The `Z_("eff")` on the last electron in Sc is______. 

Answer

To find the effective nuclear charge (Z_eff) on the last electron in Scandium (Sc), we need to follow these steps:

### Step 1: Identify the Atomic Number (Z)
The atomic number (Z) of Scandium (Sc) is 21. This means that Scandium has 21 protons in its nucleus.

**Hint:** The atomic number can be found on the periodic table and represents the number of protons in the element.

### Step 2: Determine the Electron Configuration
The electron configuration of Scandium is:
\[ \text{Sc: } 1s^2 2s^2 2p^6 3s^2 3p^6 3d^1 4s^2 \]
This shows that Scandium has a total of 21 electrons.

**Hint:** The electron configuration helps in visualizing how electrons are distributed among the various energy levels and subshells.

### Step 3: Calculate the Shielding Constant (σ)
To find the effective nuclear charge (Z_eff), we need to determine the shielding constant (σ). In general, for transition metals like Scandium, we can estimate σ based on the number of inner electrons that shield the outermost electrons.

For Scandium:
- The inner electrons are those in the 1s, 2s, 2p, 3s, and 3p orbitals.
- This gives us a total of 18 inner electrons (2 from 1s, 2 from 2s, 6 from 2p, 2 from 3s, and 6 from 3p).

Thus, we can estimate σ ≈ 18.

**Hint:** The shielding constant is an estimate of how much the inner electrons shield the outer electrons from the full charge of the nucleus.

### Step 4: Calculate Z_eff
Now we can calculate the effective nuclear charge (Z_eff) using the formula:
\[ Z_{\text{eff}} = Z - \sigma \]
Substituting the values we have:
\[ Z_{\text{eff}} = 21 - 18 = 3 \]

### Final Answer
The effective nuclear charge (Z_eff) on the last electron in Scandium is **3**.

---