In order to make sure that elements having similar properties fell in the same vertical column or group, Mendeleev left some gaps in his periodic table. These gaps were left for subsequent inclusion of elements not known at that time.
For example,
(i) Eka-aluminium (means one place below aluminium in the group); its atomic mass and chemical properties are quite similar to those of the element gallium discovered later in 1876.
(ii) Properties of eka-silicon are the properties of germanium.
Chapter Overview: The Periodic Table
The Periodic Table organises all known elements based on their properties. Dobereiner grouped elements into triads, Newlands proposed the Law of Octaves, and Mendeleev arranged elements by increasing atomic mass, predicting undiscovered elements. The Modern Periodic Table (Moseley) arranges elements by increasing atomic number. It has 7 periods and 18 groups. Periods represent the number of electron shells, while groups contain elements with similar valence electron configurations and chemical properties. The Class IX syllabus introduces students to the basic structure of the periodic table, the distinction between metals (left side), non-metals (right side), and metalloids (along the staircase line). Students learn periodic properties at an introductory level: atomic size, metallic character, and their general trends. The table is divided into s-block (Groups 1-2), p-block (Groups 13-18), d-block (Groups 3-12, transition metals), and f-block (lanthanides and actinides). Understanding the periodic table helps predict element behaviour and forms the basis for Class X chemistry.
Key Definitions
| Term | Definition |
|---|---|
| Dobereiner's Triads | Groups of three elements where the middle element's atomic mass is the average of the other two |
| Newlands' Octaves | Every eighth element has properties similar to the first (like musical octaves) |
| Mendeleev's Periodic Law | Properties of elements are periodic functions of their atomic masses |
| Modern Periodic Law | Properties of elements are periodic functions of their atomic numbers |
| Metals | Elements that are shiny, malleable, ductile, good conductors; tend to lose electrons |
| Non-metals | Elements that are dull, brittle, poor conductors; tend to gain electrons |
| Metalloids | Elements with properties intermediate between metals and non-metals (e.g., Si, Ge) |
Must-Know Concepts
- Alkali metals (Group 1): Li, Na, K - very reactive, 1 valence electron, stored in oil
- Halogens (Group 17): F, Cl, Br, I - very reactive non-metals, 7 valence electrons
- Noble gases (Group 18): He, Ne, Ar - inert, fully filled outer shells
- Metals are on the left and centre; non-metals on the upper right of the table
- Mendeleev left gaps for undiscovered elements: eka-boron (Scandium), eka-aluminium (Gallium), eka-silicon (Germanium)
- Limitations of Mendeleev's table: position of hydrogen, no place for isotopes, some elements out of order by mass
Metals vs Non-metals
| Property | Metals | Non-metals |
|---|---|---|
| State | Mostly solids (except Hg) | Solids, liquids, or gases |
| Lustre | Shiny (lustrous) | Dull (except diamond, iodine) |
| Conductivity | Good conductors | Poor conductors (except graphite) |
| Electron tendency | Lose electrons (form cations) | Gain electrons (form anions) |
Important Diagrams to Practice
- Layout of the Modern Periodic Table showing s, p, d, f blocks
- Position of metals, non-metals, and metalloids on the table
- Staircase line separating metals and non-metals
Common Mistakes
- Confusing Mendeleev's law (atomic mass) with the Modern law (atomic number)
- Thinking noble gases have 8 electrons in outermost shell (He has only 2)
- Placing hydrogen definitively in Group 1 or Group 17 (its position is debatable)
- Forgetting exceptions: Hg is a liquid metal, graphite conducts electricity despite being non-metal
Scoring Tips
- Know Mendeleev's predictions and how they were validated (eka-aluminium = Gallium)
- For comparison questions, always present in tabular form for clarity
- Remember the limitations of each classification attempt (triads, octaves, Mendeleev)
- Group names to memorise: 1 = Alkali metals, 2 = Alkaline earth, 17 = Halogens, 18 = Noble gases
Frequently Asked Questions
Why is hydrogen's position in the periodic table controversial?
Hydrogen has 1 valence electron like alkali metals (Group 1), but it is a non-metal and can gain 1 electron like halogens (Group 17). It does not fit perfectly in either group, so its position remains special.
What were the limitations of Newlands' Law of Octaves?
It worked only up to calcium (Z=20). After that, elements did not fit the pattern. He assumed all elements had been discovered and left no gaps for new elements. Also, he sometimes placed two elements in the same slot.
Why are noble gases called inert gases?
Noble gases have completely filled outermost electron shells (2 for He, 8 for others), making them extremely stable. They have no tendency to gain, lose, or share electrons, so they rarely participate in chemical reactions. However, some compounds of heavier noble gases (Xe, Kr) have been synthesised.