirely. The Iron Pillar of Delhi, cast more than 1,600 years ago, over 8 metres tall and weighing more than 6,000 kilograms, has stood in the open for over a millennium and a half, facing rain, wind and humidity the entire time, and it still shows almost no rust. Ancient Indian metallurgists, without knowing the word corrosion, had discovered a way of making iron that resisted it almost completely, a level of metallurgical skill scientists still study today.

Hold a strip of magnesium ribbon in a pair of tongs and light one end, and it does not just catch fire, it erupts into a dazzling white flame, far brighter than an ordinary flame has any right to be, before settling into a soft white ash. That ash is magnesium oxide, formed the moment magnesium metal combines with the oxygen in the surrounding air. Stir a little of that white ash into warm water and test the resulting solution with litmus paper, and red litmus paper turns blue, the telltale sign of a basic solution. This is not a coincidence unique to magnesium: metal oxides, as a general rule, are basic in nature. Sodium reacts with air and water so violently that it is not even burned in a classroom demonstration; it is instead stored fully submerged in kerosene at all times, specifically to keep it away from the oxygen and moisture it would otherwise react with immediately and dangerously. Its oxide, like magnesium's, is basic too. A pattern is emerging: metals react with oxygen, and whatever oxide forms turns out to be basic.

Take powdered sulfur and burn it the same way magnesium was burned, and the differences start immediately. Sulfur does not flare into a brilliant white flame; it burns with a comparatively dull blue flame, giving off a gas, sulfur dioxide, that stings the nose. Dissolve that gas in water and test it with litmus paper, and this time blue litmus paper turns red: this solution is acidic, not basic. Non-metals, as a general rule, form acidic oxides, the exact opposite of the basic oxides metals produce. Try adding plain sulfur to water directly, without burning it first, and nothing happens at all, no reaction, no change; sulfur is simply unreactive with water on its own. Phosphorus, another non-metal, is the opposite in one specific way: it catches fire so readily on contact with ordinary air that it has to be stored underwater at all times, just as sodium is stored in kerosene, but for the exact opposite reason: sodium reacts with water, phosphorus barely tolerates plain air. Beyond how they react, non-metals are also generally soft and dull rather than hard and lustrous, and they are neither malleable, ductile nor sonorous; a lump of coal cannot be beaten into a sheet, drawn into a wire, or made to ring. They are also poor conductors of both heat and electricity. Metals and non-metals are, in fact, both sub-categories of a bigger idea: an element, a substance that cannot be broken down into anything simpler. Of the 118 known elements, some occur naturally and some are made only in laboratories, and none of them should be confused with everyday materials like plastic, glass or rubber, which are not elements at all, but rather combinations built out of elements.

It would be easy, after everything metals can do, gleam, bend into shape, carry heat and electricity, ring like a bell, to assume non-metals are the lesser, leftover half of this story. The opposite is closer to the truth. Oxygen, a non-metal, is the single gas every breath depends on; without it, survival is measured in minutes. Carbon, another non-metal, is the literal building block of every living thing, folded into the proteins, fats and carbohydrates that make up bodies, food and growth alike. Nitrogen, mostly inert in the air itself, becomes essential the moment it is converted into fertiliser, feeding the growth of the crops an entire country depends on. Chlorine purifies drinking water at a scale metals cannot match, and iodine, dabbed onto a cut as an antiseptic, is a non-metal doing quiet, everyday first aid. Metals, meanwhile, rarely work entirely alone either: alloys, mixtures of two or more metals, or a metal combined with a non-metal, are engineered to combine the best of both. Some specialised metals do highly specific jobs no ordinary metal could: zirconium withstands the extreme demands of atomic energy plants, titanium is trusted in aerospace for being both strong and light. And in India, iron and aluminium in particular are recycled at large scale, precisely because a metal's most valuable properties, its strength, its conductivity, its shine, survive being melted down and reshaped, unlike so many other materials that degrade the moment they are reused.