S2.3Structure 2 — Models of bonding and structure
The metallic model — IB Chemistry HL Revision Notes & Topical Questions
5 min read 11 parts Lesson 3 of 4 1 quiz
Lesson summary
S2.3 The metallic model — IB CHEMISTRY HL notes: Describe metallic bonding, explain the properties and uses of metals, the trends in metallic bond strength, and the properties of transition elements (HL).Lattice — metal atoms are tightly packed in a lattice.Delocalised electrons …
- Positive ions
- Lattice stability
- Strength and hardness
- Electrical conduction
- Under a potential difference
- Across a period
Section 1 of 11Metallic bonding
Lattice
- Lattice metal atoms are tightly packed in a lattice.
Delocalised electrons
- Delocalised electrons outer electrons are free to move throughout the structure and are not bound to any one atom.
Positive ions
- Positive ions losing electrons leaves positively charged metal ions (cations).
Bonding
Bonding the strong electrostatic attraction between the positive metal ions and the "sea" of delocalised electrons.
Lattice stability
- Lattice stability the positive ions repel each other and keep the arrangement in place.
Section 2 of 11Malleability, ductility and strength
Malleable
- Malleable can be hammered into sheets.
Ductile
- Ductile can be drawn into wires.
Cause
- Cause layers of ions slide over each other under force, and the delocalised electrons keep the structure bonded.
Strength and hardness
- Strength and hardness metals are typically strong because of the strong attraction between cations and electrons and the close packing of the ions.
Section 3 of 11Electrical and thermal conductivity
Electrical and thermal conductivity
Electrical conduction
- Electrical conduction metals conduct as solids and liquids because mobile electrons can move freely.
Under a potential difference
- Under a potential difference delocalised electrons move away from the negative terminal towards the positive terminal.
Across a period
- Na, Mg and Al have 1, 2 and 3 outer electrons, so conductivity increases with the number of delocalised electrons.
Heat conduction
- Heat conduction heated cations vibrate more vigorously and pass kinetic energy to neighbours by collisions.
Electrons and heat
- Electrons and heat cations transfer kinetic energy to the delocalised electrons, which carry it quickly through the metal.
Section 4 of 11Melting and boiling points
High values
- High values strong electrostatic attraction between cations and delocalised electrons needs a lot of energy to overcome.
Across a period
- Across a period melting and boiling points rise as the number of mobile charges increases and the attraction strengthens.
Section 5 of 11Choosing metals for uses
Properties considered
- Properties considered malleability and ductility, melting and boiling point, density, reactivity, electrical and thermal conductivity, strength, toxicity, lustre, strength-to-weight ratio, corrosion resistance and sonority.
Aluminium
- Aluminium used in food cans because it is non-toxic and resists corrosion and acidic food.
Copper
- Copper used in electrical wiring because it is a good conductor and malleable and ductile.
Stainless steel
- Stainless steel used for cutlery because it is strong and resists corrosion.
Section 6 of 11Strength of metallic bonding in s- and p-block metals
Strength of metallic bonding in s- and p-block metals
Factors
- Factors the charge on the metal ion and the radius of the metal ion.
Greater charge
- Greater charge more electrons in the sea and a bigger charge difference give stronger attraction.
Smaller radius
- Smaller radius the ion attracts the sea of electrons more strongly.
Number of delocalised electrons
- Number of delocalised electrons more per atom gives stronger bonding.
Period 3 data
- Period 3 data melting points of Na, Mg and Al are 371 K, 922 K and 933 K.
Group 1 data
- Group 1 data melting points of Na, K and Rb are 371 K, 337 K and 312 K.
Section 7 of 11Trends in melting point
Trends in melting point
Across a period
- Across a period valence electrons rise (Na 3s1, Mg 3s2, Al 3s2 3p1) and ions get smaller, so melting point increases from Na to Al.
Only metals
- Only metals only the first three elements of period 3 have metallic bonding.
Down a group
- Down a group the cation grows larger, which decreases attraction to the delocalised electrons, so melting point decreases from Li to Cs.
Trends not rules
- Al melts only slightly higher than Mg, because packing structure also matters, which is beyond the IB syllabus.
Section 8 of 11Transition elements (HL)
1Definition
- Definition an element with an incomplete d subshell, or one that forms at least one stable cation with an incomplete d subshell.
2Scandium and zinc
- Scandium and zinc not transition elements, because Sc forms only Sc3+ ([Ar] 3d0) and Zn only Zn2+ ([Ar] 3d10).
3First series
- Ti to Cu in period 4, Zr to Ag in period 5.
4Bonding
- 3d and 4s subshells are close in energy, so d electrons also delocalise and take part in metallic bonding.
5High melting points
- High melting points more delocalised electrons give stronger attraction,
- so period 4 transition metals melt higher than group 1 and 2 metals.
- (Be at 1287 °C is an exception due to its small size)
6High conductivity
- High conductivity many delocalised electrons carry charge; silver, copper and gold are the best conductors,
- and copper is widely used for cables because of cost and conductivity.
Section 9 of 11More from the specification
1Metallic bonding model
- Strong electrostatic attraction between a regular lattice of positive metal cations and delocalised valence electrons.
2Electrical conductivity mechanism
- When an electric potential difference is applied, delocalised electrons drift freely through the lattice carrying electric current.
3Physical properties of metals
- High melting points, malleability,
- and ductility arise because layers of ions can slide past each other without breaking non-directional metallic bonds.
4Transition metal bond strength
- Greater number of valence d-electrons involved in delocalisation enhances electrostatic bonding forces, yielding high hardness and melting points.
5Physical property
- A property observed or measured without changing the material.
6Electrostatic force
- Force of attraction between opposite charges.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Metallic bond | Attraction between metal cations and delocalised electrons. |
| Delocalised electrons | Outer electrons free to move through the metal lattice. |
| Malleable | Able to be hammered into shape. |
| Ductile | Able to be drawn into wires. |
| Transition element | An element with an incomplete d subshell or a stable ion with an incomplete d subshell. |
| Cation | A positively charged ion. |
Section 11 of 11Exam technique
1
Saying metallic bonds are between metal atoms in pairs — they are attractions between cations and the electron sea.
2
Explaining malleability by weak bonds — the layers slide and the delocalised electrons keep the bonding intact.
3
Saying delocalised electrons carry the positive charge — the cations stay fixed in the lattice.
4
Treating scandium and zinc as transition elements — they do not meet the definition.
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