Chemistry: Atoms First 2e · Electronic Structure and Periodic Properties of Elements
Ionic and Molecular Compounds
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Nearly every substance you encounter — table salt, water, carbon dioxide, sugar — is a compound. Compounds divide into two broad classes. Ionic compounds form when electrons transfer from a metal to a nonmetal, producing oppositely charged ions that attract each other (NaCl). Molecular (covalent) compounds form when nonmetal atoms share electrons, producing discrete molecules (H₂O).
This topic teaches you to classify a formula, write correct formulas from ion Atom or group of atoms with a net charge charges, and compare the classes' physical properties. It builds on the periodic table: an element's position predicts its ion charge, and that charge dictates the formula.
Why this matters
Deciding "ionic or molecular?" is the first step in almost every chemistry task — naming a compound, predicting melting point, or judging whether it will dissolve and conduct electricity:
- Everyday chemistry. Table salt (NaCl), baking soda (NaHCO₃), and limestone (CaCO₃) are ionic; water and CO₂ are molecular.
- Body chemistry. Nerve signals depend on Na⁺ and K⁺ moving across membranes; calcium phosphate hardens bone; electrolyte drinks replace ions lost in sweat.
- Predicting properties. Ionic compounds are crystalline solids with high melting points that conduct when melted or dissolved; molecular compounds are often gases, liquids, or low-melting solids that do not. Knowing the class tells you what to expect before touching a substance.
- Formula writing. Correct subscripts (CaCl₂, not CaCl or CaCl₃) separate a right answer from a wrong one — a classic exam trap.
The college version
Core Concepts
Ions: atoms with charge
An ion is an atom or group of atoms with a net electric charge. Metals lose valence electrons to become cations (positive); nonmetals gain electrons to become anions (negative). Charges follow the periodic table: group 1 → +1, group 2 → +2, group 15 → −3, group 16 → −2, group 17 → −1; aluminum forms Al³⁺; transition metals form several (Fe²⁺/Fe³⁺, Cu⁺/Cu²⁺).
Ions from a single atom are monatomic ions (Na⁺, Cl⁻, S²⁻). Ions from several bonded atoms are polyatomic ions (OH⁻, NO₃⁻, SO₄²⁻, NH₄⁺, PO₄³⁻, CO₃²⁻); they act as one charged unit and stay intact in compounds.
Writing formulas for ionic compounds
An ionic compound Charge-neutral compound of cations and anions Full entry → is electrically neutral, so total positive charge must equal total negative charge; the formula is the simplest whole-number ratio of ions. Ionic compounds are described by formula units, not molecules: a NaCl crystal is an extended alternating lattice of Na⁺ and Cl⁻, with no discrete "NaCl molecules."
To write a formula, combine ions in the ratio that balances charge. For calcium chloride:
Ca2+ + Cl- ⇒ (+2) + 2(-1) = 0 ⇒ CaCl2
When both charges have the same magnitude (Mg²⁺ and O²⁻), the ratio is 1:1, giving MgO — not "Mg₂O₂." Always reduce to the simplest ratio.
Molecular compounds and molecules
Molecular compounds form when two or more nonmetals share electrons. The formula gives the actual number of each atom in one molecule Discrete particle of atoms held by shared electrons Full entry →: H₂O has two H and one O; glucose is C₆H₁₂O₆. Molecular substances exist as discrete particles, and the same two elements can form several compounds (CO and CO₂), which is why molecular names use prefixes (Chapter 4).
Comparing the two classes
| Property | Ionic compounds | Molecular compounds |
|---|---|---|
| Bonding | Electron transfer; ion–ion attraction | Electron sharing; covalent bonds |
| Particles | Formula units in a crystal lattice | Discrete molecules |
| State at room temperature | Usually solids | Gases, liquids, or low-melting solids |
| Melting/boiling points | High (NaCl melts ≈ 801 °C) | Low to moderate (H₂O boils at 100 °C) |
| Electrical conductivity | Good when molten or dissolved; poor as solid | Poor in all states (most do not form ions) |
| Example | NaCl, CaCO₃, KNO₃ | H₂O, CO₂, CH₄, table sugar |
Molar mass of a compound
The molar mass Mass in grams of one mole (g/mol) Full entry → of a compound is the sum of the atomic masses of all atoms in the formula, in grams per mole:
molar mass of CaCl2 = 40.08 g/mol + 2(35.45 g/mol) = 111.0 g/mol
Molar mass is the conversion factor between moles and grams — the bridge between formula units and the balance.
Worked Example: Writing Formulas from Ion Charges
Problem. Write the formula of the compound formed by aluminum (Al³⁺) and oxygen (O²⁻).
Solution. Net charge must be zero. The smallest balancing ratio uses two Al³⁺ (+6) and three O²⁻ (−6):
2(+3) + 3(-2) = 0 ⇒ Al2O3
Al₂O₃ (aluminum oxide) is charge-neutral — the ratio is 2:3, not 1:1, because the ion charges differ. This is the "criss-cross" shortcut: exchange the charge magnitudes as subscripts, and the result is already reduced.
Worked Example: Molar Mass with Dimensional Analysis
Problem. A reaction calls for 0.500 mol of calcium chloride, CaCl₂. What mass should you weigh out?
Solution. First find the molar mass (Ca = 40.08 g/mol, Cl = 35.45 g/mol):
M(CaCl2) = 40.08 + 2(35.45) = 111.0 g/mol
Then convert moles to grams, arranging the factor so "mol" cancels:
0.500 mol CaCl2 × 111.0 g CaCl21 mol CaCl2 = 55.5 g CaCl2
Check units: mol cancels, leaving grams — and 55.5 g is about half the molar mass, which makes sense for half a mole. This dimensional analysis is the same used in every stoichiometry problem later in the book.
Worked Example: Identifying the Class
Problem. Classify each compound as ionic or molecular: (a) KNO₃, (b) C₆H₁₂O₆, (c) MgO, (d) NH₃.
Solution. (a) KNO₃ has a metal plus the nitrate ion — ionic. (b) C₆H₁₂O₆ is all nonmetals — molecular. (c) MgO is metal + nonmetal — ionic. (d) NH₃ is all nonmetals — molecular, even though the ammonium ion NH₄⁺ also contains N and H; molecule and ion are different species.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Ionic compounds | "Molecules" | NaCl exists as formula units in a lattice, not discrete NaCl molecules |
| MgO formula | "Mg₂O₂" | Ionic formulas are simplest ratios; equal-magnitude charges give 1:1 |
| Calcium chloride (CaCl₂) | Calcium chlorate, Ca(ClO₃)₂ | The -ide/-ate ending marks a different anion |
| Ammonia (NH₃) | Ammonium ion (NH₄⁺) | NH₃ is a neutral molecule; NH₄⁺ is a charged polyatomic ion that forms salts |
| Conduction by ionic compounds | In solid form | Solid NaCl does not conduct; ions must be molten or dissolved to move |
| Empirical vs molecular formula | Same thing | Identical for ionic compounds, different for many molecular ones (C₆H₁₂O₆ vs CH₂O) |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine atoms as children who give away or borrow toys. In an ionic compound, one atom gives an electron to another, so one becomes positive and the other negative, and they stick together like magnets in a repeating pattern — a salt crystal. In a molecular compound, atoms hold hands and share the toys, staying together as one small group, like a water molecule.
Key takeaways
- Ionic = metal + nonmetal (or ammonium + nonmetal); molecular = nonmetal + nonmetal.
- Cations are positive (metals lose electrons); anions are negative (nonmetals gain electrons).
- Monatomic ion charges come from the group: 1 → +1, 2 → +2, 15 → −3, 16 → −2, 17 → −1.
- Ionic formulas must be charge-neutral, in the simplest whole-number ratio (MgO, not Mg₂O₂).
- Ionic compounds are crystalline solids with high melting points; they conduct only when molten or dissolved.
- Molecular compounds form discrete molecules; many are gases or liquids at room temperature and do not conduct.
- Molar mass = sum of (atomic mass × subscript) over all atoms.
- Polyatomic ions (OH⁻, NO₃⁻, SO₄²⁻, NH₄⁺, CO₃²⁻, PO₄³⁻) stay intact in formulas.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Which two conditions allow an ionic solid to conduct electricity?
Show answer
When the solid is molten or dissolved in water, its ions are free to move and carry charge.
Write the formula for the compound of magnesium (Mg²⁺) and phosphorus (P³⁻).
Show answer
Three Mg²⁺ (+6) with two P³⁻ (−6) → Mg3P2.
Why is CO₂ molecular but CaO ionic?
Show answer
CO₂ joins two nonmetals sharing electrons → molecular; CaO combines a metal with a nonmetal by electron transfer → ionic.
Calculate the molar mass of water, H₂O (H = 1.01, O = 16.00 g/mol), and the mass of 2.00 mol of water.
Show answer
M = 2(1.01) + 16.00 = 18.02 g/mol; 2.00 × 18.02 = 36.0 g.
What charge does SO₄²⁻ carry, and what compound forms with two Na⁺ ions?
Show answer
Sulfate is −2; with two Na⁺ the neutral compound is Na2SO4, sodium sulfate.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- ion
- Atom or group of atoms with a net charge
- cation
- Positive ion formed when a metal loses electrons
- anion
- Negatively charged ion formed when a nonmetal gains electrons
- monatomic ion
- Ion from a single atom (Na⁺, Cl⁻, S²⁻)
- polyatomic ion
- Charged group of bonded atoms (OH⁻, NO₃⁻, SO₄²⁻)
- ionic compound
- Charge-neutral compound of cations and anions
- formula unit
- Simplest whole-number ratio of ions in an ionic compound
- molecule
- Discrete particle of atoms held by shared electrons
- empirical formula
- Simplest whole-number ratio of atoms
- molar mass
- Mass in grams of one mole (g/mol)
Sources & references
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