What is the Modern Periodic Law and how is it different from Mendeleev's Periodic Law?

The Modern Periodic Law states that the physical and chemical properties of elements are periodic functions of their atomic numbers. This is a crucial distinction from Mendeleev's Periodic Law, which stated that properties are periodic functions of their atomic masses. Henry Moseley's work with X-rays established atomic number as a more fundamental property, leading to a more accurate and consistent arrangement of elements, resolving anomalies like the position of isotopes and certain element pairs.

How do you determine the period and group of an element from its atomic number?

To determine the period, write the element's electronic configuration. The highest principal quantum number ($n$) reached by an electron in the outermost shell corresponds to the period number. For the group, if the last electron enters the s-orbital, the group number is the number of valence s-electrons. If it enters the p-orbital, the group number is 10 + (number of valence s-electrons + number of valence p-electrons). If it enters the d-orbital, the group number is (number of $(n-1)d$ electrons + number of $ns$ electrons).

Why are lanthanoids and actinoids placed separately at the bottom of the periodic table?

Lanthanoids and actinoids, also known as f-block elements or inner transition elements, are placed separately to maintain the aesthetic and functional compactness of the main periodic table. If they were included within the main body, the table would become excessively long and difficult to read. Their distinct characteristic is the filling of their f-orbitals, which leads to similar chemical properties within each series, justifying their separate placement while still acknowledging their connection to the d-block elements.

What are the main advantages of the Modern Periodic Table over Mendeleev's Periodic Table?

The Modern Periodic Table offers several advantages: it correctly places isotopes (same atomic number, same position); it resolves anomalous pairs (e.g., Ar and K) by arranging elements strictly by atomic number; it provides a clear theoretical basis for periodicity based on electronic configuration; and it easily accommodates noble gases without disturbing the existing structure. It is a more logical and predictive classification system.

How does electronic configuration relate to an element's position in the Modern Periodic Table?

Electronic configuration is the direct determinant of an element's position. The highest principal quantum number ($n$) of the valence shell indicates the period. The number of valence electrons (or sum of valence s and p electrons, or (n-1)d and ns electrons) determines the group, as elements with similar valence configurations exhibit similar chemical properties. The type of orbital being filled (s, p, d, or f) dictates the block to which the element belongs.

What is the significance of the 'blocks' (s, p, d, f) in the Modern Periodic Table?

The s, p, d, and f blocks categorize elements based on the type of atomic orbital that receives the last electron during the Aufbau process. This classification is significant because elements within a block tend to share broad chemical and physical characteristics. For example, s-block elements are generally soft, reactive metals, while p-block elements show a wide range of properties from non-metals to metalloids and metals. d-block elements are transition metals with variable oxidation states, and f-block elements are inner transition metals with unique magnetic and optical properties.

← ALL TOPICS

Chemistry

NEXT TOPIC →

Significance of Classification

Modern Periodic Table

Chemistry

NEET UG

Version 1Updated 21 Mar 2026

Explore This Topic

Definition Deep Explanation Core Principles NEET Importance Problem Strategy Practice Problems MCQ Practice Quick Revision

The Modern Periodic Law states that the physical and chemical properties of elements are periodic functions of their atomic numbers. This fundamental principle, proposed by Henry Moseley in 1913, forms the basis of the Modern Periodic Table. Unlike Mendeleev's original classification which relied on atomic mass, the Modern Periodic Table arranges elements in increasing order of their atomic number…

Quick Summary

The Modern Periodic Table is the cornerstone of chemistry, organizing elements based on their atomic number, a concept pioneered by Henry Moseley. This arrangement, governed by the Modern Periodic Law, states that elemental properties are periodic functions of their atomic numbers.

The table features 7 horizontal periods, representing the principal quantum number of the outermost electron shell, and 18 vertical groups, where elements share similar valence electron configurations and thus similar chemical properties.

Elements are further categorized into s, p, d, and f blocks, reflecting the orbital being filled by the last electron. This systematic organization resolves issues of earlier classifications, such as the placement of isotopes and anomalous pairs, and provides a powerful predictive tool for understanding chemical behavior and periodic trends.

It's crucial for NEET aspirants to grasp how electronic configuration dictates an element's position and properties.

Vyyuha

Your 6-Month Blueprint, Updated Nightly

AI analyses your progress every night. Wake up to a smarter plan. Every. Single.…

Key Concepts

Determining Period and Block from Atomic Number

The period of an element is determined by the highest principal quantum number ( $n$ ) of its valence shell.…

Determining Group from Electronic Configuration

The group number is related to the number of valence electrons. For s-block elements, the group number is…

Periodicity and Electronic Configuration

Periodicity, the recurrence of similar properties at regular intervals, is fundamentally explained by the…

Modern Periodic Law: — Properties are periodic functions of atomic number (Z).

Basis: — Atomic number, not atomic mass.

Periods: — 7 horizontal rows,

n

= period number.

- Period 1: 2 elements ( $1s$ ) - Period 2: 8 elements ( $2s, 2p$ ) - Period 3: 8 elements ( $3s, 3p$ ) - Period 4: 18 elements ( $4s, 3d, 4p$ ) - Period 5: 18 elements ( $5s, 4d, 5p$ ) - Period 6: 32 elements ( $6s, 4f, 5d, 6p$ ) - Period 7: 32 elements ( $7s, 5f, 6d, 7p$ )

Groups: — 18 vertical columns, similar valence e- config, similar properties.

Blocks:

- s-block (Gr 1, 2): $ns^{1-2}$ - p-block (Gr 13-18): $ns^2np^{1-6}$ - d-block (Gr 3-12): $(n-1)d^{1-10}ns^{1-2}$ (Transition elements) - f-block (Lanthanoids, Actinoids): $(n-2)f^{1-14}(n-1)d^{0-1}ns^2$ (Inner transition elements)

Group Determination:

- s-block: No. of valence s-electrons. - p-block: $10 + (\text{No. of valence s-electrons} + \text{No. of valence p-electrons})$ . - d-block: $(\text{No. of } (n-1)d \text{ electrons} + \text{No. of } ns \text{ electrons})$ .

To remember the order of filling orbitals for period length: Some People Don't Feel. (s, p, d, f blocks). For period lengths: 2, 8, 8, 18, 18, 32, 32 (Remember the 32s for periods 6 and 7 due to f-block).

← ALL TOPICS

Chemistry

NEXT TOPIC →

Significance of Classification