= Trigonal pyramidal molecular geometry
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In <trigonal pyramidal molecular geometry>, three neighbouring <chemical atoms> form a triangular base and the central <chemical atom> lies outside its plane. The pyramid has four <atomic nuclei>; its central <chemical atom> is the apex, not the centre of the base. <Ammonia>, $\mathrm{NH}_3$, has this arrangement. The https://cccbdb.nist.gov/exp2x.asp?casno=7664417&charge=0[NIST CCCBDB experimental ammonia entry, geometric data, citing Herzberg (1966)]{ref} lists an H–N–H <bond angle> of $106.67^\circ$, commonly rounded to $106.7^\circ$ or about $107^\circ$.
The <Lewis structure> of <ammonia> allocates eight <valence electrons> as three <bond pairs> and one <lone pair>. Four <electron domains> therefore suggest tetrahedral <electron pair geometry>; removing the <lone pair> marker leaves three <hydrogen> <chemical atoms> below <nitrogen>. The <VSEPR model> treats the <lone pair> as more space-demanding and predicts compression relative to $109.4712^\circ$, but it does not calculate the precise $106.67^\circ$ value. See https://openstax.org/books/chemistry-2e/pages/7-6-molecular-structure-and-polarity[Chemistry 2e, §7.6: lone-pair effects].
Three equal bond-dipole contributions in this pyramid cancel sideways, but retain a component along its axis. By contrast, three equal contributions lying in one plane at $120^\circ$ would cancel completely. This shows how a small change in <molecular geometry> can permit a permanent <electric dipole moment>. <Ammonia> is a <polar molecule>; NIST lists a measured magnitude of about $1.476$ debye in the same https://cccbdb.nist.gov/exp2x.asp?casno=7664417&charge=0[experimental entry, electric dipole moment, citing Shimizu (1970)]{ref}. The vector argument predicts allowed direction and cancellation, not the full measured magnitude.
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