Modern Periodic Law and the Present Form of the Periodic Table
Learning Objective: To understand how periodic classification evolved to the modern (atomic-number–based) table, and to learn the structure and features of the present-day periodic table: periods, groups, and s-, p-, d-, f-block classification.
Quick Notes
- Mendeleev’s table used atomic mass and showed anomalies (e.g., Co/Ni).
- Modern Periodic Law (Moseley, 1913): “The physical and chemical properties of the elements are a periodic function of their atomic numbers.”
- The modern table is arranged in increasing atomic number.
- It has 7 periods (rows) and 18 groups (columns).
- Elements are classified into s-, p-, d-, and f- blocks based on electron configurations.
- Each new period begins with filling of a new principal energy level (n).
Full Notes
The Shift from Atomic Mass to Atomic Number
While Mendeleev’s periodic law (based on atomic mass) organized many elements successfully, some anomalies remained. For example, cobalt and nickel did not align perfectly by mass with their chemical similarities; isotopes also caused placement issues because they differ in mass but not in chemical properties.
In 1913, Henry Moseley showed from X-ray spectra that the fundamental property governing periodicity is atomic number (Z) – the number of protons in the nucleus – not atomic mass. Ordering by increasing Z removed the anomalies and produced a consistent, predictive framework.
Modern Periodic Law
Statement: “The physical and chemical properties of the elements are a periodic function of their atomic numbers.”
Meaning: When elements are arranged by increasing atomic number, similar properties recur at regular intervals (periodically). This corrects Mendeleev’s discrepancies and links periodicity directly to electronic structure.
Structure of the Modern Periodic Table
The present (long-form) table is arranged into periods (rows) and groups (columns). Each new period corresponds to the start of filling a new principal energy level n.
Horizontal Rows → Periods
- Total of 7 periods.
- Each period begins with the filling of a new principal energy level (n).
- The length of a period reflects the number of available orbitals being filled.
| Period | Number of Elements |
|---|---|
| 1st | 2 (H, He) |
| 2nd & 3rd | 8 |
| 4th & 5th | 18 |
| 6th | 32 |
| 7th | Incomplete (also 32) |
Vertical Columns → Groups
- 18 groups in total.
- Elements in the same group share similar valence-shell electron configurations, so they show related chemical properties.
- Group 1: Alkali metals | Group 2: Alkaline earth metals
- Groups 3–12: Transition metals
- Group 17: Halogens | Group 18: Noble gases
Classification Based on Electron Configuration (Blocks)
The periodic table can also be classified into blocks, depending on the type of orbital being filled:
| Block | Description | Examples |
|---|---|---|
| s-block | Groups 1 & 2, including helium | H, Na, Mg |
| p-block | Groups 13–18 | B, C, N, O, F, Ne |
| d-block | Transition elements (Groups 3–12) | Fe, Cu, Zn |
| f-block | Inner transition elements (lanthanides and actinides) | Ce, U |
The s- and p-block elements are often called representative elements. The d-block elements are the transition elements, and the f-block (lanthanides and actinides) is placed separately below the main table for clarity and compactness.
Significance of Modern Classification
- Arranging by atomic number aligns perfectly with electronic configurations and periodic trends.
- Resolves mass-based anomalies and places isotopes correctly within the same element.
- Accurately predicts chemical behaviour across periods and down groups.
- Provides a logical framework for new/synthetic elements and for studying trends (atomic size, ionization enthalpy, electronegativity, etc.).
Summary
- Moseley’s work established atomic number as the basis of periodicity.
- The modern table has 7 periods and 18 groups, arranged by increasing Z.
- Block classification (s, p, d, f) reflects valence orbital filling and explains trends.
- Modern classification is predictive, resolves earlier anomalies, and underpins periodic trends.