This chapter explains how atoms combine into molecules and compounds and how to describe them quantitatively. Two basic laws are established by experiment: the law of conservation of mass (matter is neither created nor destroyed in a chemical reaction, so total mass of reactants equals total mass of products, proposed by Lavoisier) and the law of constant (definite) proportions (a compound always contains its elements in a fixed ratio by mass whatever its source, e.g. water is always hydrogen to oxygen 1:8, proposed by Proust). These laws underlie Dalton’s atomic theory, whose postulates state that matter is made of indivisible atoms that combine in simple whole-number ratios. A molecule is an electrically neutral group of two or more atoms that can exist independently. Atoms bond to become stable by completing their octet: covalent bonds form by sharing electrons (single, double or triple bonds, as in \(\text{H}_2\), \(\text{O}_2\), \(\text{HCl}\), \(\text{H}_2\text{O}\)), while ionic bonds form by transfer of electrons creating cations and anions held by electrostatic attraction (as in NaCl, which forms a 3-D crystal lattice). The chapter teaches naming of covalent and ionic compounds and writing chemical formulae by criss-crossing valencies. Ionic compounds are usually water-soluble, high-melting and conduct electricity when dissolved or molten, while covalent compounds are usually low-melting and non-conducting. Finally it shows how to calculate molecular mass (for covalent compounds) and formula unit mass (for ionic compounds) by adding atomic masses.
Laws of conservation of mass and constant proportionsDalton’s atomic theoryCovalent bonding (sharing of electrons)Ionic bonding and properties of compoundsChemical formulae, molecular and formula unit mass
Key terms
Law of conservation of mass
In a chemical reaction matter is neither created nor destroyed, so the total mass of the products equals that of the reactants.
Law of constant proportions
A given compound always contains the same elements combined in a fixed ratio by mass, regardless of its source.
Covalent bond
A bond formed when atoms share one or more pairs of electrons to complete their octets, e.g. in water and chlorine molecules.
Ionic bond
The electrostatic force of attraction between oppositely charged ions formed by the transfer of electrons, e.g. in sodium chloride.
Molecular mass
The sum of the atomic masses of all atoms in a molecule of a covalent compound, e.g. water \(= 18\,\text{u}\).
Formula unit mass
The sum of the atomic masses of the ions in the simplest whole-number ratio of an ionic compound, e.g. \(\text{Na}_2\text{O} = 62\,\text{u}\).
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The law of conservation of mass states that in a chemical reaction, mass is:
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