Three ways to hold on
Tap a salt crystal with a hammer and it shatters into smaller cubes. Tap a copper wire and it bends. Drag a diamond across glass and it leaves a scratch. All three are solids made of atoms packed in a regular pattern. What differs is the chemical bond: how the electrons that sit between atoms hold them together. Almost every bond is some mix of three simple kinds: ionic, metallic and covalent.
Ionic: give and take
Sodium holds its one outer electron loosely; chlorine pulls hard on electrons. When they meet, the electron moves across, leaving a positive sodium ion, Na⁺, and a negative chloride ion, Cl⁻. Opposite charges attract, so in salt every ion is surrounded by six of the other kind. There is no single partner and no shared pair: the whole crystal is held by the pull between all the opposite charges at once. That pull is strong. Salt does not melt until 801 °C.
It is also why salt is brittle. Slide one layer of ions along by one ion’s width and every ion now faces one with the same charge. Like charges repel, and the crystal splits cleanly along that plane. Solid salt does not conduct electricity either, because its ions are locked in place; melted or dissolved in water, the ions move freely and it conducts.
Metallic: a sea of electrons
A metal’s atoms each let go of one or two outer electrons, which no longer belong to any one atom. They roam through the whole crystal, a sea of shared electrons holding a lattice of positive ions together. Push a voltage across the metal and these electrons drift along it: that is an electric current, and it is why copper is one of the best conductors there is.
Slide one layer of copper ions over the next and nothing breaks. The ions move to new neighbours, and the sea of electrons simply flows around them and holds them there too. That is why metals bend, hammer flat into sheets and draw out into wires instead of shattering.
Try it: choose Ionic: salt and slide Slide the top half to 1 step, then choose Metallic: copper and do the same. Choose Atoms under Zoom to the atoms to see why one splits and the other only bends.
Covalent: sharing pairs
Carbon atoms neither give electrons away nor set them free; they share them. Each covalent bond is a pair of electrons held between two atoms, pointing in a fixed direction. In diamond every carbon shares a pair with each of four neighbours, at the corners of a tetrahedron, 0.154 nanometres away. The result is one giant molecule as big as the whole crystal.
To bend diamond you would have to stretch those bonds, and they do not stretch far. Push hard enough and they snap instead, so diamond is very hard but still brittle. Every electron is tied up in a bond, so diamond does not conduct electricity. Smaller molecules such as water use the same kind of bond, but only inside each molecule.
What this model leaves out
Real bonds are rarely purely one kind: the bond in water is covalent but lopsided, with the oxygen pulling the shared pair closer. The slide here moves a whole half of the crystal at once. Real metals bend by moving one line defect at a time, called a dislocation, which is why they bend at forces far below what this picture suggests. Real cracks also start at tiny flaws, and once open they stay open.