Mendeleev's Periodic Law

Master Inorganic Chemistry! Before the modern periodic table was established based on atomic number, Dmitri Mendeleev achieved a monumental breakthrough by organizing elements based on their atomic masses. Let's explore his genius and his limitations!

Mendeleev's Periodic Table: Law, Merits & Demerits

In 1869, Russian chemist Dmitri Mendeleev published a periodic table that forever changed chemistry. At a time when only 63 elements were known, he brilliantly realized that if elements are arranged in order of increasing atomic mass, their chemical and physical properties repeat at regular intervals.

Mendeleev's Periodic Law states:

"The physical and chemical properties of elements are a periodic function of their atomic weights (masses)."

Periodic Classification Inorganic Chemistry Class 11 Chemistry Video Lectures

Figure 1: Outline of Mendeleev's Periodic Table and its structural characteristics.

Salient Features & Merits

Mendeleev arranged elements in horizontal rows (Periods) and vertical columns (Groups). His table was not just a compilation, but a powerful predictive tool.

1. Systematic Study of Elements

By classifying elements into specific groups based on their chemical similarities, Mendeleev made the study of chemistry highly systematic. Knowing the properties of one element allowed chemists to predict the properties of all other elements in that same group.

2. Prediction of Undiscovered Elements

This was his greatest stroke of genius. Mendeleev confidently left blank gaps in his periodic table for elements that had not yet been discovered. He even predicted their exact atomic masses and chemical properties!

• Eka-Boron: Later discovered and named Scandium (Sc).
• Eka-Aluminium: Later discovered and named Gallium (Ga).
• Eka-Silicon: Later discovered and named Germanium (Ge).

3. Correction of Doubtful Atomic Masses

Mendeleev corrected the atomic masses of several elements based on their positions in his table. For example, Beryllium (Be) was originally thought to have an atomic mass of 13.5. Based on its properties, Mendeleev placed it in Group II and corrected its atomic mass to 9. He also corrected the atomic masses of Indium, Gold, and Platinum.

Figure 2: Major demerits and limitations of Mendeleev's arrangement.

Demerits (Anomalies) of Mendeleev's Table

Because Mendeleev used Atomic Mass instead of Atomic Number as the fundamental property, several contradictions and anomalies arose in his table.

1. Position of Hydrogen

Hydrogen exhibits properties similar to both Alkali Metals (Group IA) and Halogens (Group VIIA). Mendeleev could not assign a fixed, undisputed position to Hydrogen in his table.

2. Anomalous Pairs of Elements

To ensure that elements with similar chemical properties fell into the same vertical group, Mendeleev was forced to break his own rule of "increasing atomic mass" in a few specific cases. Elements with higher atomic masses were placed before elements with lower atomic masses.

The 3 Key Anomalous Pairs:

• Argon (Ar, 39.9) was placed before Potassium (K, 39.1).
• Cobalt (Co, 58.9) was placed before Nickel (Ni, 58.7).
• Tellurium (Te, 127.6) was placed before Iodine (I, 126.9).

3. Position of Isotopes

Isotopes are atoms of the same element that have different atomic masses (e.g., Carbon-12 and Carbon-14). According to Mendeleev's law, isotopes should have been given separate places in the periodic table due to their different masses. However, they were grouped together, violating the fundamental basis of his classification.

4. Grouping of Dissimilar Elements

In some cases, highly dissimilar elements were placed in the same group. For example, highly reactive alkali metals (like Li, Na, K) were placed in Group I alongside coinage metals (like Cu, Ag, Au), which possess entirely different chemical reactivity and properties.

Frequently Asked Questions (FAQs)

What is Mendeleev's Periodic Law?

Mendeleev's Periodic Law states that the physical and chemical properties of elements are a periodic function of their atomic masses. (Note: The Modern Periodic Law uses atomic number instead).

What were the predicted elements Eka-boron, Eka-aluminium, and Eka-silicon?

Mendeleev predicted the existence of undiscovered elements and named them by prefixing 'Eka' (meaning 'one' in Sanskrit) to the preceding element in the same group. Eka-boron was later discovered as Scandium (Sc), Eka-aluminium as Gallium (Ga), and Eka-silicon as Germanium (Ge).

What are the anomalous pairs in Mendeleev's periodic table?

Anomalous pairs are elements placed out of the strict atomic mass order to ensure elements with similar properties remained together in the same group. The three main pairs are: Argon (39.9) placed before Potassium (39.1), Cobalt (58.9) placed before Nickel (58.7), and Tellurium (127.6) placed before Iodine (126.9).

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Modern Periodic Table | Classification of Elements Class 11

Modern Periodic Table

Arrangement of Elements & Periodicity | Classification of Elements

1. Modern Periodic Law

In 1913, Henry Moseley showed that the Atomic Number ($Z$) is a more fundamental property than Atomic Mass. He observed a regular pattern in X-ray spectra: $\sqrt{\nu} \propto Z$.

Law: The physical and chemical properties of elements are periodic functions of their Atomic Numbers.

2. Structure of the Periodic Table

The long form of the periodic table is based on electronic configuration.

A. Periods (Horizontal Rows)

• There are 7 Periods.
• The period number corresponds to the highest Principal Quantum Number ($n$) of the elements.
• Magic Numbers: 2, 8, 8, 18, 18, 32 (Number of elements in periods 1 to 6).

B. Groups (Vertical Columns)

• There are 18 Groups (numbered 1 to 18 by IUPAC).
• Elements in the same group have the same number of valence electrons and similar chemical properties.

3. Classification into Blocks

Based on the subshell in which the last electron enters, elements are classified into four blocks:

1. s-Block Elements

• Groups: 1 (Alkali Metals) & 2 (Alkaline Earth Metals).
• Config: $ns^{1-2}$
• Reactive metals with low ionization enthalpy.

2. p-Block Elements

• Groups: 13 to 18.
• Config: $ns^2 np^{1-6}$
• Contains Metals, Non-metals, and Metalloids.
• Representative Elements: s-block + p-block (except Group 18).

3. d-Block Elements (Transition Elements)

• Groups: 3 to 12.
• Config: $(n-1)d^{1-10} ns^{0-2}$
• Metals that form colored ions, variable oxidation states, and act as catalysts.
• Note: Zn, Cd, Hg are d-block but strictly not transition elements (full d-orbital).

4. f-Block Elements (Inner Transition Elements)

• Position: Placed separately at the bottom (belong to Group 3).
• Config: $(n-2)f^{1-14} (n-1)d^{0-1} ns^2$
• Lanthanoids ($4f$): Rare earth elements.
• Actinoids ($5f$): Radioactive elements.

4. Summary of Periodic Trends

Property	Left to Right (Period)	Top to Bottom (Group)
Atomic Radius	Decreases	Increases
Ionization Energy	Increases	Decreases
Electronegativity	Increases	Decreases
Metallic Character	Decreases	Increases

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Chemistry Bridge Course Lecture 11

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JEE & NEET Academy by Abhishek Sengar

Home | Bridge Course Series

Lecture 11 Inorganic Foundations Target: Class 10 to 11 Transition (JEE/NEET)

Introduction to Modern Periodic Table

Welcome to Lecture 11 of the CHEMCA Bridge Course! Inorganic chemistry is fundamentally driven by the arrangement of elements. In this session, Abhishek Sengar Sir introduces the Modern Periodic Table. Learn how the modern table is organized around Atomic Numbers ($Z$), discover the 4 block subshells ($s, p, d, f$), and find out how magic numbers determine the element capacities of periods.

Video Lecture Broadcast

Instructor: Abhishek Sengar Sir Published: April 23, 2026 Subject: Modern Periodic Table

Interactive Lecture Timestamps

Click any topic to skip the video directly to that specific concept explanation.

In-Depth Lecture Notes & Summary

01

Mendeleev vs. Modern Periodic Law

Early attempts at classifying elements (such as Dobereiner's Triads, Newlands' Octaves, and Mendeleev's Periodic Table) were based strictly on Atomic Mass. However, this caused numerous anomalies, such as placing heavier elements before lighter ones.

The Modern Periodic Law

"The physical and chemical properties of the elements are periodic functions of their Atomic Numbers ($Z$)."

Proposed by Henry Moseley, sorting elements by atomic number ($Z$) aligned them perfectly with their electronic configurations, resolving almost all historical classification flaws.

02

Periods, Groups & Subshell Blocks

The Modern Periodic Table organizes elements into 7 Horizontal Rows (Periods) and 18 Vertical Columns (Groups), categorized into 4 subshell blocks:

• s-block (Groups 1 & 2): Outer electrons fill the $s$-subshell. Very reactive metals.
• p-block (Groups 13 to 18): Outer electrons fill the $p$-subshell. Includes metals, metalloids, and non-metals.
• d-block (Groups 3 to 12): Outer electrons fill the inner $(n-1)d$ subshell. Known as Transition Metals.
• f-block (Placed below): Outer electrons fill the $(n-2)f$ subshell. Known as Inner Transition Elements.

03

Special Names of Chemical Families

Specific groups contain chemically similar families with special historical names:

Group 1

Alkali Metals

Group 2

Alkaline Earth

Group 11

Coinage Metals

Group 15

Pnictogens

Group 16

Chalcogens

Group 17

Halogens

Group 18

Noble Gases

04

The Magic Numbers ($2, 8, 8, 18, 18, 32, 32$)

The capacities of periods are governed by quantum subshell energy limits. Adding the capacities of filled subshells in each period reveals the Magic Numbers of elements per period:

• Period 1 (Shortest): $2$ elements ($1s$) $\implies$ $Z=1$ to $2$.
• Period 2 (Short): $8$ elements ($2s, 2p$) $\implies$ $Z=3$ to $10$.
• Period 3 (Short): $8$ elements ($3s, 3p$) $\implies$ $Z=11$ to $18$.
• Period 4 (Long): $18$ elements ($4s, 3d, 4p$) $\implies$ $Z=19$ to $36$.
• Period 5 (Long): $18$ elements ($5s, 4d, 5p$) $\implies$ $Z=37$ to $54$.
• Period 6 (Very Long): $32$ elements ($6s, 4f, 5d, 6p$) $\implies$ $Z=55$ to $86$. Includes Lanthanides ($Z = 58$ to $71$ in 4f series).
• Period 7 (Very Long): $32$ elements ($7s, 5f, 6d, 7p$) $\implies$ $Z=87$ to $118$. Includes Actinides ($Z = 90$ to $103$ in 5f series).

Magic Numbers Period Finder

Input a Period Number ($1$ to $7$) to explore subshell configurations and atomic capacities.

Period Number

Period Details: Long

Element Count: 18 elements

Subshells Filling: 4s, 3d, 4p

Atomic Number Range: Z = 19 to 36

Mnemonic Cheat Sheets

Click below to view the Hindi memorization phrases Abhishek Sir teaches in class!

Group 1: Alkali Metals

"He Li Na Ki Rub Se Faryad"

Elements: H, Li, Na, K, Rb, Cs, Fr

Group 2: Alkaline Earth

"Beta Mange Car Scooter Baap Raazi"

Elements: Be, Mg, Ca, Sr, Ba, Ra

Group 17: Halogens

"Fir Se Bahar Aayi Aunty"

Elements: F, Cl, Br, I, At

Lecture 11 Concept Test

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Stuck on Blocks?

If you are struggling to identify why certain elements are placed in the d-block or f-block, drop a message to Abhishek Sengar Sir!

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The Interactive Periodic Table

Click or hover over any element. Filter using the buttons to watch matching subshells glow.

Z = 1 s-block

H Hydrogen Mass: 1.008 u

Chemical Series: Reactive Nonmetal

Period & Group: Period 1, Group 1

Electronic Config: 1s¹

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