C7: Metallic Bonding and Alloys
Sea of delocalised electrons, properties of metals, alloys and shape memory alloys
📋 Key Definitions
Metallic bonding: The strong electrostatic attraction between positive metal ions and the sea of delocalised electrons that surrounds them. The metal ions are arranged in a regular lattice structure.
Delocalised electrons: Electrons that are not bound to any particular atom and are free to move throughout the metallic lattice. These come from the outer shell of each metal atom.
Alloy: A mixture of a metal with one or more other elements (usually other metals or carbon). Alloys are harder than pure metals because the different-sized atoms distort the regular layers and prevent them from sliding over each other.
Shape memory alloy: An alloy that can be deformed but returns to its original shape when heated. Nitinol (nickel-titanium alloy) is the most common example.
⚛️ Metallic Bonding Model
In metallic bonding, metal atoms lose their outer shell electrons to become positive ions. These electrons become delocalised - they are no longer attached to any particular atom and form a "sea" of free electrons that flows through the structure. The strong electrostatic attraction between the positive metal ions and the delocalised electrons holds the structure together.
Metal atoms → Metal ions⁺ + delocalised electrons
Key features of the metallic bonding model: Positive metal ions arranged in regular layers, surrounded by a sea of delocalised electrons. The strong electrostatic attraction between the ions and the electrons acts in all directions throughout the structure.
When describing metallic bonding, always mention three things: (1) positive metal ions in a lattice, (2) a sea of delocalised electrons, (3) strong electrostatic attraction between them. All three are needed for full marks.
📊 Properties of Metals
| Property | Explanation using metallic bonding model |
|---|---|
| High melting and boiling points | Strong electrostatic attraction between positive ions and delocalised electrons requires a lot of energy to overcome |
| Good conductors of electricity | Delocalised electrons are free to move through the structure and can carry charge when a voltage is applied |
| Good conductors of thermal energy | Delocalised electrons can transfer kinetic energy through the structure by colliding with each other and with ions |
| Malleable (can be hammered into shape) | Layers of positive ions can slide over each other because the delocalised electrons maintain the electrostatic attraction even when the layers shift |
| Ductile (can be drawn into wires) | Same reason as malleability - layers can slide while the electron sea holds the structure together |
| Shiny/lustrous | Delocalised electrons reflect light at the surface |
Explaining Malleability
When a force is applied to a pure metal, the layers of positive ions can slide over each other. The delocalised electrons move with them, so the electrostatic attraction is maintained and the metal does not break - it just changes shape. This is very different from ionic compounds, where sliding layers bring like charges together, causing the crystal to shatter.
A common question asks why metals are malleable but ionic compounds are brittle. The key difference: in metals, the electron sea maintains bonding when layers slide. In ionic compounds, sliding brings like charges together causing repulsion and the structure shatters.
🔧 Alloys
An alloy is a mixture of two or more elements where at least one is a metal. Alloys are made by mixing elements in the molten state and then allowing them to solidify. The different-sized atoms of the added element distort the regular layers of the metal lattice, making it harder for the layers to slide over each other.
Why alloys are harder than pure metals: In a pure metal, all the atoms are the same size so the layers can slide easily. In an alloy, the different-sized atoms disrupt the regular arrangement, preventing the layers from sliding. This makes the alloy harder and stronger than the pure metal.
Common Alloys and Their Composition
| Alloy | Composition | Properties and Uses |
|---|---|---|
| Brass | Copper + zinc | Hard, gold-coloured; used for musical instruments, door fittings |
| Bronze | Copper + tin | Hard, resistant to corrosion; used for statues, coins, ship fittings |
| Stainless steel | Iron + carbon + chromium (+ nickel) | Hard, resistant to corrosion; used for cutlery, surgical instruments, sinks |
| Carbon steel | Iron + carbon (small amount) | Harder than pure iron; used in construction, tools |
| Solder | Tin + lead (or tin + silver/copper) | Lower melting point than constituent metals; used in joining electrical components |
| Amalgam | Mercury + other metals | Used to be used in dental fillings (now largely replaced) |
Alloys are NOT compounds: Alloys are mixtures. The elements are not chemically combined in fixed proportions - the amounts of each element can vary. This is different from a compound like sodium chloride, which always has sodium and chlorine in a 1:1 ratio.
When asked why an alloy is harder than a pure metal, use the phrase "different-sized atoms distort the layers, preventing them from sliding over each other." You may be asked to draw or describe how smaller or larger atoms disrupt the regular arrangement.
🔄 Shape Memory Alloys
Shape memory alloys are smart materials that can return to their original shape after being deformed, simply by heating. The most well-known shape memory alloy is Nitinol.
Nitinol: An alloy of nickel and titanium. It can be bent out of shape but returns to its original form when heated. This property is useful in medical applications such as stents (tiny tubes inserted into blood vessels that expand to the correct shape at body temperature) and dental braces.
Uses of Shape Memory Alloys
- Medical stents - compressed for insertion, expand at body temperature
- Dental braces - apply constant gentle force as they try to return to their original shape
- Glasses frames - can bend and return to shape
- Actuators and sensors in aerospace engineering
📊 Comparing Bonding Types
| Property | Metallic | Ionic | Simple Covalent | Giant Covalent |
|---|---|---|---|---|
| Particles | Metal ions + delocalised electrons | Positive and negative ions | Molecules | Atoms in giant lattice |
| Bonding | Electrostatic attraction between ions and electrons | Electrostatic attraction between oppositely charged ions | Strong covalent bonds within, weak intermolecular forces between | Strong covalent bonds throughout |
| Melting point | High | High | Low | Very high |
| Electrical conductivity (solid) | Conducts (delocalised electrons) | Does not conduct (ions fixed) | Does not conduct (no free charges) | Most do not conduct; graphite does |
| Electrical conductivity (liquid) | Conducts | Conducts (ions free to move) | Does not conduct | N/A (decomposes before melting, mostly) |
| Malleability | Malleable (layers slide, electron sea maintains bonding) | Brittle (like charges align and repel) | Brittle (weak intermolecular forces) | Very hard (diamond) or soft layers (graphite) |
| Examples | Iron, copper, aluminium | NaCl, MgO | H₂O, CO₂, CH₄ | Diamond, graphite, SiO₂ |
This comparison table is one of the most important in the entire bonding topic. You may be asked to predict properties of an unknown substance based on its bonding type. Always link structure → bonding → property in your explanations.
❓ Practice Questions
Q1: Foundation Describe metallic bonding and explain why metals are good conductors of electricity.
Metallic bonding is the electrostatic attraction between positive metal ions arranged in a lattice and a sea of delocalised electrons. Metals conduct electricity because the delocalised electrons are free to move throughout the structure. When a voltage is applied, these free electrons can move in one direction, carrying charge through the metal.
Q2: Foundation Explain why an alloy such as brass is harder than pure copper.
In pure copper, all the atoms are the same size and arranged in regular layers that can slide easily over each other. In brass, zinc atoms of a different size are added. These different-sized atoms distort the regular arrangement of the layers, making it much harder for the layers to slide over each other. This makes the alloy harder and stronger than the pure metal.
Q3: Higher Explain why metals are malleable but ionic compounds such as sodium chloride are brittle. Refer to the bonding in each substance.
In metals, the positive ions are held together by delocalised electrons that can move. When a force is applied, the layers of ions can slide over each other while the delocalised electrons maintain the electrostatic attraction, so the metal bends without breaking. In ionic compounds like NaCl, the positive and negative ions are arranged in alternating positions. When a force causes the layers to slide, ions of the same charge are brought next to each other. These like charges repel each other strongly, causing the crystal to shatter.
Q4: Higher What is a shape memory alloy? Describe one use of Nitinol and explain why its shape memory property is important for this use.
A shape memory alloy is an alloy that can be deformed but returns to its original shape when heated. Nitinol (nickel-titanium alloy) is used in medical stents. The stent is compressed into a small tube for insertion into a blood vessel. At body temperature, the Nitinol returns to its original expanded shape, holding the blood vessel open. This property is important because it allows a minimally invasive procedure - the stent can be inserted in a compact form and then expands to the correct shape without surgery.
🎯 Exam Tips
When explaining metallic properties, always refer to the delocalised electrons. "Free electrons" is acceptable but "delocalised electrons" is the more precise scientific term that examiners prefer.
For alloy questions, mention that the atoms of the added element are a different size - this is the key reason layers cannot slide. Simply saying "different atoms" is not enough; you must explain that the different size disrupts the regular arrangement.
Be careful with terminology: alloys are mixtures, not compounds. The elements are not chemically bonded in fixed proportions.
🧮 Maths Skills
Maths in Metallic Bonding and Alloys
Percentage composition of alloys: Calculate the percentage of each element in an alloy. E.g. Brass is 65% copper and 35% zinc by mass. If a brass sample has a mass of 200 g, the mass of copper = 65% × 200 = 130 g and the mass of zinc = 35% × 200 = 70 g.
Converting between percentages and masses: If stainless steel contains 74% iron, 18% chromium and 8% nickel, and you have a 500 g sample: iron = 0.74 × 500 = 370 g, chromium = 0.18 × 500 = 90 g, nickel = 0.08 × 500 = 40 g. Check: 370 + 90 + 40 = 500 g ✓
Density calculations: Density = mass ÷ volume. If a gold alloy ring has a mass of 12.0 g and a volume of 0.85 cm³, density = 12.0 ÷ 0.85 = 14.1 g/cm³. Pure gold has a density of 19.3 g/cm³, so the alloy is less dense, confirming it contains other metals.
Graph skills: When given a graph of alloy hardness against percentage of added element, identify the optimum composition. Describe trends using "increases up to X% then plateaus/decreases." Extrapolate carefully from data without going beyond measured points.
❌ Common Misconceptions
Misconceptions About Metallic Bonding and Alloys
Wrong: Alloys are always stronger than pure metals
Correct: Most alloys are harder than the pure metal because different-sized atoms distort the layers and prevent them from sliding. However, not every alloy is stronger in every way — some alloys may have lower tensile strength or different properties depending on composition and treatment. The general rule is that alloys are harder, but "always stronger" is too absolute.
Wrong: Metals conduct electricity because they have free electrons that move between atoms
Correct: The correct scientific term is "delocalised electrons", not "free electrons". Delocalised electrons come from the outer shell of metal atoms and are free to move throughout the metallic lattice, but they are still attracted to the positive metal ions. "Free" suggests they are completely unattached, which is misleading. They are delocalised — not bound to any one specific ion, but still part of the metallic bonding structure.
Wrong: Alloys are compounds because they are made of more than one element
Correct: Alloys are mixtures, not compounds. The elements in an alloy are not chemically bonded in fixed proportions — the composition can vary. Compounds have fixed ratios (e.g. NaCl is always 1:1), but alloy compositions can be adjusted.
✍️ 6-Mark Extended Question
Question
Explain why metals are good conductors of electricity and heat. Use the metallic bonding model in your answer.
Electrical conductivity: In metallic bonding, metal atoms lose their outer shell electrons to become positive ions arranged in a regular lattice. These outer electrons become delocalised — they form a "sea" of mobile electrons that flows through the structure. When a voltage is applied across a metal, the delocalised electrons are attracted towards the positive terminal and move through the lattice in one direction. These moving charged particles carry electrical charge through the metal, making it a good conductor of electricity [3 marks].
Thermal conductivity: The delocalised electrons also explain why metals are good conductors of heat. When a metal is heated at one end, the ions and electrons at that end gain kinetic energy and vibrate more vigorously. The delocalised electrons can move freely through the lattice and transfer this kinetic energy rapidly by colliding with other electrons and ions throughout the structure. This transfers heat energy much faster than in non-metals, where heat can only travel by vibrations passing between neighbouring atoms [3 marks].
Mark scheme: 3 marks for electrical conductivity (positive ions in lattice, delocalised electrons, electrons carry charge when voltage applied); 3 marks for thermal conductivity (delocalised electrons gain KE, move through lattice, transfer energy by collisions). Must use the term "delocalised electrons" for full marks.🔍 AO3: Analyse and Evaluate
Evaluating Alloys for a Specific Purpose
Scenario: A company needs to choose a metal alloy for surgical implants (e.g. hip replacements). The implant must be: strong enough to support body weight, resistant to corrosion in body fluids, non-toxic, and lightweight where possible. Data for three candidate alloys:
| Alloy | Composition | Tensile Strength (MPa) | Corrosion Resistance | Density (g/cm³) | Biocompatibility |
|---|---|---|---|---|---|
| Stainless steel | Fe, Cr, Ni | 500 | Good | 7.9 | Moderate (Ni can cause allergies) |
| Titanium alloy | Ti, Al, V | 900 | Excellent | 4.5 | Excellent |
| Cobalt-chrome | Co, Cr, Mo | 1200 | Excellent | 8.3 | Good |
Evaluation: Titanium alloy is the best choice. It has high tensile strength (900 MPa, sufficient for body weight), excellent corrosion resistance in body fluids, excellent biocompatibility (no allergic reactions) and a much lower density (4.5 g/cm³) making the implant lighter and more comfortable. Cobalt-chrome is stronger but much heavier and more expensive. Stainless steel is cheapest but has the lowest strength, only good corrosion resistance, and nickel content can cause allergic reactions in some patients. The higher cost of titanium is justified by its superior combination of properties for medical use.
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