fig. 9-1 Zinc and sulfur react chemically to give zinc sulfide, a substance whose properties are different from those of zinc and sulfur.

fig. 9-2 Classification of matter.

fig. 9-3 Elements and compounds have specific boiling and freezing points. A solution, here air, can therefore be separated into the elements or compounds it contains by boiling or freezing at an appropriate temperature, but a compound, here nitric oxide, cannot. Nitrogen boils at -196° C, oxygen boils at -183° C, and nitric oxide boils at -152° C.

fig. 9-4 An example of the law of definite proportions. Elements combine in a specific mass ratio when they form a compound. The mass ratio between the oxygen and nitrogen in nitric oxide is always 100:88.

fig. 9-5 Structures of several common molecules.

fig. 9-6 Sodium chloride crystals consist of NA⁺ and Cl^- ions rather than of neutral Na and Cl atoms or individual NaCl molecules.

fig. 9-7 The elements in a group of the periodic table have similar properties, whereas those in a period have different properties.

fig. 9-8 How chemical activity varies in the periodic table.

fig. 9-9 The majority of the elements are metals.

fig. 9-10 The transition elements are metals.

fig. 9-11 Electron structures of some atoms. In this schematic illustration of table 9-7 the circles without dots represent closed inner shells.

fig. 9-12 Electron shielding in sodium (a) and chlorine (b). Each outer electron in a Cl atom is acted upon by an effective nuclear charge 7 times greater than that acting upon the outer electron in a Na atom, even though the outer electrons in both cases are in the same shell.

fig. 9-13 Covalent bonding in hydrogen. The shared electrons spend more time on the average between their parent nuclei than on the far sides of the nuclei and therefore lead to an attractive internuclear force.

fig. 9-14 Ionic bonding. Sodium and chlorine combine chemically by the transfer of electrons from sodium atoms to chlorine atoms. The resulting ions attract electrically.

fig. 9-15 Ionic bonding in Na₂S. Each sodium atom contributes one electron to the sulfur atom, and the resulting S^2- ion attracts the two Na⁺ atoms.

fig. 9-16 When magnesium sulfate (MgSO₄) and barium chloride (BaCl₂) are dissolved in water, a precipitate of the insoluble compound barium sulfate is produced. The magnesium and chlorine ions remain in solution.

fig. 9-17 Electrolysis of water. An electric current decomposes water into gaseous hydrogen and oxygen. The volume of the hydrogen evolved is twice that of the oxygen, since water contains twice as many hydrogen atoms as oxygen atoms. A trace of sulfuric acid is used to enable the water to conduct electricity.

fig. 9-18 Schematic diagram of the electrolysis of water.