Question

Compare and contrast the arrangement of elements in Mendeleev's Periodic Table and the Modern Periodic Table.

Answer 1

The arrangement of elements in Mendeleev's Periodic Table and the Modern Periodic Table has several key differences and similarities. Here's a comparison and contrast of the two:

Mendeleev's Periodic Table

1. Basis of Arrangement:

   • Mendeleev arranged elements in order of increasing atomic mass.
   • He left gaps for undiscovered elements and predicted their properties.

2. Periodicity:

   • Mendeleev's table showed periodicity (repeating patterns) in the properties of elements.

3. Grouping:

   • Elements with similar properties were placed in the same column (group).
   • Some elements were placed out of strict atomic mass order to group elements with similar properties together.

4. Gaps for New Elements:

   • Mendeleev intentionally left gaps for elements he predicted would be discovered (e.g., gallium, germanium).

5. Unknown Noble Gases:

   • Noble gases were not included in Mendeleev's table as they had not been discovered yet.

Modern Periodic Table

1. Basis of Arrangement:

   • The modern periodic table arranges elements in order of increasing atomic number (number of protons).
   • This arrangement resolves inconsistencies that appeared in Mendeleev's table when elements were ordered by atomic mass.

2. Periodicity:

   • The modern table also exhibits periodicity, with elements showing repeating patterns in properties.

3. Grouping:

   • Elements are grouped into columns (groups) based on their similar valence electron configurations, leading to similar chemical properties.
   • Groups are numbered 1 through 18.

4. Periods:

   • Elements are arranged in rows (periods) corresponding to the number of electron shells.
   • There are 7 periods in the modern table.

5. Inclusion of Noble Gases:

   • Noble gases are included in Group 18, completing the periodic table.

6. Lanthanides and Actinides:

   • These are placed separately below the main body of the periodic table to maintain its structure.

Key Differences

1. Atomic Mass vs. Atomic Number:

   • Mendeleev used atomic mass as the basis for arrangement, while the modern table uses atomic number.

2. Placement of Elements:

   • Mendeleev's arrangement sometimes placed elements with slightly incorrect masses in positions to align with their properties, whereas the modern table's use of atomic numbers avoids this issue.

3. Gaps for New Elements:

   • Mendeleev's table had gaps for undiscovered elements, which were later filled as elements were discovered.
   • The modern table is complete, with no gaps.

4. Noble Gases:

   • Mendeleev's table did not include noble gases, whereas the modern table does.

Key Similarities

1. Periodicity:

   • Both tables demonstrate periodicity in the properties of elements.

2. Grouping by Properties:

   • Both tables group elements with similar properties into the same columns (groups).

3. Predictive Power:

   • Both tables have been useful in predicting the properties of elements based on their position.

In summary, while Mendeleev's periodic table was a groundbreaking step in understanding the arrangement of elements and their properties, the modern periodic table, with its basis on atomic number, provides a more accurate and consistent framework.

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