Mendeleev's Periodic Law — Revision Notes

⚡ 30-Second Revision

Mendeleev's Law: — Properties are periodic function of atomic masses.
Basis: — Increasing atomic mass.
Merits:

- Systematic classification. - Prediction of undiscovered elements (Eka-Al $ightarrow$ Ga, Eka-Si $ightarrow$ Ge). - Correction of atomic masses (e.g., Beryllium).

Demerits:

- Position of Hydrogen. - Position of Isotopes (different mass, same properties). - Anomalous Pairs (Ar-K, Te-I, Co-Ni). - No place for Lanthanides/Actinides. - No explanation for periodicity.

Groups: — Vertical columns (similar properties).
Periods: — Horizontal rows (increasing atomic mass).

2-Minute Revision

Mendeleev's Periodic Law, proposed in 1869, states that the properties of elements are a periodic function of their atomic masses. He arranged elements in increasing order of atomic mass, placing those with similar chemical properties in vertical groups and observing recurring patterns.

His table's major strengths included providing a systematic classification for elements, accurately predicting the existence and properties of several undiscovered elements like Eka-aluminium (Gallium) and Eka-silicon (Germanium), and correcting the atomic masses of elements such as Beryllium.

However, the law had significant limitations: it couldn't assign a unique position to hydrogen, failed to accommodate isotopes (due to their varying atomic masses), and contained 'anomalous pairs' (e.g.

, Argon-Potassium, Tellurium-Iodine) where atomic mass order was violated to maintain property similarity. It also lacked a proper place for lanthanides and actinides and couldn't explain the fundamental cause of periodicity.

Despite these drawbacks, Mendeleev's work was a monumental achievement, laying the essential groundwork for the modern periodic table.

5-Minute Revision

Mendeleev's Periodic Law is a cornerstone in the development of chemistry. In 1869, Dmitri Mendeleev proposed that 'The properties of the elements are a periodic function of their atomic masses.' This meant that if elements were arranged in ascending order of their atomic masses, their chemical and physical properties would repeat at regular intervals.

He constructed a periodic table with horizontal rows (periods) based on increasing atomic mass and vertical columns (groups) for elements with similar properties.

Key Merits:

1
Systematic Classification: — It provided a logical and comprehensive way to organize the then-known elements, simplifying their study.
2
Prediction of Undiscovered Elements: — This was his greatest triumph. He left gaps in his table and predicted the properties of elements like Eka-boron (Scandium), Eka-aluminium (Gallium), and Eka-silicon (Germanium) with remarkable accuracy. The later discovery of these elements validated his law.
3
Correction of Atomic Masses: — Mendeleev used the table to correct the atomic masses of elements like Beryllium (from 13.5 to 9) and Indium, based on their observed properties and group placement.

Key Demerits:

1
Position of Hydrogen: — Hydrogen's dual nature (resembling alkali metals and halogens) made its placement ambiguous.
2
Position of Isotopes: — Since isotopes have different atomic masses but identical chemical properties, Mendeleev's mass-based law could not place them in a single position, creating a contradiction.
3
Anomalous Pairs: — To maintain chemical similarity, he sometimes placed elements with higher atomic masses before those with lower masses. Examples: Argon (39.9) before Potassium (39.1); Tellurium (127.6) before Iodine (126.9); Cobalt (58.9) before Nickel (58.7).
4
Position of Lanthanides and Actinides: — These elements could not be accommodated within the main structure of the table.
5
Cause of Periodicity: — Mendeleev's law observed periodicity but did not explain its fundamental cause, which was later attributed to electronic configuration.

Understanding these merits and demerits, along with the specific examples, is crucial for NEET. Remember the distinction between atomic mass (Mendeleev) and atomic number (Modern Periodic Law) as the fundamental basis.

Prelims Revision Notes

Mendeleev's Periodic Law (1869)

Statement: The properties of the elements are a periodic function of their atomic masses.

Basis of Classification: Increasing order of atomic masses.

Structure of Table:

Periods: — Horizontal rows, elements arranged by increasing atomic mass.
Groups: — Vertical columns, elements with similar chemical properties (initially 8 groups, often divided into A and B sub-groups).

Key Merits (Advantages):

1
Systematic Study: — Provided a logical framework for classifying and studying elements.
2
Prediction of Undiscovered Elements: — Left gaps for unknown elements and accurately predicted their properties.

* Eka-Boron (Eb) $ightarrow$ Scandium (Sc) * Eka-Aluminium (EAl) $ightarrow$ Gallium (Ga) * Eka-Silicon (ESi) $ightarrow$ Germanium (Ge) * Eka-Manganese (EMn) $ightarrow$ Technetium (Tc)

1
Correction of Doubtful Atomic Masses: — Used the table to correct the atomic masses of elements like:

* Beryllium (Be): Corrected from 13.5 to 9 (valency changed from 3 to 2). * Indium (In): Corrected from 75.6 to 113.4. * Uranium (U): Corrected from 120 to 240.

Key Demerits (Limitations):

1
Position of Hydrogen: — Ambiguous; resembles both alkali metals and halogens.
2
Position of Isotopes: — Problematic; isotopes have different atomic masses but identical chemical properties. Mendeleev's law would require separate positions for them, which is chemically illogical.
3
Anomalous Pairs (Inverted Pairs): — Elements with higher atomic mass placed before elements with lower atomic mass to maintain property similarity.

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

1
Position of Lanthanides and Actinides: — No proper place within the main table; placed outside.
2
Separation of Similar Elements: — Some similar elements (e.g., Cu, Ag, Au with alkali metals) were grouped together, while others (e.g., Cu and Hg) were separated.
3
Grouping of Dissimilar Elements: — Dissimilar elements were sometimes placed in the same group.
4
Cause of Periodicity: — Did not explain *why* properties are periodic; only observed the phenomenon.

Comparison with Modern Periodic Law:

Mendeleev: — Based on Atomic Mass.
Modern: — Based on Atomic Number (Moseley's Law).

NEET Focus: Remember specific examples for merits (predicted elements, corrected masses) and demerits (anomalous pairs, isotopes).

Vyyuha Quick Recall

My Periodic Law Always Makes Discoveries:

Mass-based (Atomic Mass)
Predicts elements (Eka-Al, Eka-Si)
Leaves gaps
Anomalous pairs (Ar-K, Te-I) - a Demerit
Mass corrections (Beryllium)
Difficult for isotopes & Hydrogen - also Demerits