Chapter 9: Atomic Foundations of Matter

In 1789 Antoine Lavoisier weighed everything before and after a reaction and found a pattern. The law of conservation of mass says that in a chemical reaction the total mass of the reactants equals the total mass of the products. When vinegar and baking soda react in an open flask, the reading falls because carbon dioxide escapes. When a balloon is tied to the flask, the gas is trapped and the reading stays the same. A reaction that forms no gas, such as sodium sulfate with barium chloride giving a white precipitate, keeps the same reading even in an open flask. The reason is that atoms are only rearranged, never lost.

Joseph Proust, a French chemist, found that a compound always contains the same elements combined in a fixed ratio by mass, whatever its source or the way it is made. This is the law of constant proportions, also called the law of definite proportions. In pure water, hydrogen and oxygen are always in the mass ratio 1 : 8, so 9 g of water gives 1 g of hydrogen and 8 g of oxygen. The ratio is by mass, not by number of atoms; in water the atoms are in the ratio 2 : 1. Mixtures do not follow the law, since sand and iron filings can be mixed in any ratio. To use a ratio, divide the given mass by its ratio number and multiply by the other.

In 1808 John Dalton proposed an atomic theory built on postulates, which are basic assumptions accepted without proof. He said that matter is made of tiny atoms; atoms are indivisible and cannot be created or destroyed in a chemical reaction; atoms of one element are identical in mass and chemical properties; atoms of different elements differ; and atoms combine in simple whole-number ratios, with fixed numbers and kinds in a given compound. The first idea explains the law of conservation of mass and the last explains the law of constant proportions. Today we know that atoms contain subatomic particles and that atoms of one element can differ in mass.

A molecule is an electrically neutral entity of more than one atom that can exist independently and shows the properties of the substance. Helium exists as single atoms because its valence shell is already full. Other atoms reach a full shell, a duplet for the K shell and an octet otherwise, by sharing electrons in a covalent bond. One shared pair is a single bond, two pairs a double bond and three pairs a triple bond. Two chlorine atoms share one pair in Cl₂, two oxygen atoms share two pairs in O₂ and two nitrogen atoms share three pairs in N₂. In compounds such as HCl, H₂O, NH₃ and CO₂ the atoms also share electrons. Covalent compounds are named with prefixes such as mono-, di-, tri-, tetra- and penta-.

A metal atom with fewer than four valence electrons tends to lose them, and a non-metal atom with more than four tends to gain or share them. A sodium atom (2, 8, 1) loses one electron and becomes Na⁺, which has 11 protons and 10 electrons. A chlorine atom (2, 8, 7) gains one electron and becomes Cl⁻. Positive ions are cations and negative ions are anions, and together they are called ions. The electrostatic attraction between them is the ionic bond. Ionic compounds form three-dimensional crystals; in sodium chloride each Na⁺ is surrounded by six Cl⁻. Names give the cation first and then the anion, ending in -ide; polyatomic ions such as NH₄⁺ are groups of atoms.

For a covalent compound, write the symbols with their valencies and criss-cross the valencies as subscripts; a subscript 1 is not written. For an ionic compound, write the cation first, put the charges under the symbols and criss-cross only the numbers, and the charges themselves are not written in the formula. Divide the subscripts by any common factor, because a formula gives the simplest ratio. When two or more polyatomic ions of the same type are needed, put them in brackets, as in Mg(NO₃)₂ and Al(OH)₃. The total positive charge must equal the total negative charge, so the compound is neutral.

An electric current in a solution is carried by moving ions. In solid sodium chloride the ions are held in fixed positions, so it does not conduct; dissolved in water or melted, its ions are free and it conducts. Ionic compounds such as sodium chloride and copper sulfate are generally soluble in water but not in petrol or kerosene, and they have high melting and boiling points because of strong attractions between ions. Covalent compounds such as camphor and naphthalene are generally insoluble in water but soluble in petrol and kerosene, and they have low melting and boiling points. Sugar is covalent and dissolves in water, but it gives no ions, so its solution does not conduct. In the test, use a low-voltage battery and keep petrol and kerosene away from flames.

The molecular mass of a covalent compound is the sum of the atomic masses of all the atoms in its formula, in the unit u. For oxygen, O₂ = 16 × 2 = 32 u, and for ammonia, NH₃ = 14 + 3 × 1 = 17 u. Ionic compounds do not form molecules; they form crystals, so we use a formula unit, which is the simplest whole-number ratio of ions. The formula unit mass is the sum of the atomic masses in the formula unit, for example NaCl = 23 + 35.5 = 58.5 u. When a bracket has a subscript, multiply everything inside it, so Ca(OH)₂ = 40 + 2 × (16 + 1) = 74 u.