Case Study:

During a classroom experiment, students were asked to analyze the atomic radii of elements across Period 2: Li, Be, B, C, N, O, and F. Surprisingly, despite each successive element having one more electron than the previous, the measured atomic size was found to decrease steadily from lithium to fluorine.

Before diving into the radii data, it helps to have the order of these elements down pat. If you're still shaky on the sequence, check out these tricks to memorize the first 20 elements specifically designed for Class 10.

A student argued that since electrons are being added, atomic size should increase due to increased electron-electron repulsion. However, experimental data showed the opposite trend. The teacher then introduced the concept of effective nuclear charge.

Understanding why the table is arranged this way makes these trends much more logical. You can explore the evolution from Mendeleev to the Modern Periodic Table to see how scientists first decoded these patterns

As protons are added to the nucleus across a period, the nuclear charge increases. Although electrons are also added, they enter the same shell, resulting in minimal additional shielding.

Thus, the increased attraction between the nucleus and the electrons pulls the electron cloud closer, reducing the atomic radius. For example, lithium has a larger atomic size compared to fluorine, even though fluorine has more electrons.

This observation raised an important question: why does increased nuclear charge dominate over electron repulsion in determining atomic size across a period?

Mastering these trends is all about spotting the rhythm of the table. For more shortcuts, these mnemonics and patterns will help you predict atomic behavior instantly during an exam.

CASE-BASED QUESTIONS

MCQ

Q1.Which factor primarily causes the decrease in atomic size across a period?
A. Increase in number of shells
B. Increase in shielding effect
C. Increase in effective nuclear charge
D. Increase in electron-electron repulsion

Q2. Which element has the smallest atomic size in Period 2?
A. Lithium
B. Carbon
C. Oxygen
D. Fluorine

Assertion -  Reason

Q3. Assertion (A): Atomic size decreases from Li to F across Period 2.
Reason (R): Electrons are added to the same shell while nuclear charge increases.

A. Both A and R are true and R is the correct explanation
B. Both A and R are true but R is not the correct explanation
C. A is true but R is false
D. A is false but R is true

Application-Based

Q4. A student compares sodium (Na) and magnesium (Mg) in Period 3 and predicts their atomic sizes. Which element is expected to be smaller and why?

Q5. Explain why electron-electron repulsion does not increase atomic size significantly across a period.

Data/Logic-Based

Q6.  Given the following trend in atomic radii (pm):
Li (152) > Be (112) > B (85) > C (77) > N (75) > O (73) > F (72)
Identify the underlying trend and explain the reason for the sharp decrease between Li and B.

ANSWER KEY WITH EXPLANATION

A1. C. Increase in effective nuclear charge
       Increasing nuclear charge pulls electrons closer, reducing atomic size despite added electrons.

A2. D. Fluorine
       Fluorine has the highest nuclear charge in Period 2, leading to the smallest atomic radius.

A3. A. Both A and R are true and R is the correct explanation
       Electrons are added to the same shell while nuclear charge increases, causing size reduction.

A4. Magnesium is smaller
       Mg has higher nuclear charge than Na, pulling electrons closer within the same shell.

A5. Electrons are added to the same shell, so shielding remains nearly constant. Nuclear attraction dominates over repulsion.

A6. Trend: Decreasing atomic size across the period
      Reason: Increase in effective nuclear charge with minimal shielding; sharper drop due to stronger nuclear pull from Li to B.

Nailed these questions? Don't stop here. Consistent practice is the secret to a high score in Chemistry. Download our Class 10 Chemistry Worksheets or test your timing with these solved and unsolved practice papers to get exam-ready.

HOTS EXTENSION QUESTIONS

1. Predict how atomic size would change if electrons were added to a new shell instead of the same shell across a period. Justify your answer.

2. If shielding effect suddenly increased significantly across a period, how would the trend of atomic size change? Explain with reasoning.

Still have a nagging question about atomic trends? Post it on our Discuss Forum and let’s talk it out! You can also challenge yourself with our interactive Chemistry quizzes to see where you stand.

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