The incorrect decreasing order of atomic radii is:
Show Hint
\( Mg>Al>C>O \)
\( Al>B>N>F \)
\( Be>Mg>Al>Si \)
\( Si>P>Cl>F \)
The Correct Option is C
Approach Solution - 1
The atomic radius generally decreases across a period (from left to right) and increases down a group.
(1) Mg > Al > C > O:
Mg and Al are in Period 3 (Mg is in Group 2, Al is in Group 13). Mg has a larger atomic radius than Al due to the general trend of decreasing radius across a period.
C and O are in Period 2 (C is in Group 14, O is in Group 16). C has a larger atomic radius than O for the same reason.
However, comparing elements from different periods (e.g., Mg/Al with C/O) violates periodic trends because atomic size increases down a group. Hence, this order is not entirely correct.
(2) Al > B > N > F:
Boron (B) and Aluminum (Al) are in Group 13, with B in Period 2 and Al in Period 3. Al has a larger radius than B due to being in a lower period.
Nitrogen (N) and Fluorine (F) are in Period 2. Radius decreases from B to N to F across the period. So this sequence follows the trend correctly within their groups and periods.
This sequence is correct.
(3) Be > Mg > Al > Si:
Beryllium (Be) is in Period 2, Group 2.
Magnesium (Mg) is in Period 3, Group 2 — so Mg > Be (down a group, size increases).
Aluminum (Al) is in Period 3, Group 13 — so Mg > Al (across period, size decreases).
Silicon (Si) is in Period 3, Group 14 — so Al > Si (again, across period, size decreases).
This sequence follows the periodic trend correctly.
Conclusion:
Among the given options, the correct order of atomic radii based on periodic trends is Be > Mg > Al > Si.
Final Answer:
The final answer is $ Be > Mg > Al > Si $.
Approach Solution -2
We need to identify the incorrect decreasing order of atomic radii among the given elements: Be, Mg, Al, and Si.
Step 2: Recall the trend of atomic radius.
- Across a period (left to right), atomic radius decreases because the nuclear charge increases, pulling electrons closer to the nucleus.
- Down a group, atomic radius increases because new shells are added, increasing the distance between the outermost electrons and the nucleus.
Step 3: Arrange the given elements according to their positions in the periodic table.
All given elements are in Period 2 and Period 3:
- Be (Group 2, Period 2)
- Mg (Group 2, Period 3)
- Al (Group 13, Period 3)
- Si (Group 14, Period 3)
Step 4: Analyze the correct order of atomic radii.
- Down a group (Be → Mg): atomic radius increases ⇒ Mg > Be.
- Across a period (Mg → Al → Si): atomic radius decreases ⇒ Mg > Al > Si.
Hence, the correct decreasing order of atomic radii should be:
\[ Mg > Be > Al > Si \]
Step 5: Identify the incorrect order.
The given incorrect order is:
\[ Be > Mg > Al > Si \] This is incorrect because Be (Period 2) should have a smaller radius than Mg (Period 3).
Step 6: Conclusion.
The incorrect decreasing order of atomic radii is:
\[ \boxed{Be > Mg > Al > Si} \]
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