Coordination numbers are a fundamental concept in chemistry used to describe the bonding environment of an atom. Specifically, they quantify the number of atoms directly bonded to a particular atom within a molecule or polyatomic ion. This number helps characterize the structural and bonding properties of chemical species.
Determining Coordination Numbers
Based on the provided reference, for molecules and polyatomic ions, the process for finding an atom's coordination number is straightforward:
- Identify the Atom of Interest: Choose the atom whose coordination number you want to determine. This is often the central atom in a complex ion or molecule.
- Count Bonded Atoms: Simply count the number of other atoms that are directly bonded to your atom of interest. This count includes bonds of any order (single, double, or triple).
It is important to note that you count the number of atoms attached, not the number of bonds if there are multiple bonds to the same atom.
Practical Application: Describing Bonding
The primary use of the coordination number, as demonstrated by the reference, is to describe the bonding environment around an atom. For example, an atom with a coordination number of 6 is often referred to as "hexacoordinate." This descriptor provides immediate information about how many neighbors the atom has.
Let's look at the example provided in the reference:
Example: The [Cr(NH3)2Cl2Br2]− Ion
Consider the polyatomic ion [Cr(NH3)2Cl2Br2]−.
- Central Atom: The central cation is Cr3+.
- Bonded Atoms: The chromium ion is bonded to:
- Two nitrogen (N) atoms from the two NH3 ligands.
- Two chlorine (Cl) atoms from the two Cl ligands.
- Two bromine (Br) atoms from the two Br ligands.
Following the determination method:
- Number of N atoms bonded to Cr = 2
- Number of Cl atoms bonded to Cr = 2
- Number of Br atoms bonded to Cr = 2
Total number of atoms bonded to Cr = 2 (N) + 2 (Cl) + 2 (Br) = 6.
Therefore, based on counting the bonded atoms, the coordination number of the Cr3+ ion in [Cr(NH3)2Cl2Br2]− is 6.
Significance in the Example
Having determined the coordination number is 6, the Cr3+ central cation in this ion is described as hexacoordinate. This description immediately tells chemists about the spatial arrangement and bonding pattern around the chromium atom, which is typically octahedral for a coordination number of 6.
In summary, coordination numbers are used by calculating the number of attached atoms and then employing this number to characterize and describe the structural environment of an atom within a chemical species.
