Periodicity is a fundamental concept in OCR A-Level Chemistry focusing on the trends and properties of elements in the periodic table. Understanding periodicity is essential for answering exam questions accurately and achieving high marks.
This topic will cover:
- The importance of periodicity in chemistry
- Common exam questions and their answers
- Key trends across periods and groups
- Tips for scoring well in periodicity questions
What is Periodicity?
Periodicity refers to the repeating trends in the physical and chemical properties of elements as you move across a period or down a group in the periodic table. These trends occur due to the arrangement of electrons and changes in atomic structure.
Key periodic trends include:
- Atomic radius
- Ionization energy
- Electronegativity
- Melting and boiling points
- Electrical conductivity
Understanding these trends will help you analyze and predict element properties—a crucial skill for exams.
Common OCR A-Level Chemistry Periodicity Exam Questions
1. Atomic Radius Trend Across a Period
Question:
Describe and explain the trend in atomic radius across Period 3 of the periodic table.
Answer:
- Trend: Atomic radius decreases across Period 3.
- Explanation:
- As you move from sodium (Na) to argon (Ar) the number of protons increases leading to a higher nuclear charge.
- Electrons are added to the same energy level so shielding remains relatively constant.
- Stronger attraction between nucleus and outer electrons pulls the electron cloud closer reducing atomic size.
2. Ionization Energy Trends
Question:
Explain why the first ionization energy generally increases across a period but shows a dip at magnesium (Mg) and aluminum (Al).
Answer:
-
Trend: Ionization energy generally increases across a period because:
- Nuclear charge increases attracting electrons more strongly.
- Atomic radius decreases making it harder to remove an electron.
-
Dip at Aluminum (Al):
- Mg: 1s² 2s² 2p⁶ 3s²
- Al: 1s² 2s² 2p⁶ 3s² 3p¹
- The outer electron in Al is in the higher-energy 3p orbital which is further from the nucleus and easier to remove causing a lower ionization energy.
3. Electronegativity Across a Period
Question:
Why does electronegativity increase across a period?
Answer:
-
Trend: Electronegativity increases from left to right across a period.
-
Explanation:
- Nuclear charge increases pulling bonding electrons closer.
- Atomic radius decreases strengthening the attraction for shared electrons.
-
Example:
- Sodium (Na): Low electronegativity (willing to lose electrons).
- Chlorine (Cl): High electronegativity (strongly attracts electrons in bonds).
4. Melting and Boiling Points in Period 3
Question:
Describe and explain the trend in melting and boiling points across Period 3.
Answer:
-
Metals (Na Mg Al):
- High melting points due to strong metallic bonding.
- Aluminum (Al) has the highest melting point among them due to more delocalized electrons.
-
Silicon (Si):
- Very high melting point due to its giant covalent structure (strong Si-Si bonds).
-
Non-metals (P S Cl Ar):
- Lower melting points as they exist as simple molecules with weak Van der Waals forces.
- Sulfur (S₈) has a higher melting point than Phosphorus (P₄) because larger molecules have stronger intermolecular forces.
5. Electrical Conductivity Across a Period
Question:
Why does electrical conductivity decrease across Period 3?
Answer:
- Metals (Na Mg Al):
- Good conductors due to free-moving delocalized electrons.
- Silicon (Si):
- Semiconductor—conductivity increases with temperature.
- Non-metals (P S Cl Ar):
- Poor conductors as they lack free electrons.
How to Answer Periodicity Exam Questions Effectively
1. Understand the Key Trends
Memorize and understand periodic trends such as atomic radius ionization energy and electronegativity.
2. Use Scientific Explanations
Always justify your answers using:
- Nuclear charge
- Electron shielding
- Atomic radius
- Bonding and structure
3. Compare and Contrast
When answering questions compare elements within a period or group to highlight clear differences in properties.
4. Use Correct Terminology
Include terms like “effective nuclear charge” “electron shielding” and “delocalized electrons” to demonstrate your understanding.
5. Practice with Past Papers
- Answer OCR past exam questions on periodicity.
- Review mark schemes to understand expected answers.
Exam Tips for Scoring High in Periodicity
- Read questions carefully. Look for key terms like explain describe or compare.
- Use clear concise sentences. Avoid vague explanations.
- Refer to specific elements. General answers may lose marks.
- Include diagrams when needed. They can help illustrate concepts like atomic size trends.
Periodicity is a crucial topic in OCR A-Level Chemistry covering trends such as atomic radius ionization energy electronegativity and conductivity. Understanding these trends and practicing exam-style questions can help students answer effectively and score high marks.
By mastering scientific explanations and comparing element properties you can excel in periodicity-related questions and improve your overall chemistry exam performance.
