Classification of Elements & Periodicity Flashcards

Dobereiner's Triad Theory

• Historical Context: In the year $1829$, Johann Wolfgang Dobereiner classified elements into groups of three, known as triads.
• Definition of Triad Theory: When elements were arranged in increasing order of their atomic masses, the atomic mass of the middle element was roughly the average of the atomic masses of the remaining two elements.
• First Triad Example:
  • Elements: Lithium ($Li$), Sodium ($Na$), and Potassium ($K$).
  • Atomic mass of Lithium ($Li$): $7$
  • Atomic mass of Potassium ($K$): $39$
  • Calculation: $\frac{7 + 39}{2} = 23$
  • Atomic mass of Sodium ($Na$): $23$
• Second Triad Example:
  • Elements: Calcium ($Ca$), Strontium ($Sr$), and Barium ($Ba$).
  • Atomic mass of Calcium ($Ca$): $40$
  • Atomic mass of Barium ($Ba$): $137$
  • Calculation: $\frac{137 + 40}{2} = 88.5$
  • Atomic mass of Strontium ($Sr$): $87.5$ (Note: $88.5$ is roughly the mass of the middle element).
• Third Triad Example:
  • Elements: Chlorine ($Cl$), Bromine ($Br$), and Iodine ($I$).
  • Atomic mass of Chlorine ($Cl$): $35.5$
  • Atomic mass of Iodine ($I$): $127$
  • Calculation: $\frac{127 + 35.5}{2} = 81.25$
  • Atomic mass of Bromine ($Br$): $80$.
• Conceptual Breakthrough: This law gave the first indications of periodicity, defined as the recurrence of properties of elements at regular intervals.

Dechancourtois Classification

• Historical Context: In $1862$, Alexandre-Émile Béguyer de Chancourtois arranged the elements.
• Cylindrical Table: Elements were arranged in increasing order of atomic mass in a cylindrical table (spiral screw).
• Vertical Alignment: Elements with similar properties were arranged in a vertical line from the center of the spiral.
• Reception: This system did not attract much attention from the scientific community.

Newland's Law of Octaves

• Historical Context: In $1864$, John Alexander Reina Newland arranged the known elements.
• Core Principle: Elements were arranged in increasing order of their atomic masses. Newland found that the $8^{th}$ element possessed properties similar to the $1^{st}$ element.
• Musical Analogy: It is compared to the musical scale, where the $8^{th}$ note resembles the first note.
• Examples of Series:
  • Series 1: $Li$ ($1$), $Be$ ($2$), $B$ ($3$), $C$ ($4$), $N$ ($5$), $O$ ($6$), $F$ ($7$), and then $Na$ ($8$, similar to $Li$).
  • Series 2: $Na$, $Mg$, $Al$, $Si$, $P$, $S$, $Cl$, and then $K$ (similar to $Na$).
• Periodicity: This law reinforced the concept of periodicity (the recurrence of properties at regular intervals).
• Defects and Limitations:
  • The law could be best applied only up to the element Calcium ($Ca$).
  • Newland assumed that only $56$ elements existed in nature and believed no more elements would be discovered.
  • Adjustment Errors: To fit elements into his table, he kept two elements in one slot ($Co$ and $Ni$). These elements ($Co, Ni$) were placed in the same group as Fluorine ($F$) and Chlorine ($Cl$), despite having different properties.
  • Distance Errors: Iron ($Fe$), Cobalt ($Co$), and Nickel ($Ni$) resemble each other in properties but were placed far away from one another.

Lothar Meyer Classification

• Historical Context: In $1869$, Lothar Meyer studied the physical properties of elements.
• Graphing Method: He plotted a graph between atomic volume and atomic masses of the elements.
• Observations on the Curve: Elements with similar properties occupy similar positions on the curve:
  • Peaks: The most electropositive alkali metals ($IA$ group) - $Li$, $Na$, $K$, $Rb$, and $Cs$.
  • Ascending Portion: The Halogens ($VIIA$ group) - $F$, $Cl$, $Br$, and $I$.
  • Descending Portion: The alkaline earth metals ($IIA$ group) - $Be$, $Mg$, $Ca$, $Sr$, and $Ba$.
• Conclusion: Physical properties like atomic volume, melting point ($MP$), and boiling point ($BP$) of elements are periodic functions of their atomic masses.

Mendeleev's Periodic Table

• Historical Context: In $1869$, Dmitri Ivanovich Mendeleev started his work with the $63$ elements known at that time.
• Mendeleev's Periodic Law: "The physical and chemical properties of the elements are the periodic functions of their atomic weights."
• Structural Organization:
  • In $1871$, he arranged elements in a table consisting of vertical columns called Groups and horizontal rows called Periods.
  • The table contains $7$ periods and $8$ groups.
  • Groups are further divided into sub-groups $A$ and $B$.
  • The first $3$ periods are "short periods" and the remaining are "long periods."
  • Each long period contains two rows of elements.
• Transitions and Gaps:
  • Group $VIII$ consists of $3$ triads known as transition triads.
  • Mendeleev left gaps for unknown elements and used the prefix "Eka" (meaning "preceding").
  • Predicted Elements (Eka) vs. Modern Discovery:
    1. Eka-Boron: Scandium ($Sc$)
    2. Eka-Aluminium: Gallium ($Ga$)
    3. Eka-Silicon: Germanium ($Ge$)
    4. Eka-Manganese: Technetium ($Tc$)
• Inversion/Anomalous Pairs: To maintain similarity in chemical properties, some elements were arranged in decreasing order of atomic mass:
  • Argon ($Ar$, weight $40$) and Potassium ($K$, weight $39$).
  • Cobalt ($Co$, weight $58.9$) and Nickel ($Ni$, weight $58.7$).
  • Tellurium ($Te$, weight $128$) and Iodine ($I$, weight $127$).
  • Thorium ($Th$, weight $232$) and Protactinium ($Pa$, weight $231$).
• Merits of Mendeleev's Table:
  • Prediction of properties and existence of then-undiscovered elements ($Sc$, $Ga$, $Ge$).
  • Correction of atomic masses based on position and equivalent weight (utilizing the formula: $\text{Atomic Weight} = \text{Equivalent Weight} \times \text{Valency}$). Elements corrected include $Be$, $Au$, $Pt$, $In$, $U$, and $Os$.
  • Accommodation of noble gases (later introduced by Ramsay and Rayleigh in a "Zero Group").
• Defects and Limitations:
  • Position of isotopes (e.g., $Cl – 35$ and $Cl – 37$) could not be explained.
  • Anomalous pairs of elements (the inverted mass order).
  • No correct position was assigned to Hydrogen.
  • Chemically different elements kept together (e.g., alkali metals $K$, $Rb$, $Cs$ with coinage metals $Cu$, $Ag$, $Au$).
  • Lanthanides ($14$ elements) and Actinides were kept in the same place despite different masses.
  • The concept of transition elements was defective.

Moseley's Experiments and Modern Periodic Law

• Experimental Set-up: Henry Gwyn Jeffreys Moseley bombarded various elements (acting as anticathodes) with cathode rays in a discharge tube, producing characteristic X-rays.
• Mathematical Correlation: Moseley showed the relationship between the frequency of X-rays ($\nu$) and the atomic number ($Z$):
  • $\sqrt{\nu} = a(Z - b)$
  • Where $a$ and $b$ are constants.
• Conclusion: A plot of $\sqrt{\nu}$ against $Z$ gives a straight line. No such linear relationship was found using mass number. This proved that atomic number is a more fundamental property than atomic mass.
• Modern Periodic Law: "The physical and chemical properties of the elements are the periodic functions of their atomic number."

Modern Periodic Table (Bohr's Table)

• Authorship: Prepared by Werner, Bury, Rang, and Bohr. It is also called the Long Form of the Periodic Table.
• Structural Features:
  • $7$ horizontal rows (Periods) and $18$ vertical columns (Groups/Families).
  • Period Breakdown:
    1. $1^{st}$ Period: $2$ elements (Shortest period).
    2. $2^{nd}$ and $3^{rd}$ Periods: $8$ elements each (Short periods).
    3. $4^{th}$ and $5^{th}$ Periods: $18$ elements each (Long/Normal periods).
    4. $6^{th}$ Period: $32$ elements (Longest period).
    5. $7^{th}$ Period: Incomplete.
  • Sub-energy levels filled per period:
    • $1 → 1s$
    • $2 → 2s, 2p$
    • $3 → 3s, 3p$
    • $4 → 4s, 3d, 4p$
    • $5 → 5s, 4d, 5p$
    • $6 → 6s, 4f, 5d, 6p$
    • $7 → 7s, 5f, 6d, 7p$
  • Special Classifications within Modern Table:
    • Bridge Elements: $2^{nd}$ period elements ($Li$, $Be$, $B$, $C$, $N$, $O$, $F$). They act as a bridge between groups.
    • Typical Elements: $3^{rd}$ period elements ($Na$, $Mg$, $Al$, $Si$, $P$, $S$, $Cl$) because they actively participate in chemical reactions.
    • Representative Elements: groups designated as "$A$ group" ($IA, IIA, IIIA, IVA, VA, VIA, VIIA$).
    • Transition Elements: groups designated as "$B$ group" ($IIIB$ to $IIB$).
    • Lanthanides: Elements Cerium ($Ce$) to Lutetium ($Lu$) with atomic numbers $58$ to $71$, following Lanthanum ($La$).
    • Actinides: Elements Thorium ($Th$) to Lawrencium ($Lr$) with atomic numbers $90$ to $103$, following Actinium ($Ac$).
• Merits:
  • Based on atomic number; rectifies Mendeleev’s weight inversions ($Ar$ and $K$).
  • Inert gases at the extreme right indicate completion of $s$ and $p$ subshells.
  • Clarifies position of transition elements like $Fe, Co, Ni$.
  • Separates active metals, transition metals, metalloids, non-metals, and radioactive metals.
• Defects:
  • Position of Hydrogen remains unjustified.
  • Lanthanides and Actinides are not included in the main body of the table.

Block Classification

• Elements are classified into $s, p, d, f$ blocks based on which orbital the "differentiating electron" (the last electron) enters.
  • s-block: Left side of the table. Includes Helium ($He$) due to $1s^2$ configuration (though placed in the zero group).
  • p-block: Right side of the table. Example: Aluminium ($Al$, $Z=13$), last electron enters $3p$.
  • d-block: Middle of the table. Example: Titanium ($Ti$, $Z=22$), differentiating electron configuration is $3d^2$.
  • f-block: Bottom of the table.