Classification of Elements and Periodicity in Properties

Need for Classification and the Periodic Table

• At present, approximately $118$ elements are known to science. These elements form a vast number of chemical compounds.

• A systematic study is only feasible when elements are organized in a manner where the arrangement is closely related to their physical and chemical properties, thereby simplifying the study.

• The periodic table is defined as an arrangement of all known elements, based on a periodic law, such that elements with similar properties fall into the same vertical columns called groups.

• Elements within a single group resemble each other in their properties and differ from elements belonging to other groups.

Earlier Developments in the Classification of Elements

• Dobereiner's Triads: German chemist Johann Wolfgang Döbereiner identified that elements could be grouped in threes, called triads.

  • The middle element of a triad possesses an atomic weight approximately equal to the average of the other two.

  • The triad members possess similar chemical properties.

  • Examples include:

    • Triad (Li, Na, K): Atomic weight of Lithium ($7$) and Potassium ($39$). Average: $\frac{7 + 39}{2} = 23$, which is the exact atomic weight of Sodium ($Na$).

    • Other triads: (Cl, Br, I) and (S, Se, Te).

• Newlands' Law of Octaves: John Newlands observed that when elements are arranged in increasing order of atomic weight, the properties of every eighth element are similar to those of the first.

  • This was termed the Law of Octaves, analogous to musical notes.

  • Examples of octaves:

    • H, Li, Be, B, C, N, O

    • F, Na, Mg, Al, Si, P, S

    • Cl, K, Ca, Cr, Ti, Mn, Fe

  • Limitations: The law failed for elements with higher atomic weights (beyond Calcium). The discovery of inert gases also broke the pattern, as they did not fit the octavalent properties.

• Lothar Meyer's Arrangement: Meyer plotted physical properties such as atomic volume, density, melting point, boiling point, and thermal conductivity against atomic weights.

  • He found these properties varied in a periodic fashion.

  • His arrangement closely resembled Mendeleev's, though he is primarily remembered for his atomic volume curves.

Mendeleev's Periodic Table

• Mendeleev's Periodic Law: The physical and chemical properties of elements are periodic functions of their atomic weights (atomic masses).

• Significance:

  • Atomic mass is regarded as the most fundamental property of an element.

  • Elements are arranged in the increasing order of atomic masses so that elements with similar properties recur after regular intervals.

• Structural Features:

  • Horizontal columns were named periods, and vertical columns were named groups.

  • The original table (1869) contained about $65$ elements organized into ten series across seven horizontal and eight vertical columns.

  • Mendeleev prioritized grouping similar properties over strict adherence to atomic mass order.

• Modified Form of Mendeleev's Periodic Table:

  • Series: Elements are arranged in ten horizontal columns where atomic masses increase regularly.

  • Groups: Nine vertical columns (I to VIII and Zero). Except for Group VIII and Zero, groups are subdivided into A and B subgroups (e.g., IA, IB).

    • Group VIII consists of three sets, each containing three elements.

    • Zero group contains noble gases (discovered later).

  • Periods: Seven horizontal periods.

    • Period 1: $2$ elements (Short period).

    • Periods 2 & 3: $8$ elements each (Short periods).

    • Periods 4 & 5: $18$ elements each (Long periods).

    • Period 6: $32$ elements (Long period).

    • Period 7: Incomplete, currently containing $19$ elements (10th series).

• Key Contributions of Mendeleev:

  1. Systematic Study: Categorized elements logically for the first time.

  2. Discovery of New Elements: Mendeleev left gaps for unknown elements and predicted their properties.

     • Examples: Ekaaluminium (later discovered as Gallium) and Ekasilicon (later discovered as Germanium).

     • Comparison of Ekasilicon (predicted 1871) vs Germanium (determined by Winkler 1886):

       • Atomic mass: $72$ (pred) vs $72.6$ (obs).

       • Density: $5.5\,g\,cm^{-3}$ (pred) vs $5.36\,g\,cm^{-3}$ (obs).

       • Melting Point: High (pred) vs $1231\,K$ (obs).

       • Formula of Oxide: $MO_2$ (pred) vs $GeO_2$ (obs).

       • Formula of Chloride: $MCl_4$ (pred) vs $GeCl_4$ (obs).

  3. Correction of Atomic Masses: For example, Beryllium ($Be$) was previously thought to have an atomic mass of $14.8$. Mendeleev determined it was bivalent with an equivalent mass of $4.5$, leading to a corrected mass: $4.5 \times 2 = 9$.

• Defects of Mendeleev's Periodic Table:

  1. Position of Hydrogen: Resembles both Group IA (alkali metals) and Group VIIA (halogens).

  2. Position of Isotopes: Isotopes have different masses but similar chemical properties; they would require different slots under Mendeleev's law.

  3. Anomalous Pairs: Some elements with higher atomic masses are placed before lighter ones (e.g., Argon $39.9$ before Potassium $39.1$; Tellurium $127.61$ before Iodine $126.91$).

  4. Dissimilar Elements in Same Group: For example, Copper ($Cu$) and Silver ($Ag$) placed in the same group as Alkali metals despite different properties.

  5. Lanthanoids and Actinoids: These $14$ element blocks are not accommodated in the main body.

Modern Periodic Law and Long Form of the Periodic Table

• Moseley's Discovery: Henry Moseley showed that atomic number, not atomic mass, is the most fundamental property determining physical and chemical behavior.

• Modern Periodic Law: The physical and chemical properties of elements are the periodic functions of their atomic numbers.

• The Long Form (Present Form):

  • This table is based on electronic configurations.

  • Horizontal Rows (Periods): There are seven periods ($1-7$).

    • Period 1: Filling $n=1$ shell. Contains $2$ elements ($H$ and $He$).

    • Period 2: Filling $n=2$ shell ($2s, 2p$). Contains $8$ elements ($Li$ to $Ne$).

    • Period 3: Filling $n=3$ shell ($3s, 3p$). Contains $8$ elements ($Na$ to $Ar$). Note: $3d$ is skipped due to higher energy.

    • Period 4: Filling $4s, 3d, 4p$. Contains $18$ elements ($K$ to $Kr$).

    • Period 5: Filling $5s, 4d, 5p$. Contains $18$ elements ($Rb$ to $Xe$).

    • Period 6: Filling $6s, 4f, 5d, 6p$. Contains $32$ elements ($Cs$ to $Rn$).

    • Period 7: Filling $7s, 5f, 6d, 7p$. Also contains $32$ elements (including Actinoids).

  • Vertical Columns (Groups): There are $18$ groups, numbered $1$ to $18$ per IUPAC (1984).

IUPAC Nomenclature for Elements with Z > 100

• For elements discovered through artificial transmutation (Z > 100), IUPAC established rules to avoid naming conflicts (e.g., American vs Soviet scientists naming $Z=104$ Rutherfordium vs Kurchatovium).

• Naming Rules:

  1. Use numerical roots for digits: $0$ = nil, $1$ = un, $2$ = bi, $3$ = tri, $4$ = quad, $5$ = pent, $6$ = hex, $7$ = sept, $8$ = oct, $9$ = enn.

  2. Roots are sequenced based on the atomic number digits and terminated with "-ium".

  3. If "enn" precedes "nil", drop one 'n'. If "bi" or "tri" precedes "ium", drop the final 'i'.

• Example: Element $106$ ($1 = un, 0 = nil, 6 = hex$). Name: Unnilhexium. Symbol: Unh.

Division of the Periodic Table into Blocks

• s-block: Groups $1$ (IA) and $2$ (IIA). Valence electrons enter the $ns$ subshell. General configuration: $ns^{1-2}$.

• p-block: Groups $13$ to $18$. Valence electrons enter the $np$ subshell. General configuration: $ns^2 np^{1-6}$. (Helium is an exception).

• d-block: Groups $3$ to $12$. Differentiating electrons enter the $(n-1)d$ subshell. General configuration: $(n-1)d^{1-10} ns^{1-2}$.

• f-block: Found at the bottom (Lanthanoids and Actinoids). Electrons enter the $(n-2)f$ subshell. General configuration: $(n-2)f^{1-14} (n-1)d^{0-1} ns^2$.

Prediction of Period, Group, and Block

• Period: Determined by the principal quantum number of the valence shell.

  • Example: $Na$ ($[Ne] 3s^1$) is in Period $3$.

• Block: Determined by the subshell where the differentiating electron (the last electron added) resides.

  • Example: $Sc$ resides in the d-block because its $21^{st}$ electron enters the $3d$ subshell.

• Group:

  • s-block: Group number = number of valence electrons.

  • p-block: Group number = $10 + \text{number of valence electrons}$.

  • d-block: Group number = $2 + \text{number of } (n-1)d \text{ electrons}$.