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 characteristics of the elements in the third period of the periodic table.

### Step-by-Step Solution:

1. **Identify the Elements in the Third Period**:
The third period of the periodic table includes the following elements: Sodium (Na), Magnesium (Mg), Aluminum (Al), Silicon (Si), Phosphorus (P), Sulfur (S), Chlorine (Cl), and Argon (Ar).

2. **Determine the Number of Electron Shells**:
All elements in the third period have their outermost electrons in the third shell (n=3). This means they all have three electron shells.

3. **Evaluate the Subshells**:
In the third period, the electrons occupy the following subshells:
- The 3s subshell (which can hold a maximum of 2 electrons)
- The 3p subshell (which can hold a maximum of 6 electrons)
- The 3d subshell (which is not filled in the third period but is present)

Thus, all elements in the third period have three subshells: 3s, 3p, and 3d.

4. **Assess the Valence Electrons**:
The number of valence electrons varies among the elements in the third period:
- Sodium (1 valence electron)
- Magnesium (2 valence electrons)
- Aluminum (3 valence electrons)
- Silicon (4 valence electrons)
- Phosphorus (5 valence electrons)
- Sulfur (6 valence electrons)
- Chlorine (7 valence electrons)
- Argon (8 valence electrons)

Therefore, not all elements have three valence electrons.

5. **Consider the Octet Rule**:
The octet rule states that atoms tend to have eight electrons in their valence shell. Since the number of valence electrons varies, it is incorrect to say that all elements in the third period have three electrons less than an octet.

6. **Conclusion**:
The correct answer to the question is that all elements in the third period have **three complete subshells** (3s, 3p, and 3d).

### Final Answer:
All elements in the third period have **3 complete subshells**.

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