Physical properties of metals & non-metals

Physical properties of metals & non-metals

Welcome to your deep dive into the physical properties of metals and non-metals! In the world of chemistry, elements are broadly classified into these two fundamental groups. Understanding their distinct physical characteristics isn't just about memorising lists; it's about recognising the patterns that govern the materials that build our world, from the copper wires in our walls to the air we breathe.

This lecture will provide a comprehensive, exam-ready breakdown of every key physical property. We will explore not just the rules, but also the crucial exceptions that are often the focus of exam questions. Let's begin by getting a clear overview of the main differences.

{{TABLE: title=At a Glance: Metals vs. Non-Metals

This table is your roadmap for the entire topic. Now, let's dissect each of these properties in detail, starting with the characteristics that define metals.

Physical Properties of Metals

Metals are a group of elements known for their strength, shine, and ability to conduct heat and electricity. These properties arise from their unique atomic structure, specifically the presence of 'free' or 'delocalised' electrons.

{{ZOOM: title=The "Sea of Electrons" Model | text=Imagine a metal as a rigid lattice of positive metal ions (atoms that have lost electrons) sitting in a "sea" of delocalised electrons. These electrons are not tied to any single atom and are free to move throughout the entire structure. This mobility of electrons is the fundamental reason behind most of the classic metallic properties like conductivity and malleability.}}

1. Lustre

Metals, in their pure state, have a characteristic shining surface. This property is called metallic lustre. When light falls on the surface of a metal, the free electrons absorb the energy and get excited. They immediately fall back to their original energy levels, re-emitting the light, which we perceive as a shine.

• Examples: Gold, silver, and platinum are famous for their brilliant lustre, making them ideal for jewellery. Freshly cut surfaces of iron, copper, and aluminium also show this shine before they react with air (tarnish or rust).
• Real-world use: The shiny surface of silver is used to make mirrors by depositing a thin layer of it on glass.

2. Hardness

Most metals are hard and have high tensile strength. This means they can resist being scratched, dented, or broken. The strong metallic bonds holding the positive ions together in their lattice structure are responsible for this property.

• Examples: Iron, cobalt, and titanium are exceptionally hard metals, used in construction and engineering.
• Exceptions: This is a key area for exam questions! Some metals are surprisingly soft. Sodium (Na) and Potassium (K) are so soft they can be easily cut with a knife. Gallium (Ga) and Caesium (Cs) have very low melting points; gallium will melt in your hand!

{{KEY: type=points | title=Key Exceptions to Metal Hardness | text=

• Sodium (Na) & Potassium (K): Alkali metals that are very soft.
• Mercury (Hg): The only common metal that is liquid at room temperature.
• Gallium (Ga) & Caesium (Cs): Have very low melting points (29.76 °C and 28.44 °C respectively). }}

3. Malleability

This is one of the most defining properties of metals. Malleability is the ability of a substance to be hammered or rolled into thin sheets without breaking.

When a force is applied to a metal, the layers of positive ions can slide over one another. The delocalised electrons act as a flexible 'glue', continuing to hold the ions together in their new positions. This prevents the structure from shattering.

• Examples: Gold and silver are the most malleable metals. Gold can be beaten into sheets just a few atoms thick, known as gold leaf. Aluminium is also highly malleable, which is why we use aluminium foil for wrapping food.

{{VISUAL: diagram: showing a block of metal being hammered flat into a thin sheet, illustrating malleability, with layers of atoms sliding past each other.}}

4. Ductility

Closely related to malleability, ductility is the ability of a metal to be drawn into thin wires. Like malleability, this is possible because the layers of metal ions can slide past each other without breaking the metallic bonds.

• Examples: Copper is highly ductile, making it the primary material for electrical wiring. Gold is so ductile that a single gram can be drawn into a wire over 2 kilometres long! Tungsten is used as the filament in incandescent light bulbs because of its high melting point and ductility.

{{KEY: type=definition | title=Malleability & Ductility | text=Malleability is the property of being beaten into thin sheets. Ductility is the property of being drawn into thin wires. Both are characteristic of metals and result from the nature of the metallic bond.}}

5. Electrical and Thermal Conductivity

Metals are excellent conductors of electricity and heat. This is arguably their most important property in modern technology. The reason, once again, is the sea of delocalised electrons.

• Electrical Conductivity: When a voltage is applied across a metal wire, the free electrons are attracted towards the positive terminal, creating a flow of charge, which is an electric current.

• Thermal Conductivity: When one end of a metal is heated, the atoms at that end vibrate more vigorously. These vibrations are passed along the lattice. More importantly, the free electrons at the hot end gain kinetic energy and move rapidly to the colder end, transferring heat much faster than vibrations alone could.

• Best Conductors: Silver is the best conductor of both heat and electricity, followed closely by copper and gold. Copper is used for wiring because it's much cheaper than silver.

• Poorest Conductors: Lead and mercury are among the poorest metallic conductors.

6. State at Room Temperature

With one famous exception, all metals are solid at standard room temperature (around 20-25°C). The strong metallic bonds require a large amount of energy to be broken, resulting in high melting points.

• The Big Exception: Mercury (Hg) is the only metal that is a liquid at room temperature. This unique property makes it useful in thermometers and barometers.

7. Sonority

Metals are sonorous, meaning they produce a deep, ringing sound when struck. This happens because the impact creates vibrations that travel easily through the rigid, tightly packed lattice structure of the metal.

• Examples: This property is why bells are made of metal alloys like bronze. School bells, musical instruments like cymbals and tuning forks all rely on the sonority of metals.

8. Density and Melting/Boiling Points

Metals generally have high densities because their atoms are packed closely together in a regular lattice. They also have high melting and boiling points due to the strength of the metallic bonds that hold the atoms together.

• High Density Examples: Osmium (Os) and Iridium (Ir) are the densest known elements.
• Low Density Exceptions: Alkali metals like Lithium (Li), Sodium (Na), and Potassium (K) have low densities and can even float on water.
• High Melting Point: Tungsten (W) has the highest melting point of any metal (3422 °C), making it ideal for bulb filaments.
• Low Melting Point Exceptions: As mentioned, Caesium, Gallium, and of course, Mercury, have very low melting points.

Physical Properties of Non-Metals

Non-metals occupy the upper right side of the periodic table. Their properties are generally the opposite of metals. They lack the sea of delocalised electrons, and their atoms are typically held together by strong covalent bonds or weak intermolecular forces, leading to vastly different characteristics.

1. State at Room Temperature

Non-metals are the most versatile group in this regard. They can exist in all three states of matter at room temperature.

• Solids: Carbon (C), Sulphur (S), Phosphorus (P), Iodine (I).
• Liquid: Bromine (Br) is the only non-metal that is a liquid at room temperature.
• Gases: Oxygen (O₂), Nitrogen (N₂), Hydrogen (H₂), Chlorine (Cl₂), and all the noble gases (Helium, Neon, Argon, etc.).

2. Appearance (Lustre)

Most non-metals have a dull appearance and are not shiny. They do not have free electrons to reflect light in the way metals do.

• Key Exceptions:
  • Iodine (I): Has a lustrous, greyish-black crystalline appearance, similar to a metal.
  • Graphite & Diamond (Allotropes of Carbon): Diamond is famously brilliant and sparkly. Graphite has a dull metallic sheen. These exceptions are very important to remember.

{{KEY: type=exam | title=Common Trap: Exceptions in Non-Metals | text=Examiners love to ask about the exceptions to the rules. Remember: Iodine is lustrous, Graphite conducts electricity, and Diamond is extremely hard. These three non-metals defy the typical expectations.}}

3. Hardness

Non-metals are generally soft and not strong. Solid non-metals are often brittle.

• Examples: Sulphur is a brittle powder. Solid phosphorus is waxy and soft.
• The Ultimate Exception: Diamond, an allotrope (different structural form) of carbon, is the hardest naturally occurring substance known. Each carbon atom in a diamond is covalently bonded to four other carbon atoms in a strong, rigid, three-dimensional tetrahedral network.

4. Malleability and Ductility

Non-metals are brittle, which is the opposite of malleable and ductile. If you try to hammer a solid non-metal like sulphur or hit a piece of coal (carbon), it will shatter into pieces rather than changing shape.

This is because the atoms in non-metals are held by rigid covalent bonds or weak forces. When a force is applied, these bonds break, and the structure fractures. They cannot be beaten into sheets or drawn into wires.

5. Electrical and Thermal Conductivity

Non-metals are typically poor conductors of heat and electricity. They are considered insulators. Their electrons are tightly held in covalent bonds between atoms or by individual atoms, so they are not free to move and carry charge or heat energy.

• Examples: Wood (mostly carbon), plastic (carbon-based polymers), rubber (sulphur and carbon), and gases like air are all excellent insulators.
• The Star Exception: Graphite, another allotrope of carbon, is a good conductor of electricity. In graphite, each carbon atom is bonded to only three others in flat layers. The fourth outer electron of each atom is delocalised within the layer and is free to move, allowing graphite to conduct electricity. This is why it's used as electrodes in batteries and industrial electrolysis.

{{VISUAL: diagram: a simple electrical circuit with a battery and a bulb. Two versions are shown side-by-side. In one, a copper rod completes the circuit and the bulb lights up. In the other, a sulphur stick is used, and the bulb does not light up.}}

6. Sonority

Non-metals are non-sonorous. They do not produce a ringing sound when struck. Hitting a piece of wood or a block of sulphur produces a dull thud.

7. Density and Melting/Boiling Points

Non-metals generally have low densities and low melting and boiling points compared to metals.

• Reasoning: The atoms of gaseous non-metals are very far apart. In solid non-metals, the forces holding the molecules together (intermolecular forces) are often weak, so not much energy is needed to melt or boil them.
• Exceptions: Diamond and graphite have very high melting and boiling points because they are giant covalent structures, and breaking the vast network of strong covalent bonds requires a huge amount of energy. Silicon, another non-metal with a similar structure, also has a high melting point.

Final Summary and Practice

Let's consolidate our understanding with a quick practice question that mirrors what you might see in an exam.

{{TABLE: title=Final Property Cheatsheet

Practice Question: An element 'X' is a brittle solid with a dull appearance. It is a poor conductor of electricity. An element 'Y' is a shiny solid that can be drawn into a wire and is an excellent conductor of heat.

1. Categorise X and Y as a metal or a non-metal.
2. Give one example for each of X and Y.
3. If a third element 'Z' is a liquid at room temperature and conducts electricity, what could it be?

Solution:

1. Categorisation:
   • Element X is brittle (non-malleable/ductile), dull, and a poor conductor. These are all characteristic properties of a non-metal.
   • Element Y is shiny (lustrous), ductile, and a good conductor. These are all characteristic properties of a metal.
2. Examples:
   • An example of X (brittle, dull, solid non-metal) could be Sulphur or Phosphorus.
   • An example of Y (shiny, ductile, solid metal) could be Copper, Aluminium, or Iron.
3. Identification of Z:
   • Element Z is a liquid at room temperature and conducts electricity. The only metal that is liquid at room temperature is Mercury (Hg). It fits both descriptions.

{{FLASHCARD: q=What are the three main exceptions to remember for non-metals? | a=1. Iodine is lustrous (shiny). 2. Graphite conducts electricity. 3. Diamond is extremely hard.}}