Notes
Why bonds form
- Atoms bond to reach a full outer level (the octet rule). Valence electrons determine bonding.
Ionic and covalent bonds
- Ionic bonds: electrons transfer from a metal to a nonmetal, forming oppositely charged ions that attract. Ionic compounds have high melting points and conduct when melted or dissolved.
- Covalent bonds: nonmetals share electron pairs to form molecules. Single, double and triple bonds share 1, 2 and 3 pairs.
- Metallic bonds share a sea of free electrons.
Formulas and names
- Subscripts count atoms; ionic charges must add to zero ($\text{Mg}^{2+}$ + 2 $\text{Cl}^-$ gives $\text{MgCl}_2$).
- Binary ionic names end in -ide. Covalent names use prefixes (carbon dioxide, dinitrogen tetroxide).
Topics in this unit
Each topic has its own short notes and practice.
Practice
It has 2 valence electrons and loses them to form $\text{Ca}^{2+}$.
Hydrogen and oxygen are both nonmetals that share electrons.
$\text{Mg}^{2+}$ needs two $\text{Cl}^-$ ions.
The subscript 2 outside the parentheses multiplies both N and O inside.
2 N atoms is dinitrogen and 4 O atoms is tetroxide.
Fixed ions in a solid can't move, but they can when melted.
Free response
Write your answer first, then open the scoring guide.
Free-response question
Sodium (Group 1) reacts with chlorine (Group 17) to form sodium chloride, NaCl.
(a) Describe what happens to the electrons when sodium and chlorine bond.
Show the answer and scoring guide
Sodium transfers its one valence electron to chlorine. Sodium becomes $\text{Na}^+$ and chlorine becomes $\text{Cl}^-$.
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(b) Identify the type of bond and explain why NaCl has a high melting point.
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It is an ionic bond. The attraction between oppositely charged ions throughout the crystal is strong, so a lot of energy is needed to separate them.
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(c) Write the formula for the compound of $\text{Mg}^{2+}$ and $\text{Cl}^-$ and explain the subscript.
Show the answer and scoring guide
$\text{MgCl}_2$. One $\text{Mg}^{2+}$ has a $+2$ charge, so two $\text{Cl}^-$ ions are needed to make the total charge zero.
Check off the points you earned: