All elements in the third period have

Options

A. Three complete shells
B. Three complete subshells
C. Three valence electrons
D. Three electrons less than octet .

Answer

To solve the question "All elements in the third period have," we need to analyze the options provided and understand the electronic configuration of the elements in the third period of the periodic table.

### Step-by-Step Solution:

1. **Identify the Elements in the Third Period:**
The elements in the third period are:
- Sodium (Na)
- Magnesium (Mg)
- Aluminium (Al)
- Silicon (Si)
- Phosphorus (P)
- Sulphur (S)
- Chlorine (Cl)
- Argon (Ar)

2. **Determine the Electron Configuration:**
The atomic numbers and electron configurations of these elements are:
- Sodium (Na) - Atomic number 11: 1s² 2s² 2p⁶ 3s¹
- Magnesium (Mg) - Atomic number 12: 1s² 2s² 2p⁶ 3s²
- Aluminium (Al) - Atomic number 13: 1s² 2s² 2p⁶ 3s² 3p¹
- Silicon (Si) - Atomic number 14: 1s² 2s² 2p⁶ 3s² 3p²
- Phosphorus (P) - Atomic number 15: 1s² 2s² 2p⁶ 3s² 3p³
- Sulphur (S) - Atomic number 16: 1s² 2s² 2p⁶ 3s² 3p⁴
- Chlorine (Cl) - Atomic number 17: 1s² 2s² 2p⁶ 3s² 3p⁵
- Argon (Ar) - Atomic number 18: 1s² 2s² 2p⁶ 3s² 3p⁶

3. **Analyze Each Option:**
- **Option A: 3 complete shells**
- The third period elements do not have three complete shells. They have two complete shells (1s² 2s² 2p⁶) and the third shell is being filled.
- **Conclusion:** This option is incorrect.

- **Option B: 3 complete subshells**
- In the third period, there are two complete shells (1s and 2s, 2p) and the third shell (3s) is partially filled. Thus, there are three complete subshells: 1s, 2s, and 2p.
- **Conclusion:** This option is correct.

- **Option C: 3 valence electrons**
- The number of valence electrons varies among the elements in the third period. For example, sodium has 1, magnesium has 2, aluminium has 3, and so on.
- **Conclusion:** This option is incorrect.

- **Option D: 3 electrons less than octet**
- This statement is misleading. The number of valence electrons varies, and not all elements have 3 electrons less than an octet.
- **Conclusion:** This option is incorrect.

4. **Final Answer:**
The correct answer is **Option B: 3 complete subshells**.