Answer:
[1] sulphur trioxide
Explanation:
Isoelectronic species have the same number of valence electrons.
Valence electrons in nitrate (NO₃⁻):
5e- (N) + (3 x 6e-)(3xO) + 1e- (charge) = 24e-
Valence electrons in sulphur trioxide (SO₃):
6e- (S) + (3 x 6e-)(3xO) = 24e-
Valence electrons in sulphite (SO₃²⁻):
6e- (S) + (3 x 6e-)(3xO) + 2e- (charge) = 26e-
Valence electrons in phosphine (PH₃):
5e- (P) + (3 x 1e-)(3xH) = 8e-
Valence electrons in water (H₂O):
6e- (O) + (2 x 1e-)(2xH) = 8e-
Valence electrons in chlorite (ClO₂⁻):
7e- (Cl⁻) + (2 x 6e-)(2xO) + 1e- (charge) = 20e-
The only species isoelectronic with nitrate is sulphur trioxide. Both have trigonal planar geoemetry.
