Why atoms stick together
Atoms bond to fill their outer shell — the octet rule. There are three main routes: transfer electrons (ionic), share them (covalent), or pool them in a sea of free electrons (metallic). The route an atom takes depends on its electronegativity — its pull on bonding electrons.
Ionic — transfer
A metal hands its outer electrons to a nonmetal. Both end up with full shells but become charged ions: Na → Na⁺, Cl → Cl⁻. Opposite charges lock into a rigid 3-D lattice. Hallmarks: high melting point, brittle, conducts electricity only when molten or dissolved.
Covalent — share
Two nonmetals share pairs of electrons. Nonpolar if the atoms pull equally (EN diff < 0.4, e.g. O₂); polar if one pulls harder (0.4–1.7, e.g. H₂O). Discrete molecules; lower melting points; usually don't conduct.
Metallic — pool
Metal atoms release outer electrons into a delocalized "sea" while their positive cores stay packed in a lattice. Free electrons explain why metals conduct, shine, bend without breaking, and transfer heat fast.
Intermolecular forces
Between separate molecules, weaker attractions act: London dispersion (all molecules), dipole–dipole (polar ones), and hydrogen bonds (H bonded to N, O or F). These set boiling points, surface tension and why ice floats.
Bond type by electronegativity difference
Linus Pauling's rule of thumb on the EN scale (his own 0.7–4.0 scale). Real bonds blend types — these cutoffs guide first-pass classification.
Property comparison
| Property | Ionic | Covalent | Metallic |
|---|---|---|---|
| Particles | + and − ions in lattice | Discrete molecules | Cations in electron sea |
| Melting point | High (600–3000 °C) | Low to moderate | Variable (Hg liquid, W > 3400 °C) |
| Electrical conductivity | Only when molten/dissolved | Usually no | Yes — solid or liquid |
| Hardness | Hard but brittle | Soft to hard | Malleable, ductile |
| Solubility in water | Many soluble | Polar in polar; nonpolar in nonpolar | Insoluble (most) |
| Examples | NaCl, MgO, CaF₂ | H₂O, CO₂, CH₄, O₂ | Cu, Fe, Al, brass |
Hands-on tools
Predict bonds, draw Lewis structures, peek inside a salt crystal, watch the electron sea.
Bond-type predictor
Pick two elements. The electronegativity difference predicts the bond type.
Pick element 1.
Lewis dot structures
Each dot is a valence electron. Pairs around an atom (bonded or lone) add up to 8 — H to 2.
3D molecular shapes
VSEPR theory says electron pairs around a central atom repel each other. Click and drag the molecule to rotate. The shape badge tells you what geometry results.
The seven canonical VSEPR shapes
Sodium chloride lattice
In NaCl, each Na⁺ is surrounded by 6 Cl⁻ and vice versa — face-centered cubic. Dragging shifts the view.
Lattice energy of NaCl: −787 kJ/mol — that's why it melts at 801 °C.
Sea of electrons
Cations sit on a regular grid. Electrons drift between them — that drift is the current you see in copper wire.
Quiz
Flashcards
Tap a card to flip. ← / → keys to navigate.
Daily challenge
Five bond-type calls. Same set worldwide, refreshes at midnight UTC. One shot per day.
For teachers
Print-ready worksheet, answer key, teaching tips and standards alignment.
Teaching tips
Standards alignment
Reference
Glossary
Photo gallery
Images sourced from Wikipedia.