Which of the following are isoelectronic and isostructural? \(\mathrm{NO}_{3}^{-}, \mathrm{CO}_{3}^{2-}, \mathrm{ClO}_{3}^{-}, \mathrm{SO}_{3}\) (a) \(\mathrm{ClO}_{3}^{-}, \mathrm{CO}_{3}{ }^{2-}\) (b) \(\mathrm{CO}_{3}{ }^{2-}, \mathrm{SO}_{3}\) (c) \(\mathrm{NO}_{3}^{-}, \mathrm{CO}_{3}^{2-}\) (d) \(\mathrm{SO}_{3}, \mathrm{NO}_{3}^{-}\)

Understanding the shape of molecules is key to grasping concepts in chemistry, especially when discussing isoelectronic and isostructural species. Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. The spatial organization of atoms helps determine a molecule’s properties, like reactivity, polarity, and phase of matter at room temperature.

The geometry can vary greatly, from simple linear structures to complex three-dimensional figures. It's dictated by the number of electron pairs, both bonding and nonbonding, surrounding the central atom. VSEPR theory (Valence Shell Electron Pair Repulsion) is a model used to predict this geometry, based on the idea that electron groups around a central atom will repel each other and thus adopt the most stable, minimum-energy shape.

When comparing species to determine if they are isoelectronic, we look at their electron count. Isoelectronic means that two atoms, ions, or molecules have the same number of electrons, even if they are composed of different elements. To figure out the electron count, one must account for the electrons in each atom, as well as any additional charges. Charges can add or subtract from the total electron count: negatively charged ions have extra electrons while positively charged ions have fewer.

Electron count is not only crucial in establishing isoelectronicy but also plays a significant role in defining the chemical and physical properties of a species. The electron configuration impacts how atoms bond, their reactivity, and the magnetic properties of the compound.

The trigonal planar structure is a molecular geometry that results when a central atom forms bonds with three other atoms, which are usually located at the corners of an equilateral triangle, with the central atom in the same plane. This arrangement happens in species with a steric number of three, which indicates three regions of electron density (bonding or nonbonding pairs) around the central atom.

In such structures, usually, the central atom participates in sp2 hybridization, which will be explained further in the next section. Compounds with a trigonal planar molecular geometry are characterized by an angle of 120 degrees between the bonded atoms. Understanding this specific geometry provides insights into the molecule's reactivity and the nature of the bonds that hold the atoms together.

Sp2 hybridization occurs when one s orbital mixes with two p orbitals from the same shell of an atom to form three sp2 hybrid orbitals. This mixing, or hybridization, results in orbitals of equal energy that are arranged in a trigonal planar fashion about the central atom, leading to bond angles of approximately 120 degrees.

The process of sp2 hybridization is significant because it explains the bonding and the molecular geometry in planar triatomic species, like those mentioned in the exercise. When a molecule has sp2 hybridization, it features one unhybridized p orbital, which could be involved in pi (π) bonding if double or triple bonds are present. Recognizing when and how hybridization occurs allows chemists to predict molecular structure and bonding patterns, laying the foundation for understanding molecular reactivity and interactions.