Unit 1 - Lesson 2: Gradation of Properties in MPT (Atomic size, Electronegativity, Polar compounds, Metallic & Non-metallic properties)
The gradation of elemental properties across the Modern Periodic Table demonstrates the profound atomic harmony governing chemical nature, intrinsically connecting macroscopic behavior to orbital geometry and effective nuclear charge. Atomic size, measured in sub-nanometer picometers, systematically decreases across horizontal periods due to increasing nuclear electrostatic attraction on valence electrons, while expanding steadily down vertical groups as new principal energy levels are filled. Electronegativity provides the definitive metric for bond polarity, explaining the polar character of water and ammonia molecules. Furthermore, the periodic shift from strong alkali metals through metalloids to reactive halogens, along with the formation of basic, acidic, and amphoteric oxides, establishes the fundamental principles of chemical reactivity in full harmony with national curricula.
1. Gradation of Atomic Size
Atomic size is determined by atomic radius, measured in picometers (pm) where 1 pm = 1 × 10⁻¹² m:
- Across Periods:
Atomic size decreasesfrom left to right as atomic number increases, due to greater positive nuclear charge attracting valence electrons inward. - Down Groups:
Atomic size increasesfrom top to bottom as atomic number increases, due to the addition of complete principal energy levels. - Largest Atom: Cesium (Cs), located at the bottom left of the table (Group 1A).
- Smallest Atom: Fluorine (F), located at the top right of the table (Group 7A).
2. Electronegativity & Polar Compounds
Electronegativity: The ability of an atom in a covalent molecule to attract the bonding electrons toward itself:
- Polar Covalent Molecule: A covalent compound in which the difference in electronegativity between bonded atoms is relatively large.
- Key Polar Examples:
• Water (H₂O): Highly polar because electronegativity difference between O and H is large.
• Ammonia (NH₃): Polar because electronegativity difference between N and H is significant.
• Water is more polar than ammonia because the electronegativity difference (O - H) > (N - H).
3. Metallic, Non-metallic & Metalloid Elements
Elements are divided into four primary categories based on electronic structure and behavior:
| Category | Valence Electrons | Chemical Behavior | Examples |
|---|---|---|---|
| Metals | < 4 electrons | Tend to lose electrons forming positive cations | Na, Mg, Al |
| Non-metals | > 4 electrons | Tend to gain electrons forming negative anions | O, N, Cl |
| Metalloids | Intermediate properties | Possess properties of both metals and non-metals | Silicon Si, Boron B, Germanium Ge |
Period Gradation: Periods begin with strong metals, decrease in metallic character to metalloids, transition into non-metals, and end with inert noble gases.
4. Chemical Properties of Oxides
Chemical reactions of metallic and non-metallic oxides:
- Basic Oxides: Metal oxides, some of which dissolve in water forming alkaline solutions (turn litmus blue):
2Mg + O₂ ➔ 2MgO(Magnesium oxide)
MgO + H₂O ➔ Mg(OH)₂(Magnesium hydroxide - alkali). - Acidic Oxides: Non-metal oxides that dissolve in water forming acidic solutions (turn litmus red):
C + O₂ ➔ CO₂(Carbon dioxide)
CO₂ + H₂O ➔ H₂CO₃(Carbonic acid). - Amphoteric Oxides: Oxides that react with acids as bases and with bases as acids, yielding salt and water, such as aluminum oxide Al₂O₃.
Exercise 1: Comparing Atomic Radii in a Period
Problem: Arrange in descending order of atomic size: ¹¹Na, ¹²Mg, ¹⁷Cl.
• All three belong to Period 3.
• Across a period, atomic size decreases as atomic number increases.
• Descending order: ¹¹Na > ¹²Mg > ¹⁷Cl.
Exercise 2: Polarity of Water vs Ammonia
Problem: Explain why water (H₂O) is more polar than ammonia (NH₃).
• The electronegativity of oxygen (3.5) is greater than that of nitrogen (3.0).
• Consequently, the electronegativity difference between oxygen and hydrogen in water is larger than that between nitrogen and hydrogen in ammonia.
Exercise 3: Reaction of Metals with Dilute Acids
Problem: What happens when a magnesium ribbon is placed in dilute hydrochloric acid? Write the balanced equation.
• Magnesium displaces hydrogen, effervescing hydrogen gas that pops when exposed to a flame, forming magnesium chloride salt.
• Equation: Mg + 2HCl ➔ MgCl₂ + H₂ ↑.
Exercise 4: Non-metals with Dilute Acid
Problem: What happens when a piece of carbon is placed in dilute hydrochloric acid?
• No reaction occurs because non-metals do not react with dilute acids under standard conditions.
Exercise 5: Producing an Alkaline Solution from Magnesium
Problem: Write balanced equations showing how to obtain an alkaline solution turning litmus blue from magnesium metal.
• Step 1: Burning magnesium in oxygen: 2Mg + O₂ ➔ 2MgO.
• Step 2: Dissolving magnesium oxide in water: MgO + H₂O ➔ Mg(OH)₂ (magnesium hydroxide turns litmus blue).
Exercise 6: Nature of Amphoteric Oxides
Problem: What is an amphoteric oxide? Provide a primary example.
• An oxide that reacts with acids as a basic oxide and with bases as an acidic oxide, producing salt and water in both scenarios.
• Primary example: Aluminum oxide Al₂O₃.
Exercise 7: Copper Behavior with Dilute Acids
Problem: Explain why copper metal does not react with dilute hydrochloric acid.
• Because copper follows hydrogen in the chemical activity series, so it cannot displace hydrogen from dilute acids.
Exercise 8: Litmus Test for Non-metal Oxides
Problem: Describe the effect of dissolving carbon dioxide in water on litmus solution.
• Carbon dioxide reacts with water forming carbonic acid (H₂CO₃), which turns litmus solution red due to its acidic nature.
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