Physical Properties of Metals and Non-Metals for CBSE Class 10 Science

All chemical elements can be broadly classified into two principal categories: metals and non-metals. From the copper wiring powering our electrical appliances and the iron girders supporting bridges, to the oxygen we breathe and the carbon forming the backbone of living tissue, metals and non-metals display distinctly contrasting physical properties.

In CBSE Class 10 Science, Chapter 3 (Metals and Non-Metals) begins with a comparative analysis of these physical characteristics. While general rules exist, the true test of board exam preparation lies in mastering the fascinating exceptions that deviate from textbook norms.

What You Will Learn

• General physical properties of metals: lustre, hardness, malleability, ductility, and conductivity
• General physical properties of non-metals
• The crucial exceptions to physical rules (mercury, bromine, gallium, caesium, iodine, alkali metals)
• The unique allotropes of carbon: diamond vs. graphite
• High-yield 1-mark board exam questions, observations, and error-prevention tips

1. Physical Properties of Metals

Metals possess a distinctive combination of physical attributes that make them indispensable for engineering, construction, and electrical applications:

1. Metallic Lustre: In their pure state, metals have a shiny, light-reflecting surface. This property is known as metallic lustre. When exposed to air over time, metals react with atmospheric gases and lose their shine, but rubbing with sandpaper restores their original lustre.
2. Hardness: Most metals are hard solids at room temperature. The hardness varies from metal to metal (e.g., iron and copper are very hard, whereas aluminium is moderately hard).
3. Malleability: The property by which metals can be beaten into thin sheets without fracturing is called malleability.
   • Gold and silver are the most malleable metals known. Gold can be hammered into sheets only a few nanometers thick (gold leaf).
4. Ductility: The ability of a metal to be drawn into thin wires is called ductility.
   • Gold is the most ductile metal. Remarkably, a wire of about $2\text{ km}$ length can be drawn from just $1\text{ g}$ of gold!
5. Thermal Conductivity: Metals are excellent conductors of heat and have high melting points.
   • Silver and copper are the best conductors of heat.
   • Lead and mercury are comparatively poor conductors of heat.
6. Electrical Conductivity: Metals conduct electricity efficiently due to the presence of free, mobile valence electrons. Electric transmission cables are made of copper and aluminium, coated with polyvinylchloride (PVC) or rubber-like material for electrical insulation.
7. Sonority: Metals make a ringing sound when struck hard against a surface. This property is called sonority, which is why school bells and musical instrument strings are made of metal.
8. High Density and High Melting Points: Most metals have high densities and melt only at very elevated temperatures (e.g., iron melts at $1538^\circ\text{C}$, tungsten melts at $3422^\circ\text{C}$).

2. Physical Properties of Non-Metals

Non-metals are far fewer in number compared to metals (about 22 non-metals exist), but they form the major portion of the Earth's atmosphere, oceans, and living biomass.

1. State: Non-metals exist either as solids (carbon, sulphur, phosphorus) or as gases (oxygen, nitrogen, hydrogen, chlorine) at room temperature.
2. Lustre: Non-metals have a dull appearance and do not possess lustre.
3. Brittleness: Solid non-metals are brittle; they shatter into powder when hammered and cannot be beaten into sheets or drawn into wires (they are neither malleable nor ductile).
4. Poor Conductivity: Non-metals are poor conductors of heat and electricity.
5. Low Density and Low Melting Points: Non-metals generally have low densities and relatively low melting and boiling points.

3. High-Frequency Board Exam Exceptions

Elements cannot be classified based on physical properties alone because several striking exceptions exist. CBSE board examinations frequently test these exceptions in multiple-choice and 1-mark assertion-reason questions:

                          Critical Physical Exceptions
                                       |
       +-------------------------------+-------------------------------+
       |                               |                               |
Liquid Elements                 Abnormal Melting               Anomalous Lustre & Hardness
- Metal: Mercury (Hg)           - Low Melting Metals:          - Lustrous Non-Metal: Iodine (I2)
- Non-Metal: Bromine (Br2)        Gallium (Ga) & Caesium (Cs)   - Soft Metals: Li, Na, K (cut with knife)
                                  (melt on palm)                - Hardest Natural Substance: Diamond

Exception 1: Physical State at Room Temperature

• Rule: Metals are solids; non-metals are solids or gases.
• Exceptions:
  • Mercury ($\text{Hg}$) is the only metal that is a liquid at room temperature ($25^\circ\text{C}$).
  • Bromine ($\text{Br}_2$) is the only non-metal that is a liquid at room temperature (a reddish-brown fuming liquid).

Exception 2: Melting and Boiling Points

• Rule: Metals have very high melting points.
• Exception: Gallium ($\text{Ga}$) and Caesium ($\text{Cs}$) have extremely low melting points ($29.8^\circ\text{C}$ and $28.5^\circ\text{C}$ respectively). If kept on your palm, they melt simply from body heat!

Exception 3: Metallic Lustre

• Rule: Non-metals are dull and non-lustrous.
• Exception: Iodine ($\text{I}_2$) is a non-metal, but it possesses a dark purple-black crystalline surface with a brilliant metallic lustre.

Exception 4: Hardness and Density of Metals

• Rule: Metals are hard and have high density.
• Exception: Alkali metals—Lithium ($\text{Li}$), Sodium ($\text{Na}$), and Potassium ($\text{K}$)—are so soft that they can be easily sliced with a kitchen knife. They have exceptionally low densities (sodium and potassium float on water!) and low melting points.

Exception 5: Allotropes of Carbon (The Greatest Non-Metal Anomaly)

Carbon is a non-metal that can exist in different structural forms, called allotropes:

1. Diamond: An allotrope of carbon, diamond is the hardest naturally occurring substance known and has an extremely high melting and boiling point ($> 3500^\circ\text{C}$).
2. Graphite: Another allotrope of carbon, graphite is soft, slippery, and is a good conductor of electricity because of free delocalized electrons in its layered hexagonal lattice.

Important: <u>Elements cannot be categorized into metals and non-metals definitively on the basis of physical properties alone due to these extensive exceptions. Chemical properties provide the true, unambiguous classification.</u>

4. Solved Board Examination Questions

Question 1: Identifying Elements by Unusual Properties

Problem: Give one example of each:

1. A metal which is a liquid at room temperature.
2. A metal which can be easily cut with a knife.
3. A metal which is the best conductor of heat.
4. A non-metal which is lustrous.
5. A non-metal which conducts electricity.

Solution:

1. Metal liquid at room temperature: Mercury ($\text{Hg}$)
2. Metal cut with a knife: Sodium ($\text{Na}$) (or Potassium / Lithium)
3. Best conductor of heat: Silver ($\text{Ag}$)
4. Lustrous non-metal: Iodine ($\text{I}_2$)
5. Non-metal that conducts electricity: Graphite (allotrope of carbon)

5. Master Comparison Table

Exam Tip: Whenever CBSE asks for a metal that melts when placed on the palm, write Gallium or Caesium. Never write mercury, as mercury is already a liquid at room temperature before you touch it!

Common Mistake: Stating that graphite is a metal because it conducts electricity. Graphite is an allotrope of carbon, which is strictly a non-metal!