Molecular Structure (Ps 27)

A molecule will be trigonal bipyramidal when it has five electron pairs around the central atom. This molecular shape occurs when there are three bonding pairs and two lone pairs of electrons. The central atom is located at the center of a trigonal bipyramid, with the three bonding pairs forming a triangular base and the two lone pairs occupying the axial positions above and below the base.

HNO3, also known as nitric acid, contains a central nitrogen atom bonded to three oxygen atoms and one hydrogen atom. The nitrogen atom has a formal charge of +1 and forms a double bond with one of the oxygen atoms, while the other two oxygen atoms each have a single bond with the nitrogen atom. This arrangement can be represented by drawing two resonance structures, showing the double bond alternating between the nitrogen and different oxygen atoms. Resonance structures are needed to accurately describe the delocalization of electrons in the molecule.

ClF3 has more than one unshared pair of valence electrons on the central atom. This is because chlorine (Cl) has 7 valence electrons and forms 3 single bonds with fluorine (F) atoms, leaving 2 unshared pairs of electrons on the chlorine atom.

The correct answer is "I and III only". This is because a regular tetrahedron has four equal sides and four equal angles. In molecule I (CH4), the carbon atom is bonded to four hydrogen atoms, forming a regular tetrahedral shape. In ion III (NH4+), the nitrogen atom is bonded to four hydrogen atoms, also forming a regular tetrahedral shape. Molecule II (SF4) does not have a regular tetrahedral shape as it has a trigonal bipyramidal shape with one lone pair.

The species in the given set do not include the trigonal pyramidal geometry. The other geometries mentioned, such as square planar, tetrahedral, octahedral, and linear, can be found in the set. However, the trigonal pyramidal geometry is not present.