Explain giving reason why although the nuclear charge in atoms increases in moving from left to right in a period as well as in moving from top to bottom in a group in the Modern periodic table, but the size of the atoms does not vary similarly in both situations.
Explain giving reason why although the nuclear charge in atoms increases in moving from left to right in a period as well as in moving from top to bottom in a group in the Modern periodic table, but the size of the atoms does not vary similarly in both situations.
In the Modern Periodic Table, the variation of atomic size differs in periods and groups due to the following reasons:
- Along a Period (Left to Right):
As we move from left to right in a period, the nuclear charge (number of protons) increases, and electrons are added to the same valence shell. Because the shielding effect remains relatively constant, the effective nuclear charge increases. This stronger pull of the nucleus draws the outermost electrons closer, resulting in a decrease in atomic size.
- Down a Group (Top to Bottom):
As we move down a group, although the nuclear charge increases, a new electronic shell is added at each successive step. This increases the distance between the outermost shell and the nucleus. The effect of the addition of new shells outweighs the increased nuclear charge, resulting in an increase in atomic size.
Marking Scheme
- 11 Mark: Explanation of decrease in atomic size along a period due to increased effective nuclear charge pulling electrons closer in the same shell.
- 21 Mark: Explanation of increase in atomic size down a group due to the addition of new shells which outweighs the increased nuclear charge.
Hint
Think about how the number of shells changes down a group versus along a period, and how the nuclear pull affects the outermost electrons in each case.
Quick Oral Answer
Along a period, atomic size decreases because the effective nuclear charge increases, pulling the same shell closer. Down a group, atomic size increases because new shells are added, which increases the distance from the nucleus.
Analysis & Explanation
The atomic size (or atomic radius) is determined by the distance between the nucleus and the outermost shell. Along a period, the shell number remains constant (e.g., all elements in Period 2 have electrons in the and shells). Since the positive charge in the nucleus increases with each step, it exerts a stronger electrostatic pull on the valence electrons, shrinking the cloud. Down a group, each step represents the start of a completely new shell (e.g., ). This physical addition of a shell increases the atomic radius, shielding the outer electrons from the increased nuclear charge.
Common Mistakes
- 1Confusing nuclear charge with effective nuclear charge.
- 2Stating that atomic size increases along a period because the number of electrons increases.
- 3Forgetting to mention that down a group, the addition of new shells outweighs the increase in nuclear charge.
Interesting Facts
Helium is the smallest atom in the periodic table, while Cesium and Francium are among the largest.
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Frequently Asked Questions
How many marks does this question carry in CBSE Class 10 Science 2022?
This question carries 2 marks in the CBSE Class 10 Science 2022 examination.
Which chapter does this question come from in Science?
This question is from the chapter "Metals and Non-metals" in the CBSE Class 10 Science syllabus.
What topic does this question cover in Science?
This question covers the topic "Modern Periodic Table - Gradation in Properties" from CBSE Class 10 Science.
What type of question is this in the CBSE Class 10 Science 2022 paper?
This is a Short Answer question from Section A in the CBSE Class 10 Science 2022 paper.