原子结构与元素周期表

All substances are made of atoms 原子 — the smallest part of an element that can exist. Each element has a chemical symbol (O, Na, …); about 100 elements exist, shown in the periodic table.

Compounds 化合物 form when elements combine chemically in fixed proportions; they are represented by formulae and can only be separated back into elements by chemical reactions. Chemical reactions always form one or more new substances, often with a detectable energy change. Equations: word equations and symbol equations with balanced formulae; (HT) balanced half-equations and ionic equations.

Mixtures 混合物: two or more elements or compounds not chemically combined; each substance keeps its properties. Separated by physical processes only — no new substances:

• filtration 过滤 — insoluble solid from liquid;
• crystallisation 结晶 — dissolved solid from solution;
• simple distillation 蒸馏 简单蒸馏 — liquid from dissolved solid (and solvents);
• fractional distillation 分馏 — liquids with different boiling points;
• chromatography 色谱法 — soluble substances in a mixture.

Development of the atom model: atoms were first thought to be indivisible spheres → the electron's discovery gave the plum pudding model 布丁模型 (ball of positive charge with negative electrons embedded) → the alpha-scattering experiment showed mass and positive charge concentrated in a central nucleus (nuclear model) → Bohr: electrons orbit at specific distances → protons identified in the nucleus → Chadwick: the neutron 中子. New evidence changes or replaces models.

In an atom protons = electrons, so no overall charge. Atomic number = protons; mass number = protons + neutrons. Atoms of the same element with different neutron numbers are isotopes 同位素. Atom radius ≈ 0.1 nm (1 × 10⁻¹⁰ m); the nucleus is less than 1/10 000 of that. Almost all mass is in the nucleus.

Relative atomic mass (Ar) averages over the isotopes' abundance — calculate it from percentage abundances. Electronic structure: electrons fill the lowest levels first — sodium is 2,8,1; represent the first twenty elements both as numbers and diagrams.

The table orders elements by atomic (proton) number 原子序数, so that elements with similar properties fall in groups (columns). For main-group elements, group position reflects outer-shell electrons 最外层电子 and similar chemical properties; helium has two outer electrons, while the other Group 0 elements have eight. Explain an element's position from its electronic structure; predict probable reactivity from position.

Development: early tables ordered by atomic weight — incomplete, with some elements misplaced. Mendeleev left gaps for undiscovered elements and sometimes changed the order; his predicted elements were found, supporting his table. Isotopes later explained why atomic-weight order was sometimes wrong.

Metals and non-metals: metals usually lose electrons to form positive ions; many non-metals gain electrons to form negative ions or share electrons in covalent bonds. Metals sit left and lower; non-metals right and upper. Know the characteristic physical and chemical differences, and link atomic structure to position.

Group 0 — noble gases: unreactive — stable outer-shell arrangements (8 outer electrons; helium 2), so they do not easily form molecules. Boiling points increase down the group (with relative atomic mass).

Group 1 — alkali metals: one outer electron. Reactions of Li, Na, K with oxygen, chlorine and water (e.g. 2 Na + 2 H₂O → 2 NaOH + H₂). Reactivity increases down the group — the outer electron is lost more easily further from the nucleus.

Group 7 — halogens: seven outer electrons; non-metals, diatomic molecules (Cl₂, Br₂, I₂). Reactivity decreases down the group; melting and boiling points increase. A more reactive halogen displaces 置换 a less reactive one from a salt solution (chlorine displaces bromine from potassium bromide).

Compared with Group 1, the transition elements (exemplify with Cr, Mn, Fe, Co, Ni, Cu) have higher melting points, densities, strength and hardness, and are less reactive with oxygen, water and halogens.

Typical properties: ions with different charges (Fe²⁺/Fe³⁺), coloured compounds, and use as catalysts 催化剂 (iron in the Haber process).