What is Coordination Number?

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Grade Level:

Class 12

AI/ML, Physics, Biotechnology, FinTech, EVs, Space Technology, Climate Science, Blockchain, Medicine, Engineering, Law, Economics

Definition

What is it?

Coordination number tells us how many atoms, ions, or molecules are directly connected to a central atom or ion in a chemical structure. It essentially counts the number of 'neighbors' a central particle has.

Simple Example

Quick Example

Imagine you are the 'central' player in a kabaddi team. Your coordination number would be the number of other players you are directly holding hands with or linked to at that moment. If you are holding hands with 4 other players, your coordination number is 4.

Worked Example

Step-by-Step

Let's find the coordination number of the central metal ion in [Cu(NH3)4]SO4.
---Step 1: Identify the central atom or ion. Here, the central atom is Copper (Cu).
---Step 2: Identify the ligands (the atoms/molecules directly attached to the central atom). Here, NH3 (ammonia) molecules are attached to Cu.
---Step 3: Count how many ligands are directly attached. There are 4 NH3 molecules attached to Cu.
---Step 4: The number of ligands directly attached is the coordination number. So, the coordination number of Cu in [Cu(NH3)4]SO4 is 4.
---Answer: The coordination number is 4.

Why It Matters

Understanding coordination number helps scientists design new materials with specific properties, like catalysts for chemical reactions or better batteries for EVs. It's crucial for engineers developing new drugs in medicine and for chemists creating advanced materials for space technology.

Common Mistakes

MISTAKE: Counting all atoms in the compound, not just those directly bonded to the central atom. | CORRECTION: Only count the atoms or groups (ligands) that are directly attached to the central atom/ion.

MISTAKE: Confusing the oxidation state of the central atom with its coordination number. | CORRECTION: Oxidation state is about charge, while coordination number is about the number of bonds. They are different concepts, though sometimes related.

MISTAKE: Assuming coordination number is always 4 or 6. | CORRECTION: Coordination number can vary widely (e.g., 2, 3, 5, 7, 8) depending on the central atom and the ligands involved.

Practice Questions

Try It Yourself

QUESTION: What is the coordination number of Cobalt (Co) in [Co(en)2Cl2]Cl? (Note: 'en' is ethylenediamine, which is a bidentate ligand, meaning each 'en' molecule attaches at two points). | ANSWER: 6 (2 'en' ligands x 2 attachment points each = 4, plus 2 Cl ligands x 1 attachment point each = 2. Total = 4+2=6)

QUESTION: In a simple cubic unit cell, what is the coordination number of an atom at a corner? | ANSWER: 6 (It is touched by 1 atom in its own cell, 1 in the cell above, 1 in the cell below, and 3 in adjacent cells on the same plane.)

QUESTION: A complex ion has a central metal M. It is bonded to three water (H2O) molecules and two chloride (Cl-) ions. What is the coordination number of M? | ANSWER: 5 (3 H2O + 2 Cl- = 5 ligands directly bonded)

MCQ

Quick Quiz

What is the coordination number of the central metal ion in [Ag(NH3)2]Cl?

The Correct Answer Is:

In [Ag(NH3)2]Cl, the central metal ion is Ag (Silver). It is directly bonded to two NH3 (ammonia) molecules. Therefore, its coordination number is 2.

Real World Connection

In the Real World

In medicine, coordination number is key to how certain drugs, especially chemotherapy drugs, bind to target molecules in the body. For example, some anti-cancer drugs like cisplatin work by binding to DNA, and their effectiveness depends on their coordination number and how they interact with the DNA structure.

Key Vocabulary

Key Terms

LIGAND: An ion or molecule that binds to a central metal atom to form a coordination complex. | CENTRAL ATOM: The atom (usually a metal) in a coordination complex to which ligands are attached. | COMPLEX ION: A charged species consisting of a central metal ion bonded to one or more ligands. | CHELATE: A ligand that binds to a central metal atom at more than one point.

What's Next

What to Learn Next

Now that you understand coordination number, you're ready to explore 'Isomerism in Coordination Compounds'. This will show you how different arrangements of ligands around the central atom can lead to completely different properties, just like how different ingredients make different dishes!