Properties of metals
Physical Properties: Metals vs Non-Metals
To understand the properties of metals, we must first define what a metal is in terms of its structure. Metals consist of a giant lattice of positive ions surrounded by a 'sea' of delocalised electrons. This unique bonding structure dictates their physical properties.
Thermal and Electrical Conductivity
Metals are excellent conductors of both heat and electricity. This is because the delocalised electrons are free to move throughout the lattice, carrying energy (heat) and charge (electricity). In contrast, most non-metals are poor conductors (insulators) because they lack these free-moving charged particles.
Metals are excellent conductors of both heat and electricity. This is because the delocalised electrons are free to move throughout the lattice, carrying energy (heat) and charge (electricity). In contrast, most non-metals are poor conductors (insulators) because they lack these free-moving charged particles.
Malleability and Ductility
Metals are malleable (can be hammered into sheets) and ductile (can be drawn into wires). This is possible because the layers of positive ions can slide over each other without breaking the metallic bond, as the sea of electrons adjusts to hold the ions together. Non-metals, which often form simple molecular structures or rigid giant covalent lattices, are typically brittle and will shatter if force is applied.
Metals are malleable (can be hammered into sheets) and ductile (can be drawn into wires). This is possible because the layers of positive ions can slide over each other without breaking the metallic bond, as the sea of electrons adjusts to hold the ions together. Non-metals, which often form simple molecular structures or rigid giant covalent lattices, are typically brittle and will shatter if force is applied.
Melting and Boiling Points
Generally, metals have high melting and boiling points because a large amount of energy is required to overcome the strong electrostatic forces between the positive ions and the delocalised electrons. However, there are exceptions: Group 1 metals (alkali metals like sodium) have relatively low melting points compared to transition metals.
Generally, metals have high melting and boiling points because a large amount of energy is required to overcome the strong electrostatic forces between the positive ions and the delocalised electrons. However, there are exceptions: Group 1 metals (alkali metals like sodium) have relatively low melting points compared to transition metals.
| Property | Metals |
|---|---|
| Non-Metals |
| Electrical Conductivity | Good conductors (solid and molten) |
|---|---|
| Poor conductors (insulators) |
| Thermal Conductivity | Good conductors |
|---|---|
| Poor conductors |
| Mechanical Property | Malleable and ductile |
|---|---|
| Brittle (if solid) |
| Melting/Boiling Point | Generally high (except Group 1) |
|---|---|
| Generally low |
| Density | Generally high |
|---|---|
| Generally low |
Malleability and Ductility
Malleability is the ability of a metal to be hammered or pressed into thin sheets without breaking. Ductility is the ability of a metal to be drawn out into thin wires. Both properties arise from the non-directional nature of metallic bonding, allowing layers of atoms to slide past one another.
Chemical Properties: Reactivity of Metals
Metals tend to lose electrons to form positive ions (cations). Their reactivity varies, but we can describe general reactions with three common substances: dilute acids, water, and oxygen.
Reaction with Dilute Acids
Most metals react with dilute acids (such as hydrochloric acid or sulfuric acid) to produce a salt and hydrogen gas. The general word equation is:
\text{Metal} + \text{Dilute Acid} \rightarrow \text{Salt} + \text{Hydrogen}
For example, iron reacts with dilute hydrochloric acid to form iron(II) chloride and hydrogen gas:
\text{Fe}(s) + 2\text{HCl}(aq) \rightarrow \text{FeCl}_2(aq) + \text{H}_2(g)
Note: The salt formed contains the metal ion and the anion from the acid (e.g., chloride from HCl, sulfate from H_2SO_4).
Most metals react with dilute acids (such as hydrochloric acid or sulfuric acid) to produce a salt and hydrogen gas. The general word equation is:
\text{Metal} + \text{Dilute Acid} \rightarrow \text{Salt} + \text{Hydrogen}
For example, iron reacts with dilute hydrochloric acid to form iron(II) chloride and hydrogen gas:
\text{Fe}(s) + 2\text{HCl}(aq) \rightarrow \text{FeCl}_2(aq) + \text{H}_2(g)
Note: The salt formed contains the metal ion and the anion from the acid (e.g., chloride from HCl, sulfate from H_2SO_4).
Reaction with Water
The reactivity of metals with water depends on their position in the reactivity series.
- Cold Water: Highly reactive metals (like Group 1 alkali metals, e.g., sodium) react vigorously with cold water to form a metal hydroxide and hydrogen gas.
2\text{Na}(s) + 2\text{H}_2\text{O}(l) \rightarrow 2\text{NaOH}(aq) + \text{H}_2(g) - Steam: Less reactive metals (like iron or magnesium) do not react with cold water but will react with steam to form a metal oxide and hydrogen gas.
3\text{Fe}(s) + 4\text{H}_2\text{O}(g) \rightarrow \text{Fe}_3\text{O}_4(s) + 4\text{H}_2(g)
Note: The product with steam is often a metal oxide, whereas the product with cold water for reactive metals is a metal hydroxide.
Reaction with Oxygen
All metals react with oxygen to form metal oxides. These oxides are typically basic or amphoteric, meaning they react with acids to form salts and water, unlike non-metal oxides which are acidic.
2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s)
This is why metals corrode or rust over time.
All metals react with oxygen to form metal oxides. These oxides are typically basic or amphoteric, meaning they react with acids to form salts and water, unlike non-metal oxides which are acidic.
2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s)
This is why metals corrode or rust over time.
Identifying Metals from Properties
Scenario: You are given four unknown solids: P, Q, R, and S.
- P is a shiny silver solid that conducts electricity and can be bent into shape.
- Q is a brittle black solid that does not conduct electricity.
- R is a soft metal that reacts violently with cold water.
- S is a hard metal with a very high melting point.
Analysis:
- P exhibits typical metallic properties: shiny, conductive, malleable. It is likely a transition metal like iron or copper.
- Q lacks conductivity and is brittle. These are non-metallic properties (e.g., sulfur or carbon).
- R is soft and reacts with cold water. This indicates a Group 1 alkali metal, such as sodium or potassium.
- S is hard with a high melting point. This suggests a transition metal like tungsten or iron.
Conclusion: P, R, and S are metals. Q is a non-metal.
⚠︎ Confusing Physical and Chemical Properties
The Error: Students often list chemical reactivity (e.g., 'reacts with acid') when asked for physical properties.
The Correction: Physical properties are those observed without changing the substance's chemical identity. Examples include melting point, density, conductivity, and malleability. Chemical properties describe how a substance reacts to form new substances.
The Correction: Physical properties are those observed without changing the substance's chemical identity. Examples include melting point, density, conductivity, and malleability. Chemical properties describe how a substance reacts to form new substances.
The Error: Stating that metals conduct electricity 'in solution'.
The Correction: Metals conduct electricity in the solid state due to delocalised electrons. Ionic compounds (salts) only conduct when molten or dissolved in water, but pure metals do not need to be molten to conduct.
The Correction: Metals conduct electricity in the solid state due to delocalised electrons. Ionic compounds (salts) only conduct when molten or dissolved in water, but pure metals do not need to be molten to conduct.
Describing Differences Between Metals
When to use: When asked to compare two different metals (e.g., sodium vs. iron) or a metal vs. a non-metal.
Why examiners accept this: Examiners look for specific comparative adjectives rather than just listing properties. Using words like 'higher', 'lower', 'softer', or 'more reactive' directly addresses the command word 'compare'.
Example: Instead of saying 'Sodium has a low melting point and Iron has a high melting point', write: 'Sodium has a significantly lower melting point than iron.' This explicitly highlights the difference.
Why examiners accept this: Examiners look for specific comparative adjectives rather than just listing properties. Using words like 'higher', 'lower', 'softer', or 'more reactive' directly addresses the command word 'compare'.
Example: Instead of saying 'Sodium has a low melting point and Iron has a high melting point', write: 'Sodium has a significantly lower melting point than iron.' This explicitly highlights the difference.
When to use: When writing equations for metals reacting with steam.
Why examiners accept this: Students often incorrectly write metal hydroxides as the product. Examiners require the correct oxide formula for high-temperature reactions with steam.
Example: For iron and steam, the correct product is magnetic iron oxide (Fe_3O_4), not FeO or Fe(OH)_2. Ensure your equation is balanced: 3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2.
Why examiners accept this: Students often incorrectly write metal hydroxides as the product. Examiners require the correct oxide formula for high-temperature reactions with steam.
Example: For iron and steam, the correct product is magnetic iron oxide (Fe_3O_4), not FeO or Fe(OH)_2. Ensure your equation is balanced: 3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2.
Past Paper Style Questions
Q:
Describe two physical properties that are typical of non-metals.
A:
- Poor conductor of electricity (or insulator). 2. Brittle (or not malleable/ductile). (Other acceptable answers: low melting point, poor thermal conductor).
Q:
State one difference in the physical properties of silver and sodium.
A:
Silver has a higher melting point than sodium. (Or: Silver is harder than sodium; Silver has a higher density than sodium).
Q:
Name the property of metals that allows them to be stretched into wires.
A:
Ductility.
Q:
Complete the symbol equation for the reaction between iron and steam:
3Fe + \dots H_2O \rightarrow Fe_3O_4 + 4\dots
3Fe + \dots H_2O \rightarrow Fe_3O_4 + 4\dots
A:
4H_2O; H_2
Q:
Which of the following is a property true for both sodium and iron?
A) Form acidic oxides
B) Form negative ions
C) Form basic oxides
D) Are gases at room temperature
A) Form acidic oxides
B) Form negative ions
C) Form basic oxides
D) Are gases at room temperature
A:
C. Both sodium and iron are metals, so they form positive ions and their oxides are basic.