H2O Lewis structure, worked step by step
Water, H2O: a bent, polar molecule. Every step below is worked from the atoms up; the diagram is drawn from coordinates, not copied from anywhere.
1. Count the valence electrons
Each hydrogen brings 1 valence electron, and oxygen, in group 16, brings 6. With 2 hydrogens, the total is 2 × 1 + 6 = 8 valence electrons to place.
2. Draw the structure
Oxygen sits in the middle with a single bond to each hydrogen. Each single bond uses 2 electrons, so the 2 bonds use 4 of the 8 electrons. The remaining 4 electrons sit on oxygen as 2 lone pairs, shown as the dot pairs in the diagram. Hydrogen keeps no lone pairs; its single electron is entirely in the bond.
Formal charge on oxygen: 6 valence electrons − 4 non-bonding − 2 (half of 4 bonding electrons) = 0. Formal charge on each hydrogen: 1 valence electron − 0 non-bonding − 1 (half of 2 bonding electrons) = 0. No charges to mark.
3. VSEPR shape and bond angle
Oxygen has 2 bonding domains and 2 lone pairs, 4 electron domains in total, so it is AX2E2 in VSEPR terms. Four electron domains arrange themselves toward the corners of a tetrahedron, an idealized angle of 109.5° between domains (this is plain geometry, arccos(−1/3), not a looked-up value). derived by rule Because 2 of those 4 corners are lone pairs rather than atoms, the shape traced by the atoms themselves is bent, not tetrahedral.
Lone pairs repel more strongly than bonding pairs do, so they push the two O–H bonds closer together than the idealized 109.5° figure. This site does not print a specific measured H–O–H angle here, because a precise experimental value is not part of the two sources (NIST, PubChem) this build draws numbers from — only the qualitative direction (smaller than the idealized angle) follows from the rule itself.
4. Is it polar?
Each O–H bond is polar: oxygen's Pauling electronegativity is 3.44 and hydrogen's is 2.2, a difference of 1.24. calculated Because the molecule is bent rather than linear, the two O–H bond dipoles do not point in opposite directions, so they do not cancel; they add up to a net dipole pointing from between the hydrogens toward the oxygen. The molecule as a whole is polar.
5. Molar mass, atom by atom
NIST's standard atomic weight for hydrogen is the interval [1.00784,1.00811]; this site takes the midpoint of that interval, rounded to three decimal places, as hydrogen's conventional atomic weight: 1.008. calculated For oxygen, NIST's interval is [15.99903,15.99977], with midpoint (rounded the same way) 15.999. calculated
Adding two hydrogens and one oxygen: 2 × 1.008 + 15.999 = 18.015 g/mol. calculated PubChem's own computed molecular weight for water is 18.015 g/mol, from its Computed Properties section — the two independent routes, NIST atomic weights summed by hand and PubChem's own computed value, agree.