The correct order of atomic radius of Li, Na, Be and O is
Na > Li > Be > O
Understanding Atomic Radius Trends
The question asks us to determine the correct order of atomic radius for the elements Lithium (Li), Sodium (Na), Beryllium (Be), and Oxygen (O). To answer this, we need to consider how atomic size changes based on an element's position in the periodic table.
Periodic Trends Influencing Atomic Radius
Atomic radius is primarily influenced by two main trends in the periodic table:
- Trend Across a Period: Moving from left to right across a period, the atomic radius generally decreases. This is because the number of protons in the nucleus increases, leading to a greater effective nuclear charge. This stronger positive charge attracts the valence electrons more powerfully, pulling them closer to the nucleus and shrinking the atom.
- Trend Down a Group: Moving from top to bottom down a group, the atomic radius generally increases. This occurs because each element down a group has one additional electron shell compared to the element above it. The valence electrons are farther from the nucleus, and the shielding effect from inner electrons reduces the nucleus's attraction, resulting in a larger atomic size.
Analyzing the Elements Li, Na, Be, and O
Let's locate these elements in the periodic table to understand their relationships:
- Li (Lithium): Located in Period 2, Group 1.
- Na (Sodium): Located in Period 3, Group 1.
- Be (Beryllium): Located in Period 2, Group 2.
- O (Oxygen): Located in Period 2, Group 16.
Comparing Atomic Radii Step-by-Step
We can now compare the elements based on the periodic trends:
- Sodium (Na) vs. Lithium (Li): Both Na and Li belong to Group 1 (alkali metals). Na is in Period 3, directly below Li in Period 2. According to the trend down a group, atomic radius increases. Therefore, Na has a larger atomic radius than Li.
Comparison:
Na > Li - Lithium (Li) vs. Beryllium (Be): Both Li and Be are in the same period, Period 2. Li is in Group 1, and Be is in Group 2. Moving across a period from left to right, atomic radius decreases. Thus, Li has a larger atomic radius than Be.
Comparison:
Li > Be - Beryllium (Be) vs. Oxygen (O): Both Be and O are also in Period 2. Be is in Group 2, while O is significantly further across in Group 16. As we move across the period from Be to O, the atomic radius continues to decrease due to the increasing effective nuclear charge. Therefore, Be has a larger atomic radius than O.
Comparison:
Be > O
Combining the Comparisons to Find the Order
By combining the individual comparisons, we can establish the overall order of atomic radius:
- From the Na vs. Li comparison:
Na > Li - From the Li vs. Be comparison:
Li > Be - From the Be vs. O comparison:
Be > O
Linking these together gives us the final sequence in decreasing order of atomic radius:
Na > Li > Be > O
Final Conclusion
Sodium (Na) is the largest because it is in the third period, having an extra electron shell compared to Li, Be, and O, which are all in the second period. Within the second period, the atomic radius decreases from left to right. Lithium (Group 1) is larger than Beryllium (Group 2), and Beryllium is larger than Oxygen (Group 16). This confirms the order Na > Li > Be > O.
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