Ionic and covalent bonds
Atoms bond to reach more stable electron arrangements. Whether they transfer or share electrons determines the properties of the substance.
Why atoms bond
Atoms bond because bonded arrangements have lower energy than separate atoms. Many main-group atoms are most stable with eight valence electrons (an octet), like the noble gases. They reach it by transferring electrons (ionic bonding) or sharing them (covalent bonding).
Ionic bonds
Metals with few valence electrons transfer them to nonmetals. Sodium gives an electron to chlorine, forming \(\text{Na}^+\) and \(\text{Cl}^-\), held together by electrostatic attraction in a crystal lattice. Ionic compounds have high melting points, are often brittle, and conduct electricity when melted or dissolved, because their ions can then move.
Writing ionic formulas
Ionic formulas balance charges to zero. Magnesium (\(\text{Mg}^{2+}\)) and chloride (\(\text{Cl}^-\)) form \(\text{MgCl}_2\). Aluminium (\(\text{Al}^{3+}\)) and oxide (\(\text{O}^{2-}\)) form \(\text{Al}_2\text{O}_3\): the charges cross over to give subscripts, then reduce to lowest terms.
Covalent bonds
Nonmetals share electron pairs. In \(\text{H}_2\), each hydrogen contributes one electron to a shared pair. Oxygen in \(\text{O}_2\) shares two pairs (a double bond); nitrogen in \(\text{N}_2\) shares three (a triple bond, very strong). More shared pairs mean shorter, stronger bonds.
Lewis structures
Lewis structures show valence electrons as dots and shared pairs as lines. To draw one: count total valence electrons, connect atoms with single bonds, complete octets on outer atoms, place leftovers on the central atom, and form multiple bonds if the central atom lacks an octet. For \(\text{CO}_2\): \(\text{O}{=}\text{C}{=}\text{O}\), with two lone pairs on each oxygen.
Electronegativity and polarity
When two atoms with different electronegativities share electrons, the more electronegative atom pulls the pair closer, creating a polar covalent bond with partial charges. In \(\text{H–Cl}\), chlorine carries a partial negative charge. A large difference (roughly above 2) produces ionic bonding; equal atoms give nonpolar bonds.
Metallic bonding
In metals, valence electrons are delocalised in a 'sea' shared by all the metal ions. This explains why metals conduct electricity and heat, are malleable (layers can slide without breaking bonds), and shine (free electrons interact with light).
Worked example
Draw the Lewis structure of water. Valence electrons: \(2(1) + 6 = 8\). Connect O to each H (4 electrons used), then place the remaining 4 as two lone pairs on oxygen. Oxygen has an octet, each hydrogen has two electrons, and the lone pairs will matter for the molecule's shape.
Key takeaways
Atoms bond to reach lower energy, often an octet. Ionic bonds transfer electrons between metals and nonmetals; covalent bonds share pairs between nonmetals; metallic bonds use delocalised electrons. Lewis structures track valence electrons, and electronegativity differences determine bond polarity.
Flashcard
Atoms tend toward eight valence electrons
Check your understanding
What is the formula of the ionic compound formed by calcium (Ca^(2+)) and chloride (Cl^-)?