The Periodic Table
Introduction
The periodic table organises all known elements based on their properties. ICSE Class 9 covers the historical development of the periodic table, the modern periodic law, and periodic trends.
Early Attempts at Classification
Dobereiners Triads (1817)
Groups of three elements with similar chemical properties where the atomic mass of the middle element is approximately the average of the other two.
| Triad | Element 1 | Mass | Element 2 | Mass | Element 3 | Mass | Average |
|---|---|---|---|---|---|---|---|
| 1 | Li | 7 | Na | 23 | K | 39 | (7+39)/2 = 23 |
| 2 | Ca | 40 | Sr | 88 | Ba | 137 | (40+137)/2 = 88.5 |
| 3 | Cl | 35.5 | Br | 80 | I | 127 | (35.5+127)/2 = 81.25 |
Limitation: Could only classify a few elements; not all elements fit into triads.
Newlands Law of Octaves (1864)
When elements are arranged in increasing order of atomic mass, every eighth element has properties similar to the first (like musical notes).
Limitation: Only worked up to calcium; could not accommodate new elements.
Mendeleevs Periodic Table (1869)
Mendeleevs Periodic Law: The properties of elements are a periodic function of their atomic masses.
Features:
- Arranged 63 known elements by increasing atomic mass
- Left gaps for undiscovered elements (predicted their properties)
- Corrected atomic masses of some elements
Achievements:
- Predicted properties of gallium (eka-aluminium), scandium (eka-boron), and germanium (eka-silicon)
- Grouped elements with similar properties together
Limitations:
- Position of isotopes (different masses, same chemical properties)
- Position of some elements violated mass order (e.g., Te and I)
- No place for noble gases (discovered later)
Modern Periodic Table
Modern Periodic Law
The properties of elements are a periodic function of their atomic numbers (not atomic masses). Proposed by Henry Moseley (1913).
Structure of the Modern Periodic Table
Periods (Horizontal rows):
- 7 periods in total
- Period 1: 2 elements
- Periods 2 and 3: 8 elements each (short periods)
- Periods 4 and 5: 18 elements each (long periods)
- Period 6: 32 elements
- Period 7: Incomplete
Groups (Vertical columns):
- 18 groups in total
- Elements in the same group have the same number of valence electrons
- Elements in the same group have similar chemical properties
| Group | Valence Electrons | Elements | Properties |
|---|---|---|---|
| 1 | 1 | Li, Na, K, Rb, Cs, Fr | Alkali metals |
| 2 | 2 | Be, Mg, Ca, Sr, Ba, Ra | Alkaline earth metals |
| 17 | 7 | F, Cl, Br, I, At | Halogens |
| 18 | 8 | He, Ne, Ar, Kr, Xe, Rn | Noble gases |
Periodic Trends
Atomic Size
- Across a period (left to right): DECREASES (nuclear charge increases, pulling electrons closer)
- Down a group (top to bottom): INCREASES (new shells are added)
Valency
- Across a period: First increases from 1 to 4, then decreases to 0
- Down a group: Remains constant (same number of valence electrons)
Metallic Character
- Across a period: DECREASES (left metals → right non-metals)
- Down a group: INCREASES
| Property | Across Period (→) | Down Group (↓) |
|---|---|---|
| Atomic size | Decreases | Increases |
| Valency | Varies (1-4-0) | Remains same |
| Metallic character | Decreases | Increases |
| Electronegativity | Increases | Decreases |
Common Mistakes With Fixes
| Mistake | Correction |
|---|---|
| Mendeleev used atomic number | Mendeleev used ATOMIC MASS; Modern table uses atomic number |
| All elements in a group have same properties | Same group elements have SIMILAR (not identical) properties |
| Atomic size increases across a period | Atomic size DECREASES across a period |
| Valency depends on total electrons | Valency depends on VALENCE electrons |
ICSE Exam Focus
| Topic | Marks (approx.) | Frequency |
|---|---|---|
| Modern periodic table (groups and periods) | 4-5 marks | Very common |
| Periodic trends | 3-4 marks | Very common |
| Mendeleevs contributions | 3-4 marks | Common |
| Dobereiners triads and Newlands octaves | 2-3 marks | Occasionally asked |
Self-Test
Q1: State the Modern Periodic Law.
Q2: How many periods and groups are there in the modern periodic table?
Q3: Explain the trend in atomic size across a period and down a group.
Q4: Why were noble gases not included in Mendeleevs original periodic table?
Q5: Give one example of Dobereiners triad and verify the law.
