Atomic Foundations of Matter · Lesson 14 of 14
Chapter Summary and Practice
“Every law, bond, formula and mass calculation returns for one connected review.”
• Connect the chemical laws with Dalton's atomic theory. • Compare covalent and ionic bonding using electron behaviour. • Write and name chemical formulae accurately. • Compare ionic and covalent properties. • Calculate molecular and formula unit masses. • Solve integrated problems involving electronic configuration, ions and bonding.
This chapter begins with measurements of matter and ends with a particle-level explanation of compounds. Conservation of mass tells us that matter is not created or destroyed during a chemical reaction. Constant proportions tells us that a pure compound has a definite mass composition. Dalton's atomic theory connects these observations to atoms that rearrange and combine in definite ratios.
Detailed Chapter Summary
Chemical Laws and Atomic Theory
In a physical change such as dissolving salt, the total mass remains unchanged when all material is included. In a chemical reaction, the same principle holds. An open setup can appear to lose mass when a gaseous product escapes, but a closed setup that retains the gas shows that total mass before and after is equal.
Matter can neither be created nor destroyed in a chemical reaction; total reactant mass equals total product mass.
The Law of Constant Proportions states that a given compound contains its elements in a fixed mass ratio. Pure water, for example, has hydrogen and oxygen in the mass ratio 1:8. Changing the amount of sample scales both parts while preserving the ratio.
A given compound contains its constituent elements in a fixed ratio by mass, independent of its source.
Dalton used atomic ideas to explain these patterns. Chemical reactions rearrange atoms rather than creating or destroying them, and a fixed relative number and kind of atoms gives a compound its definite composition.
Molecules and Chemical Bonds
A molecule is a neutral entity made from more than one atom that can exist independently and shows the properties of the substance. Atoms can become more stable by sharing electrons or by transferring electrons. The force holding atoms together in a stable arrangement is a chemical bond.
A bond formed by sharing electron pairs between atoms.
The electrostatic attraction between oppositely charged ions.
Hydrogen and chlorine form single covalent bonds by sharing one pair. Oxygen forms a double bond by sharing two pairs. Different elements can also share electrons, as in HCl and H₂O. In ionic bonding, sodium loses an electron to form Na⁺ while chlorine gains an electron to form Cl⁻. Opposite charges then attract.
Ions and Ionic Structures
A cation is positively charged because it has fewer electrons than protons. An anion is negatively charged because it has more electrons than protons. Ionic compounds form repeating three-dimensional crystals rather than separate molecules in the description used here.
Writing and Naming Formulae
Covalent compounds can be named using prefixes that indicate atom counts. Ionic compounds are named with the cation first and the anion second. Formula writing uses valencies or ionic charges. The criss-cross method must be followed by simplification to the smallest whole-number ratio and a charge-balance check.
Polyatomic ions must remain intact when repeated. This is why magnesium hydroxide is written Mg(OH)₂ and aluminium sulfate is Al₂(SO₄)₃.
Properties of Ionic and Covalent Compounds
Ionic solids do not conduct electricity because their ions are fixed. When an ionic compound dissolves in water or melts, the ions can move and therefore can carry charge. A covalent substance such as sugar can dissolve in water without producing ions, so its solution does not conduct in the same way. Ionic compounds generally have higher melting and boiling points than covalent compounds in the comparison made here.
Mass of Compounds
Molecular mass is used for covalent molecules and is obtained by adding the atomic masses of every atom in the molecule. Formula unit mass is used for the simplest ion ratio represented by an ionic formula. In both calculations, subscripts and brackets must be expanded correctly before atomic-mass contributions are added.
Important Concepts and Formulas
| Concept | Core idea |
|---|---|
| Conservation of mass | Total reactant mass = total product mass |
| Constant proportions | A pure compound has a fixed elemental mass ratio |
| Covalent bond | Electrons are shared |
| Ionic bond | Oppositely charged ions attract after electron transfer |
| Neutral ionic formula | Total positive charge = total negative charge |
| Molecular mass | Add atomic masses for all atoms in one molecule |
| Formula unit mass | Add atomic masses represented by the simplest ionic formula ratio |
At a Glance
Mass can neither be created nor destroyed in a chemical reaction. A compound contains the same elements in a fixed ratio by mass. Molecules are electrically neutral entities made of more than one atom and capable of independent existence. Atoms combine because stable bonded arrangements can have lower energy than the separated atoms.
Covalent bonds form by electron sharing. Ionic bonds follow electron transfer, producing cations and anions that attract each other. Covalent formulae show the elements and numbers of atoms present in a molecule. Ionic formulae show the simplest whole-number ratio represented in the ionic compound.
Molecular mass is the total mass of the atoms in a molecule. Formula unit mass is the sum of atomic masses represented by one formula unit of an ionic compound.
Revise, Reflect, Refine
Problem
Element A has one electron in its third shell as its valence electron. Element B has six electrons in its second shell as valence electrons. Predict the electron changes, ion types, bond type and formula pattern.
- 1.A has one valence electron and tends to lose one to reach a stable configuration, so it forms a +1 cation.
- 2.B has six valence electrons and needs two more to reach an octet, so it tends to gain two and form a 2− anion.
- 3.Electron transfer produces ions, so the bond is ionic.
- 4.Two A⁺ ions are needed to balance one B²⁻ ion.
- 5.The neutral formula pattern is A₂B.
Problem
An element X has six electrons in its outer shell and forms a diatomic molecule. Explain the bonding.
- 1.Each X atom needs access to two additional electrons to reach an octet.
- 2.Two X atoms can each share two electrons.
- 3.This creates two shared electron pairs.
- 4.The bonding is covalent and is represented as a double bond in the same way the chapter treats oxygen.
Quiz
Which description best matches Law of Conservation of Mass?
Which description best matches Law of Constant Proportions?
Which term matches this description: Matter can neither be created nor destroyed in a chemical reaction; total reactant mass equals total product mass.
Which term matches this description: A given compound contains its constituent elements in a fixed ratio by mass, independent of its source.
Which expression represents Conservation of Mass?
Practice Problems
- Choose which combinations correctly give equal total positive and negative charge: two Al³⁺ with three Cl⁻; three Mg²⁺ with one PO₄³⁻; two Fe³⁺ with three O²⁻; three Ca²⁺ with two SO₄²⁻.
- Correct the statement: 'A molecule of a compound is always made of atoms of the same element.'
- Write formulae for aluminium nitrate, calcium oxide and ferric oxide.
- Write formulae for Ca²⁺ with Br⁻, Al³⁺ with CO₃²⁻, K⁺ with SO₄²⁻, and NH₄⁺ with Cl⁻.
- Determine which electron-shell diagram correctly represents a chloride ion when chlorine has atomic number 17: it must contain 18 electrons.
- Calculate the mass represented by NH₄NO₃, H₃PO₄ and NaHCO₃ using appropriate atomic masses supplied with the problem.
- Write formulae for compounds formed by magnesium and nitrogen, lithium and nitrogen, sodium and sulfur, and aluminium and oxygen.
- Construct a cation-anion table using NH₄⁺, Li⁺, Al³⁺ and Cu²⁺ with nitrate, sulfate and phosphate.
- Verify conservation for 5.3 g sodium carbonate plus 6.0 g acetic acid forming 2.2 g carbon dioxide, 0.9 g water and 8.2 g sodium acetate.
- A species has 11 protons, 12 neutrons and 10 electrons. Determine its atomic number, mass number, charge type, electronic configuration and identity.
The Journey Beyond
Problem
Two elements have electronic configurations A: 2, 8, 5 and B: 2, 8, 7. Use the chapter's electron ideas to predict how they may combine.
- 1.A needs access to three additional electrons to complete an octet.
- 2.B needs access to one additional electron.
- 3.Both are described as atoms that tend to gain or share rather than readily donate the required electrons.
- 4.A combination between them is therefore reasoned through electron sharing rather than transfer.
- 5.The exact shared arrangement should be built so that the valence requirements of both atoms are satisfied.
Problem
Evaluate the claim that copper sulfate conducts when molten but not when solid because its ions are fixed when molten and mobile when solid.
- 1.The conductivity observation is consistent with ionic behaviour: the solid does not conduct and the molten material can conduct.
- 2.However, the reason reverses the particle mobility.
- 3.In the solid lattice, ions are fixed in position.
- 4.In the molten state, ions can move.
- 5.Therefore the assertion is true while the stated reason is false.
Problem
For species such as ²⁷Al, ⁸⁰Br⁻ and ²⁰¹Hg²⁺, determine electrons and neutrons when proton numbers are supplied.
- 1.Use neutron number = mass number − proton number.
- 2.For a neutral atom, electron number equals proton number.
- 3.For a negative ion, add electrons equal to the magnitude of the negative charge.
- 4.For a positive ion, subtract electrons equal to the magnitude of the positive charge.
- 5.Apply these rules separately to each species using its given mass and proton numbers.
- Design an investigation that compares purified water samples from different sources to test whether hydrogen and oxygen occur in the same mass ratio.
- Compare atoms and ions of any three elements using bar graphs of electron number before and after ion formation.
- Design a card game in which cation and anion cards must be matched in combinations that give neutral compounds.
- Use a molecule-building simulation to compare molecules of elements with molecules of compounds.
Are there any chemical changes that do not obey the Law of Conservation of Mass? Use the law as the starting point for further inquiry about how scientists define the complete system being studied.
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Formula Unit Mass of Ionic Compounds
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