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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