01Electron sea
Metals conduct and are malleable because delocalized electrons move freely and let layers slide without breaking bonds.
Substitutional alloys swap similar-sized atoms (brass: Cu + Zn). Interstitial alloys tuck small atoms into gaps (steel: Fe + C), making the metal harder and less malleable.
02Notebook box
Substitutional = similar radii. Interstitial = much smaller atoms in holes → stronger, less ductile.
Worked example: Why is steel harder than iron? Carbon in the gaps blocks layers from sliding.