Lessons · Bonding & structure
Why water is bent: the electron ledger of a single molecule
Water is the most familiar molecule there is, and almost everyone first pictures it as a straight line: H–O–H. It isn't. Water is bent — and that bend is the reason it dissolves salt, climbs up plant stems, and boils at 100 °C instead of somewhere below −80 °C like a nonpolar molecule its size. We'll build H₂O one accounting step at a time — count its valence electrons, place them into bonds and lone pairs, read the shape off the electron domains, and let the shape decide the polarity — and watch where the 'straight line' picture breaks.
- 1
Count the valence electrons
machine-checkedH 2×1 = 2 + O 1×6 = 6 → 8 valence electrons
Start with the electrons you have to work with. Only the valence electrons — the outer shell — form bonds and lone pairs, so those are the only ones we count. Each hydrogen brings 1 (2 in all); oxygen brings 6. That is 8 valence electrons, or four pairs, to place. A tempting first mistake is to count all the electrons in the molecule — that would be 10, including oxygen's two inner (core) electrons — but core electrons are locked in tight and never bond. The engine derives this total from each atom's group in the periodic table and holds every later step to it.
- 2
Build the Lewis structure
machine-checked4 bonding (2 shared pairs) + 4 nonbonding (2 lone pairs) = 8 ✓
Atom Lone pairs Bonds (Σ order) Formal charge OO1 2 2 0 HH1 0 1 0 HH2 0 1 0 Now place the four pairs so every atom completes its shell. Two pairs become O–H bonds — one shared pair each, which is all a hydrogen needs (its complete shell is a duet, just 2 electrons). The remaining two pairs stay on oxygen as lone pairs. Count oxygen's shell: 2 bonding pairs + 2 lone pairs = 8 electrons — a full octet. Every atom's formal charge works out to 0 (oxygen: 6 valence − 4 lone-pair electrons − 2 bonding = 0), and they sum to 0, the neutral molecule. The ledger closes exactly: 4 bonding + 4 nonbonding = the 8 electrons we counted. ChemKernel refuses to emit any structure where that accounting doesn't balance — the same discipline that makes it refuse an unbalanced equation.
- 3
Predict the shape (VSEPR)
rule-sourced4 electron domains (2 bonding + 2 lone) → electron geometry tetrahedral, molecular shape bent.
ideal angle 109.5° — two lone pairs compress the actual angle to about 104.5°
This is the step the formula H₂O quietly hides. VSEPR predicts shape from the number of electron domains around the central atom — and a domain is any group of electrons that takes up space: a bond or a lone pair. Oxygen has four domains (2 bonds + 2 lone pairs), so they spread toward the corners of a tetrahedron (electron geometry: tetrahedral, ideal 109.5°). But you only see the atoms, and the atoms trace a bent line: the two lone pairs are invisible in the drawn shape yet they occupy space and press the two O–H bonds together, squeezing the real H–O–H angle down to about 104.5°. The lone pairs are the hidden variable — leave them out and you'd wrongly predict a straight line.
- 4
Decide the polarity
model-assumed- H–O ×2
Net: polar — The molecule is bent, so the two polar O–H bond dipoles (ΔEN 1.24) add rather than cancel — a net dipole points from the H side toward oxygen.
Shape now decides polarity. Each O–H bond is polar: oxygen is far more electronegative (ΔEN 1.24), so it pulls the shared electrons toward itself, and each bond becomes a little arrow pointing at oxygen. If water were linear the two arrows would point exactly opposite and cancel — which is precisely why CO₂, genuinely linear, is nonpolar despite two polar bonds. But water is bent, so the two arrows don't cancel; they add to a net dipole pointing from the two hydrogens toward oxygen. Water is polar, and that one fact carries an enormous amount of chemistry: the dipoles let water surround and pull apart ionic salts (dissolving them), tug on neighboring water molecules (hydrogen bonding — the high boiling point), and make water the medium nearly all of biochemistry runs in.
- ✓ Electrons conserved: 4 bonding + 4 nonbonding = 8 valence [electron ledger]
- ✓ Every atom completes its shell — octet (a duet for H) [octet check]
- ✓ Formal charges sum to the molecular charge (0) [formal-charge sum]
- ✓ Electron-domain count keys the sourced VSEPR geometry [VSEPR table]
The shape depends on electron domains, not just the atoms you can see. The central atom carries 2 lone pairs, and each one is an electron domain that takes up space — so there are 4 electron domains in all (2 bonding + 2 lone), the electron geometry is tetrahedral, and the visible atoms trace a bent shape (about 104.5°) — not a straight line. And because the molecule is bent, the bond dipoles do not cancel: the molecule is polar. Leave the lone pairs out and you predict the wrong shape and the wrong polarity.
Modeling assumptions — author-asserted, disclosed not discharged
- model The Lewis localized-pair model: the valence electrons sit either in shared bonding pairs between two atoms or in lone pairs on a single atom. It is a model of where electrons are — useful and predictive, not a photograph.
- model VSEPR: electron domains — bonds and lone pairs alike — arrange around the central atom to get as far apart as possible, and that arrangement sets the geometry. Lone pairs are treated as slightly 'larger' than bonds, which is why they compress the H–O–H angle below the ideal 109.5°.
- model Molecular polarity is the vector sum of the individual bond dipoles taken over the molecule's geometry. Here the bent shape leaves a net dipole; in a symmetric shape (like linear CO₂) equal bond dipoles cancel and the molecule is nonpolar even with polar bonds.
Concepts in this lesson
Linked into the Chemical Atlas where an entry exists; the rest fill in as the Atlas grows.
Practice this
The lesson goes deep on one scenario; the gym builds fluency by repetition. Drill these: