📘 Detailed Explanation
🔵 Trend Across a Period
The atomic size generally decreases from left to right across a period in the periodic table.
The main reason is that the number of protons in the nucleus increases while the newly added electrons enter the same principal energy level.
🟣 Main Reason: Increase in Effective Nuclear Charge
As we move from left to right, the atomic number increases. Therefore, the number of protons increases.
At the same time, the additional electrons are added to the same shell. The shielding effect does not increase enough to cancel the increasing positive charge of the nucleus.
🟢 What Happens to the Electrons?
| Quantity | Left → Right |
|---|---|
| Atomic number | Increases |
| Number of protons | Increases |
| Electrons in same shell | Increase |
| Effective nuclear charge | Increases |
| Attraction between nucleus and electrons | Increases |
| Atomic radius | Decreases |
⚡ Example: Period 3
Consider the third period:
The nuclear charge continuously increases from Na to Ar, while electrons are added to the same third shell. Hence, the electron cloud is pulled closer to the nucleus and the atomic radius generally decreases.
✅ Final Answer
Atomic size decreases from left to right across a period because the nuclear charge increases while electrons are added to the same principal energy level. The increased effective nuclear charge attracts the electrons more strongly towards the nucleus, thereby decreasing the atomic radius.
📝 20 MCQs with Answers
1. Atomic size generally ______ from left to right across a period.
✅ Answer
B) Decreases
2. The main reason for the decrease in atomic size across a period is:
✅ Answer
B) Increase in effective nuclear charge
3. From left to right across a period, the number of protons:
✅ Answer
B) Increases
4. The added electrons across a period generally enter:
✅ Answer
B) The same principal energy level
5. Effective nuclear charge across a period generally:
✅ Answer
B) Increases
6. Greater effective nuclear charge causes stronger:
✅ Answer
A) Nuclear-electron attraction
7. Which element generally has a smaller atomic radius?
✅ Answer
B) Cl
8. Atomic size is usually measured using:
✅ Answer
A) Atomic radius
9. Which period shows the sequence Na → Ar?
✅ Answer
C) Period 3
10. From Na to Cl, atomic radius generally:
✅ Answer
B) Decreases
11. Which factor does NOT increase significantly across a period?
✅ Answer
C) Number of principal shells
12. Across a period, shielding effect due to inner shells:
✅ Answer
A) Changes only slightly
13. Which statement is correct?
✅ Answer
B)
14. The attraction between nucleus and valence electrons across a period:
✅ Answer
A) Generally increases
15. Which has a larger atomic size?
✅ Answer
A) Na
16. What happens to the electron cloud across a period?
✅ Answer
A)
17. Which quantity increases from Na to Cl?
✅ Answer
B) Effective nuclear charge
18. The general periodic trend of atomic radius across a period is:
✅ Answer
B)
19. Which is responsible for pulling electrons closer to the nucleus?
✅ Answer
A)
20. The correct relationship is:
✅ Answer
B)
Q. Why does atomic size decrease from left to right across a period?
Solution: Atomic size decreases because the nuclear charge increases while electrons are added to the same shell. The increased effective nuclear charge attracts the electrons more strongly towards the nucleus.
Q. Explain the decrease in atomic radius across a period.
Solution:
- The number of protons increases from left to right.
- Electrons are added to the same principal energy level.
- Effective nuclear charge increases, pulling the electrons closer to the nucleus.
Q. Explain the periodic trend of atomic size across a period.
| Property | Left → Right |
|---|---|
| Atomic number | Increases |
| Number of protons | Increases |
| Principal energy levels | Same |
| Effective nuclear charge | Increases |
| Attraction on electrons | Increases |
| Atomic radius | Decreases |
Q. Discuss the factors responsible for the decrease in atomic size from left to right across a period.
Solution:
- Increasing atomic number: The number of protons increases successively.
- Same principal shell: The added electrons enter the same main energy level.
- Effective nuclear charge increases: The increased nuclear charge is not completely cancelled by shielding.
- Stronger attraction: The nucleus attracts the electrons more strongly.
- Atomic radius decreases: The electron cloud is pulled closer to the nucleus.
Left → Right across a period:
Effective Nuclear Charge ↑ → Atomic Radius ↓
Q. Explain in detail why atomic size decreases from left to right across a period. Give a suitable example.
Detailed Solution:
Atomic size refers to the distance from the nucleus to the outermost occupied electron shell. Across a period, atomic size generally decreases from left to right.
The main reason is the progressive increase in effective nuclear charge. As we move from left to right, the atomic number increases, so the number of protons in the nucleus increases.
At the same time, the newly added electrons enter the same principal energy level. Therefore, the shielding effect does not increase enough to offset the increasing positive charge of the nucleus.
| Factor | Trend Left → Right |
|---|---|
| Atomic number | ↑ |
| Protons | ↑ |
| New principal shell | Not added |
| Effective nuclear charge | ↑ |
| Nuclear attraction | ↑ |
| Atomic radius | ↓ |
Example: Third Period
From Na to Cl, the nuclear charge increases continuously while the electrons are added to the same third shell. Therefore, the effective nuclear attraction increases and the atomic radius generally decreases.
The atomic size decreases from left to right across a period because the number of protons increases while the added electrons enter the same principal energy level. Thus, effective nuclear charge increases, causing stronger attraction between the nucleus and the electrons. Consequently, the electron cloud is pulled closer to the nucleus and the atomic radius decreases.
🎯 Quick Revision
Atomic Number ↑
Protons ↑
Same Principal Shell
Effective Nuclear Charge ↑
Nuclear Attraction ↑
Atomic Radius ↓
