The Hardest and Softest Elements

Cesium has a Mohs hardness of about 0.2. Diamond has 10. Both are elements in their standard form, and the scale between them is so lopsided that the numbers actively mislead: the gap from corundum at 9 to diamond at 10 is larger, in any measurement that uses real units, than the whole of the rest of the scale put together.

But the sharper problem with "which element is hardest" is that the question has at least four meanings, and they do not have the same answer.

Four properties that all get called hardness

• Hardness proper is resistance to being scratched or indented — a surface property, measured ordinally by Mohs and quantitatively by Vickers or Knoop.
• Stiffness is resistance to being deformed elastically, expressed as a modulus. A stiff material springs back rather than bending.
• Strength is the stress at which a material yields or breaks, which is the number an engineer usually means.
• Toughness is resistance to a crack propagating. It is nearly the opposite of hardness, and the tradeoff between them shapes most of materials engineering.

Diamond is the hardest thing in the first sense and unimpressive in the fourth: it will scratch anything, and it will also cleave cleanly along a crystal plane if struck in the right direction, which is the entire basis of gem cutting. Osmium wins a different contest altogether — its bulk modulus is around 460 GPa against diamond's 440, which makes it arguably the least compressible element there is, and it is nonetheless brittle enough to be difficult to work at all. Being hard to squeeze and being hard to scratch turn out to be different achievements.

Carbon holds both records

The most useful fact in this whole subject is that the hardest element and one of the softest are the same element.

Carbon as diamond is a three-dimensional covalent lattice: each atom bonded to four neighbours in a tetrahedron, every bond short and strong, no weak direction anywhere. To scratch it you have to break covalent bonds, and there is no cheaper way through.

Carbon as graphite is the same atoms bonded in flat hexagonal sheets, three neighbours each, with the sheets held to one another by nothing but dispersion forces. Within a sheet, graphite is extraordinarily strong — stronger, bond for bond, than diamond. Between sheets it is so weak that the layers slide over each other under finger pressure, which is why graphite scores about 1 on Mohs, marks paper, and works as a dry lubricant.

Same element, same covalent bonding, opposite ends of the scale. The variable is geometry, and the full comparison of the two is worth reading on its own.

Second place among the elements goes to boron, at roughly 9.3 to 9.5 on Mohs, for a related reason: its icosahedral clusters are joined by a rigid covalent framework. Beyond those two, everything harder is a compound — boron carbide, boron nitride, tungsten carbide — and compounds get to combine bonding types that no single element can.

Among the metals, chromium wins and it is not close

The hardest pure metallic element is chromium, at about 8.5 on Mohs, which puts it above quartz and within reach of topaz. Nothing else metallic comes near.

The reason is in the bonding. In a metallic lattice, hardness tracks how many electrons each atom commits to the shared bonding and how strongly they are held. Chromium's configuration leaves it with an unusually generous set of unpaired d electrons available, and the resulting bond is both strong and partly directional — which also makes chromium brittle, because directional bonds do not tolerate the atomic-plane slippage that lets ductile metals deform instead of cracking.

That combination is why chromium is almost never used as a structural metal on its own and is enormously important as an alloying addition. A few per cent of it in steel raises hardness and wear resistance; more of it gives stainless steel its passivating oxide layer.

Tungsten is the other metal that dominates the strength conversation, but for a different property — it has the highest tensile strength of any pure metal, and the highest melting point, which is a separate consequence of the same strong d-orbital bonding. Its Mohs hardness is around 7.5, below chromium's. The familiar "tungsten is the hardest thing in the toolshop" belief is really about tungsten carbide, a compound with roughly twice the hardness of the metal. The practical differences between tungsten and titanium turn on exactly this kind of distinction.

The soft end is the alkali metals, and it is a straight line

Go to the other extreme and the pattern is far tidier. The soft elements are almost all in group 1, and they get softer going down:

• Lithium, around 0.6 on Mohs
• Sodium, around 0.5
• Potassium, around 0.4
• Rubidium, around 0.3
• Cesium, around 0.2 — the softest element that is solid at room temperature

The mechanism is the mirror image of chromium's. Each alkali metal atom contributes exactly one electron to the metallic bond, and as you descend the group the atoms get substantially larger, so that single electron is spread over more volume and shielded from the nucleus by more inner shells. One weakly held electron per big atom is about the feeblest metallic bond it is possible to construct.

Two elements sit oddly nearby. Gallium is soft and cuttable because its crystal structure is peculiar — it forms pairs of atoms rather than a conventional close-packed lattice, and that structure gives up at just under 30 °C — a melting point low enough that on a warm day gallium is not reliably a solid at all. Mercury has no hardness at all at room temperature, being liquid, and the reasons for that are genuinely strange.

The soft metals people actually handle

Most of the everyday soft metals are not alkali metals at all. Lead and tin sit around 1.5, gold and silver around 2.5, copper around 3. Their softness is why they were the first metals worked by human beings: they can be hammered into shape cold, without a furnace hot enough to melt iron.

Gold is the extreme case of ductility rather than of softness. A single gram can be beaten into a sheet of about a square metre, thin enough to transmit greenish light, because gold's atoms slip past each other along crystal planes almost indefinitely without the metal fracturing. That is toughness in the technical sense, coexisting with very low hardness — the two are independent, and gold proves it.

It comes down to how many bonds, and which way they point

There is no single hardest element, because hardness is not one property. Diamond wins on scratching, osmium on resisting compression, chromium among the metals, tungsten on tensile strength, and gold on refusing to break while being deformed.

What actually sets all of them is how many electrons hold each atom to its neighbours and in which directions. Four rigid covalent bonds per atom in three dimensions gives diamond. The same bonds in flat sheets with nothing much between them gives graphite. A crowd of d electrons in a metal gives chromium and tungsten. One lonely s electron on a very large atom, shielded by five full shells beneath it, gives cesium — a metal that yields under a pressure most people would not describe as pressure.