Notes
Types of bonds
| Bond | Between | What happens | Properties |
|---|---|---|---|
| ionic | metal + nonmetal | electrons transferred | high melting point, brittle, conducts when melted or dissolved |
| covalent | nonmetals | electrons shared | molecules, lower melting points |
| metallic | metal atoms | sea of shared electrons | conductive, shiny, malleable |
- Ionic formulas balance charges to zero: $\text{Al}^{3+}$ and $\text{O}^{2-}$ give $\text{Al}_2\text{O}_3$.
Lewis structures and shapes
- Count valence electrons, connect atoms, complete octets (H needs 2), then use double or triple bonds if needed.
- VSEPR: electron groups spread apart. 2 groups: linear 180°. 3: trigonal planar 120°. 4: tetrahedral 109.5°. With lone pairs: trigonal pyramidal ($\text{NH}_3$), bent ($\text{H}_2\text{O}$).
Polarity and intermolecular forces
- Electronegativity difference: under 0.4 nonpolar, 0.4 to 1.7 polar covalent, over 1.7 ionic.
- Symmetric molecules ($\text{CO}_2$, $\text{CH}_4$) are nonpolar even with polar bonds.
- IMFs from weakest to strongest: London dispersion < dipole-dipole < hydrogen bonding (H on N, O or F). Stronger IMFs raise boiling points.
Topics in this unit
Each topic has its own short notes and practice.
Practice
Magnesium is a metal and oxygen is a nonmetal.
Nitrogen has 3 bonds and 1 lone pair, so the atoms form a pyramid.
$4 + 2(1) + 6 = 12$.
Water's bent shape means its O–H dipoles do not cancel.
The O–H group lets ethanol molecules hydrogen bond, which takes more energy to overcome.
Freely moving electrons carry current through a solid metal.
Free response
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Free-response question
Carbon dioxide and water are both made of small molecules with three atoms. Yet $\text{CO}_2$ is a gas at room temperature and $\text{H}_2\text{O}$ is a liquid.
(a) Find the total valence electrons in each molecule and describe each Lewis structure.
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$\text{CO}_2$: $4 + 2(6) = 16$. Structure O=C=O, with two double bonds and two lone pairs on each O. $\text{H}_2\text{O}$: $2(1) + 6 = 8$. Structure H–O–H, with two single bonds and two lone pairs on O.
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(b) Give the shape and bond angle of each molecule using VSEPR.
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$\text{CO}_2$ has 2 electron groups on C and no lone pairs: linear, 180°. $\text{H}_2\text{O}$ has 2 bonds and 2 lone pairs on O: bent, about 104.5°.
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(c) Use polarity and intermolecular forces to explain the difference in states.
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$\text{CO}_2$ is linear, so its polar C=O bonds cancel and the molecule is nonpolar; only weak dispersion forces hold its molecules together, so it is a gas. $\text{H}_2\text{O}$ is bent and polar, and H bonded to O allows strong hydrogen bonds between molecules, so more energy is needed to separate them and it is a liquid.
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