Question

What is effective atomic number of Pt in $[\text{Pt}(\text{NH}_3)_4]^{2+}$ ? (Given atomic number of Pt = 78)

• A. 76
• B. 84
• C. 72
• D. 86

Hint:

To remember EAN easily, just think of it as a simple electron accounting ledger: start with the neutral atom's electrons ($Z$), subtract the electrons lost to form its ionic charge, and add all the incoming electrons handed over by the ligands!

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks us to compute the Effective Atomic Number (EAN) of the central metal Platinum (Pt) within the coordination complex ion $[\text{Pt}(\text{NH}_3)_4]^{2+}$.

Step 2: Key Formula or Approach:
The Effective Atomic Number (EAN) represents the total number of electrons surrounding a central metal atom or ion in a coordination compound. It is calculated using the standard formula:
$$\text{EAN} = Z - X + Y$$ Where:
$Z =$ Atomic number of the central metal atom.
$X =$ Oxidation state (charge) of the central metal ion.
$Y =$ Total number of electrons donated by the surrounding ligands.

Step 3: Detailed Explanation:
Let's determine the values for each parameter step-by-step:
1. The atomic number of Platinum is given as $Z = 78$.
2. To find the oxidation state ($X$) of Pt, let it be $x$. Ammine ($\text{NH}_3$) is a neutral molecule carrying a charge of 0. Equating the sum of charges to the overall complex charge:
$$x + 4(0) = +2 \implies x = +2$$ Therefore, the oxidation state is $X = 2$.
3. Each ammine ($\text{NH}_3$) ligand behaves as a monodentate Lewis base and donates exactly 1 lone pair (2 electrons) to form a coordinate covalent bond. Since there are 4 ammine ligands, the total number of donated electrons is:
$$Y = 4 \times 2 = 8$$ Now, substitute these derived values back into our main EAN equation:
$$\text{EAN} = 78 - 2 + 8$$ $$\text{EAN} = 76 + 8 = 84$$ This result matches perfectly with option (B).

Step 4: Final Answer:
The effective atomic number of Platinum in the complex is 84, which corresponds to option (B).