Electronic Configuration of Elements in the Fourth to Seventh Periods
Anand Classes Notes explain the arrangement of elements in the periodic table follows a specific order based on the filling of electrons in atomic orbitals. From the fourth period onward, the pattern becomes more complex due to the involvement of d- and f-block orbitals. These periods include not only s- and p-block elements but also transition and inner transition elements. Understanding how these orbitals are filled explains why some periods have 18 or even 32 elements, unlike the shorter first three periods.
Table of Contents
🔹 Elements of the Fourth Period
The fourth period corresponds to principal quantum number n = 4. In this period, the filling of the following orbitals occurs:
- 4s-orbital
- Five 3d-orbitals
- Three 4p-orbitals
Though 4d and 4f orbitals exist, they are higher in energy and are filled later. Since a total of nine orbitals (1 × 4s, 5 × 3d, 3 × 4p) are being filled, the fourth period contains 18 elements, ranging from:
- Potassium (Z = 19) to Krypton (Z = 36)
A special feature of this period is the appearance of the first transition series, which involves the filling of the 3d-orbitals. It begins with:
- Scandium (Z = 21) – configuration:
3d¹ 4s² - and ends with
- Zinc (Z = 30) – configuration:
3d¹⁰ 4s²
These are known as transition elements.
🔹 Elements of the Fifth Period
Like the fourth period, the fifth period also consists of 18 elements. It starts with:
- Rubidium (Z = 37) – filling begins in the 5s-orbital
And ends at:
- Xenon (Z = 54) – with the filling of the 5p-orbitals
This period also includes the 4d transition series, starting with:
- Yttrium (Z = 39)
🔹 Elements of the Sixth Period
The sixth period is unique and contains a total of 32 elements – the longest period of the periodic table.
- Starts with Cesium (Z = 55)
- Ends with Radon (Z = 86)
Successive electron filling occurs in the following orbitals:
- 6s, 4f, 5d, and 6p
This period includes:
- Ten 5d transition elements
- Fourteen 4f elements, starting from Cerium (Z = 58) to Lutetium (Z = 71)
These 4f elements form the lanthanoid series, also known as inner transition elements.
🔹 Elements of the Seventh Period
The seventh period is similar in structure to the sixth, with electrons entering:
- 7s, 5f, 6d, and 7p orbitals
It includes most of the synthetic radioactive elements, and currently ends with:
- Element 118, which is a noble gas
This period includes:
- Fourteen 5f elements, starting from Actinium (Z = 89)
These elements form the actinoid series (or 5f-inner transition series).
🧠 Interesting Fact: Magic Numbers
The periodicity in the number of elements in each period follows a pattern:
2, 8, 8, 18, 18, 32
These are called magic numbers, and they provide a theoretical explanation for the structure of the periodic table.
📌 Short and Long Periods
- Short Periods: 1st to 3rd periods (2, 8, 8 elements)
- Long Periods: 4th to 6th periods (18, 18, and 32 elements)
❓ FAQs
Q1. Why does the fourth period have 18 elements?
Answer: Because it includes filling of 4s, 3d, and 4p orbitals — a total of nine orbitals that can hold 18 electrons.
Q2. What are lanthanoids and actinoids?
Answer: Lanthanoids (Z = 58 to 71) are elements that involve filling of 4f orbitals. Actinoids (Z = 89 to 103) involve filling of 5f orbitals. Both are part of the inner transition series.
Q3. Why does the sixth period have 32 elements?
Answer: It involves the filling of 6s, 4f, 5d, and 6p orbitals — which together can accommodate 32 electrons.
Q4. What is the significance of magic numbers in the periodic table?
Answer: Magic numbers like 2, 8, 18, and 32 explain the number of elements that appear in each period due to the filling of electron orbitals.
Q5. What would be the IUPAC name and symbol of the element with atomic number 120 ? [NCERT]
Answer: The roots for 1, 2 and 0 are un, bi and nil respectively.
Name of element : Unbinilium
Symbol : Ubn
Q6: How would you justify the presence of 18 elements in the 5th period of the periodic table? [NCERT]
Answer: The 5th period of the periodic table corresponds to the principal quantum number n = 5. This period includes the filling of the following orbitals:
- 5s orbital
- 4d orbitals (five in total)
- 5p orbitals (three in total)
🔬 These orbitals provide a total of 9 orbitals (1 from 5s + 5 from 4d + 3 from 5p).
Since each orbital can hold 2 electrons, the maximum number of electrons that can be accommodated is: 9 orbitals × 2 = 18 electrons
👉 Therefore, 18 elements are needed to accommodate these 18 electrons — one element for each additional electron.
📘 Conclusion:
The presence of 18 elements in the 5th period is justified by the availability of nine orbitals (5s, 4d, 5p) that can together hold 18 electrons, corresponding to 18 elements.
Q7: The elements with atomic numbers Z = 107, 108, and 109 have been synthesized recently. Indicate the group in which you would place these elements. [NCERT]
Answer: Let’s first write their electronic configurations:
- Z = 107 → [Rn] 5f¹⁴ 6d⁵ 7s²
- Z = 108 → [Rn] 5f¹⁴ 6d⁶ 7s²
- Z = 109 → [Rn] 5f¹⁴ 6d⁷ 7s²
🧬 These elements involve the filling of 6d-orbitals, which classifies them as d-block elements.
Now, let’s analyze the group placement based on their outer electronic configuration:
| Atomic Number | Configuration (Valence) | Group |
|---|---|---|
| Z = 107 | 6d⁵ 7s² | Group 7 |
| Z = 108 | 6d⁶ 7s² | Group 8 |
| Z = 109 | 6d⁷ 7s² | Group 9 |
🧩 Conclusion
➡️ These elements belong to the d-block (transition elements) and are placed in the:
- Group 7 – Z = 107
- Group 8 – Z = 108
- Group 9 – Z = 109
They are part of the 7th period of the periodic table.
📌 Key Points
- All three elements are in period 7.
- All are transition metals due to filling of the 6d orbitals.
- Belong to Groups 7, 8, and 9 respectively.
Q8. What block do the elements Z = 107 to Z = 109 belong to?
Ans: They belong to the d-block because their last electron enters the 6d orbital.
Q9. Why is Z = 107 placed in Group 7?
Ans: Z = 107 has the configuration 6d⁵ 7s². According to group prediction rules for d-block, Group = (n-1)d electrons + ns electrons = 5 + 2 = 7.
Q10. Are these elements transition metals?
Ans: Yes, they are part of the transition metal series of the periodic table.
📝 Keep practicing electronic configurations to master the periodic table! For complete study material and solved examples, visit Anand Classes.
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