Elements are classified by the subshell into which the last (differentiating) electron enters: s-block (last electron in s), p-block (in p), d-block (in d) and f-block (in f). d-block elements are called transition elements because they lie between the s-block (metals) and p-block (non-metals) and show a gradual transition in properties; their atoms or common ions have partially filled d orbitals.

BF3: central B has 3 bond pairs and no lone pair -> trigonal planar, bond angle 120 deg. NH3: central N has 3 bond pairs and 1 lone pair -> pyramidal (trigonal pyramidal), bond angle about 107 deg. H2O: central O has 2 bond pairs and 2 lone pairs -> bent/angular, bond angle about 104.5 deg. Lone pairs repel more than bond pairs, reducing the angle from the ideal 109.5 deg in NH3 and H2O.

In ethyne each carbon is sp hybridised. The two sp hybrid orbitals on each carbon form sigma bonds: one C-C sigma bond and one C-H sigma bond, giving a linear molecule with a bond angle of 180 deg. The remaining two unhybridised p orbitals on each carbon overlap sideways to form two pi bonds, so there is a triple bond (1 sigma + 2 pi) between the carbon atoms.

H-C(triple bond)C-H