Unit 1 - Lesson 1: Attempts of Elements Classification (Mendeleev, Moseley, Modern Periodic Table: 7 periods, 18 groups, 4 blocks s, p, d, f)
Throughout chemical history, scientists continuously endeavored to classify elements to facilitate their study and unveil the fundamental relationships governing their physical and chemical behaviors. Dmitri Mendeleev constructed the first genuine periodic table, arranging sixty-seven elements in ascending order of atomic weight, audaciously predicting undiscovered elements and correcting erroneously evaluated atomic weights. Following Henry Moseley's breakthrough discovery through X-ray spectroscopy that periodic properties correlate with atomic numbers rather than atomic weights, the elements were reordered and the zero group of inert gases was established. This evolutionary journey culminated in the Modern Periodic Table, structured according to the Aufbau principle and energy sublevel filling, organized into seven horizontal periods, eighteen vertical groups, and four distinct blocks: s, p, d, and f.
1. Mendeleev's and Moseley's Periodic Tables
| Aspect | Mendeleev's Table | Moseley's Table |
|---|---|---|
| Classification Basis | Ascending order of atomic weight | Ascending order of atomic number |
| Number of Elements | 67 elements | Included newly discovered elements |
| Advantages | Predicted new elements, corrected atomic weights | Added the zero group (noble gases) |
| Defects / Modifications | Disrupted order for property grouping, clustered elements | Allocated a bottom section for Lanthanides & Actinides |
Moseley's X-ray Discovery: Revealed that the periodicity of element properties is fundamentally governed by atomic number (Z), not atomic mass.
2. Architecture of the Modern Periodic Table
Elements in the Modern Periodic Table (MPT) are classified on two dual scientific foundations:
- Horizontal Periods:
7 periods; each period begins with filling a new principal energy level. - Vertical Groups:
18 groupsin modern numbering (1 to 18), corresponding to 16 traditional groups. - Total Known Elements: 118 elements (92 naturally occurring in the Earth's crust; the remainder synthesized in laboratories).
3. The Four Blocks of the Periodic Table (s, p, d, f)
| Block | Location | Number of Groups | Traditional Notation | Modern Numbering |
|---|---|---|---|---|
| s-block | Far left | 2 groups | 1A, 2A | Groups 1, 2 |
| p-block | Far right | 6 groups | 3A, 4A, 5A, 6A, 7A, Zero Group | Groups 13 to 18 |
| d-block | Middle (Transition Elements) | 10 groups | Letter B (3B to 7B, Group VIII consisting of 3 columns, 1B, 2B) | Groups 3 to 12 |
| f-block | Bottom (Separated) | 2 series | Lanthanides (14 elements) and Actinides (14 elements) | Detached to prevent excessive widening |
Key Rule: Transition elements (d-block) start appearing in the 4th period, physically separating s-block and p-block.
Exercise 1: Locating an Element in MPT
Problem: Determine the position of calcium (²⁰Ca) in MPT (period, group, and block).
• Electronic configuration: K=2, L=8, M=8, N=2.
• Number of occupied principal energy levels = 4 ⟹ Period 4.
• Outermost valence electrons = 2 ⟹ Group 2A (Modern Group 2).
• Block: s-block.
Exercise 2: Deducing Atomic Number from Position
Problem: Element X lies in the 3rd period and Group 5A. Calculate its atomic number and name its block.
• Period 3 ⟹ 3 energy levels (K, L, M).
• Group 5A ⟹ outermost level M has 5 electrons.
• Complete levels: K=2, L=8, M=5.
• Atomic number = 2 + 8 + 5 = 15 (Phosphorus), belonging to p-block.
Exercise 3: Position of Helium
Problem: Why is helium (²He) placed in the zero group (Group 18) rather than Group 2A?
• Helium configuration: K=2.
• Its single energy level is completely saturated with electrons, giving it chemically inert noble gas properties.
• Thus, it is placed in Period 1, Group 18 (Zero Group), and belongs to p-block.
Exercise 4: Atomic Numbers in Periods and Groups
Problem: An element has atomic number 11. Find the atomic number of the element directly following it in: (a) the same period | (b) the same group.
• (a) In the same period: atomic number increases by 1 ⟹ 11 + 1 = 12.
• (b) In the same group: atomic number increases by a full shell (8 electrons) ⟹ 11 + 8 = 19.
Exercise 5: Onset of Transition Elements
Problem: In which period do transition elements appear, and which block do they constitute?
• Transition elements begin appearing in Period 4.
• They constitute the d-block, comprising 10 vertical columns located in the middle of the table.
Exercise 6: Traditional vs Modern Group Numbering
Problem: State the modern numbers for groups: 3A, 7A, 1B, and Zero Group.
• Group 3A ⟹ Group 13.
• Group 7A ⟹ Group 17.
• Group 1B ⟹ Group 11.
• Zero Group ⟹ Group 18.
Exercise 7: Reason for Separating f-block
Problem: Give reason: Lanthanides and Actinides are separated below the main periodic table.
• To prevent the periodic table from becoming excessively wide and distorted in layout, while preserving structural symmetry.
Exercise 8: Similarity in Chemical Properties
Problem: Explain why sodium (¹¹Na) and potassium (¹⁹K) exhibit highly similar chemical properties.
• Na: 2, 8, 1 | K: 2, 8, 8, 1.
• Both possess 1 valence electron in their outermost energy level and belong to Group 1A. Chemical properties are determined by valence electrons.
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