Which of the following species does not have electrons equal to 18?

Options

A. `K^(+)`
B. `Cl^(-)`
C. `Ca^(2+)`
D. K

Explanation

To determine which species does not have 18 electrons, we will analyze the given species based on their atomic numbers and charges.

### Step-by-Step Solution:

1. **Identify the Species**: We need to analyze the following species:
- K⁺ (Potassium ion)
- Cl⁻ (Chloride ion)
- Ca²⁺ (Calcium ion)
- K (Neutral Potassium atom)

2. **Determine the Atomic Numbers**:
- Potassium (K) has an atomic number of 19.
- Chlorine (Cl) has an atomic number of 17.
- Calcium (Ca) has an atomic number of 20.

3. **Calculate the Number of Electrons**:
- For K⁺:
- Atomic number = 19 (protons)
- Since it has a +1 charge, it has lost 1 electron:
- Electrons = 19 - 1 = 18.
- For Cl⁻:
- Atomic number = 17 (protons)
- Since it has a -1 charge, it has gained 1 electron:
- Electrons = 17 + 1 = 18.
- For Ca²⁺:
- Atomic number = 20 (protons)
- Since it has a +2 charge, it has lost 2 electrons:
- Electrons = 20 - 2 = 18.
- For K (neutral potassium):
- Atomic number = 19 (protons)
- As a neutral atom, it has the same number of electrons as protons:
- Electrons = 19.

4. **Conclusion**:
- K⁺ has 18 electrons.
- Cl⁻ has 18 electrons.
- Ca²⁺ has 18 electrons.
- K (neutral potassium) has 19 electrons.
- Therefore, the species that does not have 18 electrons is **K (neutral potassium)**.

### Final Answer:
The species that does not have electrons equal to 18 is **K (neutral potassium)**.