The Periodic Table

A. Dobereiner's Law of Triads (1817):

Elements with similar chemical properties arranged in groups of three (triads). The atomic weight of the middle element was approximately the arithmetic mean of the other two:

Limitation: Only three triads could be identified at the time.

B. Newlands' Law of Octaves (1866):

When elements are arranged in increasing order of atomic mass, the properties of every eighth element are similar to the first (like musical notes: sa, re, ga, ma, pa, dha, ni, sa).

Limitations: Worked only up to Calcium ($Z = 20$); failed for heavier elements; did not accommodate newly discovered noble gases.

C. Mendeleev's Periodic Law (1869):

"The physical and chemical properties of elements are periodic functions of their atomic masses."

• Merits: Left vacant spaces predicting unknown elements ($\text{Scandium}, \text{Gallium}, \text{Germanium}$); corrected doubtful atomic weights (e.g., Beryllium).
• Defects & Anomalies:
  1. Anomalous pairs: Higher atomic weight elements placed before lower ones (e.g., Argon $39.9$ before Potassium $39.1$; Cobalt $58.9$ before Nickel $58.7$; Tellurium $127.6$ before Iodine $126.9$).
  2. Position of Hydrogen was ambiguous (resembled both Group 1 alkali metals and Group 17 halogens).
  3. Isotopes (having different atomic masses) had no separate positions.

A. Moseley's Experiment (1913):

Henry Moseley bombarded elements with high-speed electrons and measured the frequency ($\nu$) of emitted characteristic X-rays. He discovered:

The frequency was directly proportional to the Atomic Number ($Z$), NOT atomic mass! Hence, atomic number is the true fundamental property of an atom.

B. Modern Periodic Law:

"The physical and chemical properties of elements are periodic functions of their atomic numbers."

C. Structure of the Long Form:

• 7 Horizontal Periods:
  • Period 1: 2 elements ($\text{H}, \text{He}$) — Shortest period.
  • Periods 2 and 3: 8 elements each — Short periods.
  • Periods 4 and 5: 18 elements each — Long periods.
  • Period 6: 32 elements (including 14 Lanthanides placed below) — Longest period.
  • Period 7: Incomplete / 32 elements (including 14 Actinides placed below).
• 18 Vertical Groups:
  • Group 1: Alkali Metals ($\text{Li}, \text{Na}, \text{K}, \text{Rb}, \text{Cs}$) — highly reactive monovalent metals.
  • Group 2: Alkaline Earth Metals ($\text{Be}, \text{Mg}, \text{Ca}, \text{Sr}, \text{Ba}$).
  • Groups 3 to 12: Transition Elements (variable valency, colored ions).
  • Group 16: Chalcogens (Ore-forming: $\text{O}, \text{S}, \text{Se}, \text{Te}$).
  • Group 17: Halogens (Salt-forming: $\text{F}, \text{Cl}, \text{Br}, \text{I}$).
  • Group 18: Zero Group / Noble Gases ($\text{He}, \text{Ne}, \text{Ar}, \text{Kr}, \text{Xe}, \text{Rn}$) — stable octet/duplet.

Problem: An element $X$ belongs to Period 3 and Group 16 of the Modern Periodic Table. State: (i) Number of valence electrons, (ii) Valency of $X$, (iii) Is $X$ a metal or non-metal? (iv) Name the element.

Solution:

1. Group 16 elements have $(16 - 10) = \mathbf{6\text{ valence electrons}}$.

2. To complete its octet, it needs $8 - 6 = 2$ electrons. Hence, its valency is $2$ (Divalent).

3. Since it has 6 valence electrons and gains electrons to form anions, it is a Non-metal.

4. Electronic configuration: Period 3 (3 shells: $K, L, M$) with 6 valence electrons $\implies 2, 8, 6 \implies Z = 16$. The element is Sulphur ($\text{S}$).