= Tetrahedral molecular geometry
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In <tetrahedral molecular geometry>, four neighbouring <chemical atoms> surround a central <chemical atom> in the directions of the vertices of a <tetrahedron>. In the regular idealization, the central <atomic nucleus> lies at the centre of a <regular tetrahedron>, and every pair of <chemical bonds> makes the same <bond angle>. <Methane>, $\mathrm{CH}_4$, provides the standard example: its <carbon> has four bonding <electron domains> and no <lone pair>. See https://openstax.org/books/chemistry-2e/pages/7-6-molecular-structure-and-polarity[Chemistry 2e, §7.6: tetrahedral methane].
The exact ideal <bond angle> follows from four directions proportional to $(1,1,1)$, $(1,-1,-1)$, $(-1,1,-1)$, and $(-1,-1,1)$. Each has squared <Euclidean norm> $3$, and each distinct pair has <dot product> $-1$. Hence
$$
\theta=\arccos(-1/3)\approx109.4712^\circ.
$$
This is a statement about a <regular tetrahedron>, rather than a universal exact value for every <molecule> labelled tetrahedral. It also explains why a cross-shaped flat drawing with $90^\circ$ gaps is misleading: four equal directions can spread out more evenly in three dimensions.
The same <vectors> sum to zero. Consequently, four equal bond-dipole contributions along those directions cancel. The symmetric <methane> <molecule> is <nonpolar>, but a tetrahedral shape alone does not imply a <nonpolar molecule>: replacing one neighbour can make the electronic distribution and bond contributions unequal. <Chemical polarity> depends on both <molecular geometry> and <electron density>. See https://openstax.org/books/organic-chemistry/pages/2-2-polar-covalent-bonds-and-dipole-moments[McMurry, Organic Chemistry, §2.2: molecular dipoles and methane].
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