Chemistry: Atoms to AP Chemistry · Module 2 of 15
Chemical Bonding
How and why atoms bond, how to name and write formulas for the results, and what shape they take.
Ionic Bonding & Ionic Compounds
- Ionic bonds form when electrons transfer from a metal to a nonmetal, creating oppositely charged ions (a cation and an anion) that attract each other.
- Ionic compounds form crystal lattices, have high melting points, and conduct electricity when dissolved in water or melted.
Explain why sodium (Na) and chlorine (Cl) form an ionic bond in NaCl.
Solution. Sodium (1 valence electron) loses that electron to become Na⁺; chlorine (7 valence electrons) gains it to become Cl⁻, and both achieve a full outer shell. The oppositely charged ions attract, forming an ionic bond.
Covalent Bonding & Molecular Compounds
- Covalent bonds form when two nonmetal atoms share one or more pairs of electrons so each reaches a stable outer shell.
- Single, double and triple bonds share 1, 2, or 3 electron pairs respectively; more shared pairs generally mean shorter, stronger bonds.
In a water molecule (H₂O), how many bonds does the oxygen atom form, and what type?
Solution. Oxygen forms two single covalent bonds, one with each hydrogen atom, sharing one electron pair with each.
Naming Compounds & Writing Formulas
- Ionic compounds: name the cation first, then the anion (ending in "-ide" for simple anions); use Roman numerals for transition metals with variable charge.
- Molecular (covalent) compounds use prefixes (mono-, di-, tri-, tetra-...) to indicate the number of each atom.
- To write an ionic formula, cross the ions' charges into subscripts, then reduce to the simplest ratio.
Write the formula for the ionic compound formed between calcium (Ca²⁺) and chlorine (Cl⁻).
Solution. Cross the charges: Ca²⁺ needs 2 Cl⁻ to balance, giving CaCl₂.
Molecular Shapes & Polarity
- VSEPR theory predicts molecular shape from electron pairs around the central atom repelling each other to be as far apart as possible.
- A bond is polar when atoms of different electronegativity share electrons unevenly; a molecule is polar overall only if its polar bonds don't cancel out due to symmetry.
Is CO₂ a polar or nonpolar molecule, even though its C=O bonds are polar?
Solution. CO₂ is linear and symmetric, so the two polar C=O bond dipoles point in opposite directions and cancel out, making the overall molecule nonpolar.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What type of bond forms between a metal and a nonmetal?
2. How many electron pairs are shared in a double covalent bond?