The correct order of atomic radius of Li, Na, Be and O is 

The correct answer is Na > Li > Be > O.

Key PointsAtomic Radius Trends on the Periodic Table

• The atomic radius refers to the size of an atom, indicating the distance from the center of the nucleus to the boundary of the surrounding cloud of electrons.
• As we move down a group in the periodic table, the atomic radius increases. This is because each successive element has an additional energy level, which places the outermost electrons further from the nucleus.
• As we move across a period from left to right, the atomic radius decreases. Despite the addition of more electrons, the increasing positive charge in the nucleus pulls the electron cloud closer to the nucleus.
• The elements given, Li (Lithium), Na (Sodium), Be (Beryllium), and O (Oxygen), belong to the second and third periods of the periodic table. Lithium and Beryllium are in the second period, while Sodium is in the third period, and Oxygen is also in the second period but towards the right side.
• Given these trends, Sodium (Na), being in the third period and the farthest left among the given elements, has the largest atomic radius. Following Sodium, Lithium (Li), which is in the same group but in the second period, has the next largest atomic radius.
• Beryllium (Be), despite being in the second period like Lithium, has a smaller atomic radius than Lithium because it is to the right of Lithium in the periodic table.
• Oxygen (O), being far to the right in the second period, has the smallest atomic radius among the given elements.
• Hence, the correct order of atomic radius from largest to smallest is Na > Li > Be > O.

Additional Information

• The concept of atomic radius is crucial in understanding the chemical behavior of elements. For instance, elements with larger atomic radii tend to be more reactive in the alkali metals group.
• The trend of atomic radius also plays a significant role in the ionization energy and electronegativity of elements. Generally, as the atomic radius decreases, the ionization energy increases, and the element becomes more electronegative.
• This knowledge is applied in various scientific fields, including chemistry, physics, and materials science, to predict and explain the behavior of elements and compounds.
• Understanding the periodic trends, including atomic radius, helps in the systematic study of elements and their properties, making it easier to predict the characteristics of unknown or newly discovered elements.