How do atomic radius vary in a period and in a group? How do you explain the variation?

🔷 Variation of Atomic Radius in a Period (Left to Right):

✅ Trend:
Atomic radius decreases across a period.

🧠 Why?

• As we move from left to right, electrons are added to the same shell.

• But at the same time, the nuclear charge (number of protons) increases.

• This increased attraction pulls the electron cloud closer to the nucleus, reducing the atomic size.

📌 Example:

• Na (Z=11) has a larger radius than Cl (Z=17), though both are in Period 3.

🔷 Variation of Atomic Radius in a Group (Top to Bottom):

✅ Trend:
Atomic radius increases down a group.

🧠 Why?

• As we move down a group, each element has one more electron shell than the one above.

• Though nuclear charge also increases, the effect of added shells (shielding effect) is greater.

• So, the outermost electrons are farther from the nucleus, making the atom larger.

📌 Example:

• Li (Z=3) has a smaller radius than Cs (Z=55), though both are in Group 1.

✅ Conclusion: