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Wrong-sized atoms get in the way

Why Is Pure Gold Too Soft for a Ring?

Metals bend because their atoms slide in neat rows, and mixing in a different-sized atom jams the rows.

Pure metal Tidy rows slide easily— softAn alloy Odd atoms jam the rows— hard
A little impurity is the whole trick.

  1. Step 1 of 5

    Bending metal is rows of atoms sliding.

    1 Atoms in tidyrows2 A push arrives3 Rows slide overeach other

    When you bend a paperclip, the atoms do not stretch. Whole layers of them slip past each other, one row at a time, like a deck of cards shearing.

  2. Step 2 of 5

    A pure metal has nothing to stop the slide.

    A bar of pure gold Identical atoms, smoothrows

    Every atom is the same size, so the rows are smooth and slipping is easy. That is why pure gold, pure copper and pure iron are all disappointingly soft.

  3. Step 3 of 5

    A different-sized atom is a boulder in the road.

    A row trying to slideOdd-sized atoms blocking itThe push you are giving

    Drop in atoms of another metal and they do not fit the grid neatly. Now a sliding row hits a bump, and it takes far more force to move it.

  4. Step 4 of 5

    Almost every metal you meet is a mixture.

    Bronzecopper + about12% tinSteeliron + under 2%carbon18-karat gold75% gold, therest copper andsilver

    Bronze is copper with tin. Steel is iron with a little carbon. Jewellery gold is gold with copper and silver, and the karat number says how much.

  5. Step 5 of 5

    A tiny amount changes everything.

    Pure ironsoft — bends byhandMild steel — 0.2% carbonstructuralTool steel — 1% carbonhard enough tocut the others

    Steel is over 98% iron. That last sliver of carbon is the difference between a bar you can bend and a blade that will cut it.

    Bronze was such an improvement on copper that we named an age after it. It arrived around 3300 BCE, roughly two thousand years before anyone could get a furnace hot enough for iron.

The short version

Mixing a second metal in jams the tidy rows of atoms that let a pure metal bend, which is why alloys are so much stronger than what went into them.

Try it yourself

Bend a paperclip back and forth in one spot. It gets stiffer and then snaps — the sliding rows have tangled themselves solid.

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