Compound Structure and Properties
Chemical Bonding — AP CHEMISTRY Revision Notes & Topical Questions
5 min read 10 parts Lesson 4 of 13 Unit quiz
Lesson summary
Chemical Bonding — AP CHEMISTRY notes: Classify bonds using electronegativity, compare ionic, covalent and metallic bonding, and interpret potential energy curves and Coulombic forces.Electronegativity = the tendency of an atom to attract the shared pair in a covalent bond, mainl…
- Nonpolar covalent
- Sodium chloride
- Bond energy
- Polar bond
- Ionic bond
- Bond length
Section 1 of 10Electronegativity
Electronegativity
Electronegativity the tendency of an atom to attract the shared pair in a covalent bond, mainly involving valence electrons.
It increases across a period because more protons raise the effective nuclear charge.
It decreases down a group because shielding and distance increase.
Fluorine is the most electronegative element.
Noble gases do not appear on the Pauling scale because they form no covalent bonds.
Section 2 of 10Covalent bonds
A covalent bond forms when two nonmetals share a pair of electrons to gain a full outer shell.
Single, double and triple bonds share one, two and three pairs.
The diatomic elements are H2, N2, F2, O2, I2, Cl2 and Br2.
Nonpolar covalent
- Nonpolar covalent electronegativity difference below about 0.4, as in Cl2.
Polar covalent
- Polar covalent difference between about 0.4 and 2.0.
- The more electronegative atom has a partial negative charge (δ−) and the other a partial positive charge (δ+).
- A larger difference gives a larger bond dipole.
Ionic character increases as the electronegativity difference increases.
Section 3 of 10Ionic bonding
An ionic bond forms when a metal transfers electrons to a nonmetal; the difference is greater than about 2.0.
The metal becomes a cation and the nonmetal an anion, both with the configuration of the nearest noble gas.
Sodium chloride
- Na+ has the configuration of neon and Cl− that of argon.
Strong Coulombic attraction between the ions holds the lattice together.
Section 4 of 10Distinguishing ionic and covalent compounds
| Property | Ionic | Covalent |
|---|---|---|
| Melting and boiling point | High | Low |
| Electrical conductivity | Only when molten or in solution | Do not conduct |
| Solubility | Generally soluble in water | Usually insoluble unless polar |
| Hardness | Hard, brittle | Soft |
| Forces | Strong Coulombic attraction | Weak forces between molecules |
| Electronegativity difference | Greater than 2.0 | Below 2.0 |
Section 5 of 10Metallic bonding
Metallic bonding
Metal atoms are tightly packed in a lattice and lose valence electrons to become positive ions.
The electrons are delocalised and not associated with any one atom.
The attraction between the cations and the sea of delocalised electrons is metallic bonding.
Section 6 of 10Potential energy and bond length
A potential energy diagram plots potential energy against internuclear distance.
- Far apart, attraction is small and the potential energy is near zero.
- Very close, nuclear and electron repulsion makes the energy very high.
- At the minimum the bond is stable and the distance is the bond length.
Bond energy
Bond energy the depth of the minimum, the energy needed to break the bond; it is positive because bond breaking is endothermic.
The minimum potential energy is always negative; H2 has −436 kJ/mol at 74 pm.
Bond length increases with atomic radius and decreases with bond order, while bond energy increases with bond order.
| Bond | Bond order | Length (pm) | Energy (kJ/mol) |
|---|---|---|---|
| C–C | 1 | 154 | 347 |
| C=C | 2 | 133 | 620 |
| C≡C | 3 | 120 | 812 |
Section 7 of 10Coulomb's law and ionic attraction
F
F kq1q2/r2; the force increases with ion charge and decreases with ionic radius.
What the symbols mean: F = electric force (N) · Q1, Q2 = charges (C) · r = separation (m) · k = 1/4πε0 = 8.99 × 109 N m2 C−2
A greater force means stronger interaction, higher lattice energy and a more negative minimum potential energy.
For potassium halides KCl, KBr and KI, the halide radius increases, so the internuclear distance rises and the lattice energy falls.
Section 8 of 10More from the specification
Compound
- Two or more elements chemically combined in a fixed ratio.
Metallic solid
- Lattice of cations in a sea of delocalised electrons.
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Electronegativity | Attraction of an atom for the electrons in a covalent bond. |
| Covalent bond | Sharing of an electron pair between nonmetals. |
| Polar bond | Covalent bond with unequal sharing of electrons. |
| Ionic bond | Electrostatic attraction between cation and anion. |
| Bond length | Internuclear distance at minimum potential energy. |
| Bond energy | Energy needed to break a bond. |
Section 10 of 10Exam technique
1
Saying electronegativity increases down a group — it decreases
2
Treating bond energy as negative — it is positive
3
Classifying bonds by electronegativity difference alone — metal and nonmetal character matters
4
Forgetting charge and distance when applying Coulomb's law
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