Ammonia has nitrogen in the center, three N–H single bonds and one lone pair on nitrogen. The four electron domains give a tetrahedral electron arrangement and a trigonal pyramidal molecular shape.
Nitrogen supplies five valence electrons and the three hydrogens supply one each. Three electron pairs form N–H bonds, leaving one pair on nitrogen. The structure uses all eight electrons and gives nitrogen a complete octet.
How to draw the NH3 Lewis structure
Count 8 valence electrons.
Use 5 from nitrogen and 3 from the hydrogen atoms.
Put nitrogen in the center.
Hydrogen is always terminal because it can form only one bond.
Connect three hydrogens.
Three N–H single bonds use 6 electrons.
Place the remaining pair.
The final 2 electrons become one lone pair on nitrogen.
Check octets and charges.
Nitrogen has eight electrons around it; each hydrogen has two. Formal charges are zero.
Formal charges
Nitrogen owns two nonbonding electrons and half of the six bonding electrons, for five assigned electrons. That matches neutral nitrogen's five valence electrons. Each hydrogen also has formal charge zero.
Geometry and polarity
The four electron domains around nitrogen are approximately tetrahedral. With one lone pair hidden from the molecular-shape name, NH3 is trigonal pyramidal. The lone pair compresses the H–N–H angles to about 107°, and the molecule is polar.
Common mistakes
Forgetting the lone pair on nitrogen.
Calling NH3 trigonal planar; the lone pair pushes the atoms out of a plane.
Drawing a double bond to hydrogen, which violates hydrogen's duet rule.
Using 10 electrons instead of the available 8.
Frequently asked questions
Why does NH3 have one lone pair?
After three N–H bonds use six electrons, two of the eight available electrons remain and form one lone pair on nitrogen.
Is NH3 polar?
Yes. Its trigonal pyramidal shape means the N–H bond dipoles do not cancel.
What is nitrogen's hybridization in NH3?
In the common valence-bond model, nitrogen is described as sp³ hybridized because it has four electron domains.