结构、成键与有机化学入门

An atom 原子 is one particle of an element 元素. An element contains only one type of atom, defined by proton number. An ion 离子 has an electrical charge because its electron count differs from its proton count. A molecule 分子 is a discrete group of covalently bonded atoms. A compound 化合物 contains different elements chemically combined. An empirical formula gives the simplest whole-number atom ratio; a molecular formula 分子式 gives actual atom numbers in one molecule. Ionic lattices have formula units, not separate molecules.

The mole 摩尔 is the unit of amount of substance. The particle count is $N=nL$, where $L=6.02\times10^{23}\ \mathrm{mol^{-1}}$. Thus one mole contains $6.02\times10^{23}$ of the specified entities. Always say whether you count atoms, molecules, ions or formula units. A mole of methane molecules contains five moles of atoms.

Relative atomic mass 相对原子质量, $A_r$, compares the abundance-weighted mean atom mass with one-twelfth of a carbon-12 atom's mass. Relative molecular mass 相对分子质量, $M_r$, adds the relative atomic masses in a molecule. Relative formula mass 相对式量 applies the same sum to a formula unit, including giant structures. Both are ratios without units. Molar mass 摩尔质量, $M$, is mass per mole, in $\mathrm{g\,mol^{-1}}$.

Use $n=m/M$ for a mass, $n=cV$ for a solution with $V$ in $\mathrm{dm^3}$, and $pV=nRT$ for a gas with pressure in Pa, volume in $\mathrm{m^3}$ and temperature in K. A stated molar gas volume can replace the gas equation only under its stated conditions. At the usual classroom RTP approximation, use $24\ \mathrm{dm^3\,mol^{-1}}$. Divide $\mathrm{cm^3}$ by 1000 to obtain $\mathrm{dm^3}$.

Worked reacting-mass method. The equation is $\mathrm{Mg+2HCl\rightarrow MgCl_2+H_2}$. There is excess acid and $0.480\ \mathrm g$ magnesium, with $M(\mathrm{Mg})=24.0\ \mathrm{g\,mol^{-1}}$.

The coefficient ratio is Mg:H₂ = 1:1, so $n(\mathrm{H_2})=0.0200\ \mathrm{mol}$. At the stated RTP approximation:

If both reactant amounts are given, compare amount divided by coefficient. The smaller value identifies the limiting reagent 限量试剂. Do not compare masses alone.

Mass concentration 质量浓度 is $m/V$ in $\mathrm{g\,dm^{-3}}$; amount concentration 物质的量浓度 is $n/V$ in $\mathrm{mol\,dm^{-3}}$. Divide mass concentration by molar mass to convert between them. Parts per million 百万分率, ppm, is a fraction multiplied by $10^6$; state whether it is a mass fraction or, for gases at the same temperature and pressure, a volume fraction.

To find an empirical formula 实验式, divide each element's mass by its $A_r$ and divide the resulting amounts by the smallest amount. Multiply the whole ratio if necessary to obtain integers. Do not round 1.5 to 2: multiply all ratios by 2. To find a molecular formula, divide measured $M_r$ by empirical-formula mass and multiply every subscript by this integer.

Worked check. A compound contains 2.40 g carbon and 0.600 g hydrogen. Carbon amount is $n=m/M=2.40/12.0=0.200\ \mathrm{mol}$; hydrogen atom amount is $n=m/M=0.600/1.00=0.600\ \mathrm{mol}$. C:H = 1:3, so the empirical formula is CH₃. If $M_r=30.0$, the factor is $30.0/15.0=2$, giving C₂H₆. The element masses add to the sample mass, 3.00 g.

Balance full equations without changing substance formulae, and include states: (s), (l), (g), (aq). For a precipitation, $\mathrm{Ag^+(aq)+Cl^-(aq)\rightarrow AgCl(s)}$ omits spectator ions. For an acid and carbonate, $\mathrm{CO_3^{2-}(aq)+2H^+(aq)\rightarrow CO_2(g)+H_2O(l)}$ conserves both atoms and charge. A precipitate or gas is an observation; an equation explains the change. Zinc in copper(II) sulfate gives a reddish copper deposit while the blue solution fades: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). Magnesium in dilute acid dissolves with hydrogen bubbles: Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g). An acid with a soluble base can warm without gas or precipitate: H⁺(aq) + OH⁻(aq) → H₂O(l). Match each equation to the actual observation rather than treating every acid reaction as gas formation.

Percentage yield 产率 compares actual with theoretical product amount. For a theoretical 4.00 g and actual isolated dry 3.20 g, yield = 3.20/4.00 × 100% = 80.0%. Atom economy 原子经济 compares the balanced desired-product mass with all balanced product masses. Include equation coefficients. A high atom economy does not guarantee a high experimental yield.

In the molar-gas-volume practical, use a known limiting amount and collect the gas under measured conditions. Account for leaks, gas dissolving, incomplete reaction and dead space. A leak lowers measured gas volume and therefore the calculated molar volume. To confirm a formula by reaction with oxygen, heat to constant mass, cool before weighing and explain how loss of solid or incomplete oxidation changes the inferred ratio. Practical reasoning remains part of this theory unit.

Protons and neutrons are in the nucleus; electrons occupy orbitals around it. Relative particle masses are approximately 1, 1 and $1/1836$; charges are +1, 0 and −1. Atomic number 原子序数 $Z$ counts protons; mass number 质量数 $A$ counts protons plus neutrons. Thus neutrons = $A-Z$. A positive ion has fewer electrons than protons. Isotopes 同位素 have the same proton number but different neutron numbers.

A mass spectrometer forms gaseous positive ions, separates them by mass-to-charge ratio 质荷比, $m/z$, and detects relative abundance 相对丰度, the proportion of the sample contributing each ion type. Vaporisation, ionisation, acceleration, separation and detection have distinct roles. In magnetic separation at a given accelerating potential, a lower $m/z$ ion bends more; a higher charge lowers $m/z$, rather than making bending smaller. A doubly charged mass-40 ion appears at $m/z=20$.

Worked isotope mean. For 60% mass-69 and 40% mass-71 gallium, $A_r=(69\times60+71\times40)/100=69.8$. The mean lies between the isotope masses, closer to the more abundant isotope. Molecular-ion peaks can identify $M_r$ when charge is known. With chlorine isotope proportions 3:1, Cl₂ molecular-ion peaks at 70, 72 and 74 have probabilities $9/16$, $6/16$ and $1/16$: the middle combination occurs in two orders.

An orbital 轨道 holds at most two electrons with opposite spins. An s orbital is spherical; a p orbital has two lobes. There are 1, 3 and 5 orbitals in s, p and d subshells, holding at most 2, 6 and 10 electrons. Fill equal-energy orbitals singly before pairing. The first four quantum shells have maximum capacities 2, 8, 18 and 32 electrons. Their available subshells are 1s; 2s/2p; 3s/3p/3d; and 4s/4p/4d/4f. An f subshell has seven orbitals and holds fourteen electrons. The periodic table has s, p and d blocks according to the subshell receiving the differentiating electron. Write configurations through krypton, including the chromium and copper exceptions: Cr is [Ar]3d⁵4s¹ and Cu is [Ar]3d¹⁰4s¹. For their positive ions, remove 4s electrons before 3d electrons. Electron-box notation must show both occupancy and spin.

First ionisation energy 电离能 is the energy per mole for $\mathrm{X(g)\rightarrow X^+(g)+e^-}$. Second ionisation removes an electron from X⁺(g); third ionisation is X²⁺(g) → X³⁺(g) + e⁻. All ionisation steps are endothermic. Greater nuclear charge strengthens attraction; more shielding and greater electron distance weaken it. A large successive-energy jump reveals that all outer-shell electrons have been removed. This identifies the main-group outer electron count but does not alone count every occupied shell.

Across Period 3, nuclear charge rises while inner-shell shielding changes little, giving a broad first-ionisation increase. Al loses a higher-energy 3p electron more easily than Mg loses a 3s electron. S has a paired 3p electron with greater repulsion than the singly occupied 3p orbitals in P. Down a group, increased distance and shielding outweigh increased nuclear charge. Recurring trends across periods are periodicity 周期性; use labelled axes, and interpret a logarithmic axis as ratios rather than equal energy increments.

An ionic bond 离子键 is the net electrostatic attraction between oppositely charged ions throughout a giant lattice. Electron transfer forms ions; the attraction is the bond. Conductivity when molten or dissolved, ion migration and electron-density evidence support the ionic model. Coloured cations move toward the negative electrode in solution. Electron-density maps show electron distribution around nuclei; compare the distribution with the ion model, rather than assuming a contour directly records the history of electron transfer. Ionic solids lack mobile ions. Smaller ions and higher charges usually strengthen lattice attraction. In an isoelectronic series, greater proton number pulls the same electron population closer: the radius decreases from N³⁻ toward Al³⁺. Down a group, extra occupied shells increase radius.

A covalent bond 共价键 is attraction between two nuclei and a shared electron pair. Dot-and-cross diagrams identify electron origin. Include lone pairs and brackets/charge for ions. A coordinate bond 配位键 uses a shared pair initially supplied by one atom, as when NH₃ donates a pair to H⁺ to form NH₄⁺. Aluminium chloride can form Al₂Cl₆ with two chloride bridges and coordinate bonding; electron diagrams must account for every outer electron.

Electron pairs around a central atom repel and arrange apart. Lone pairs repel more strongly than bonding pairs, but there is no universal fixed angle reduction per lone pair. Count regions around the central atom; a multiple bond occupies one region. Bond length 键长 is the distance between nuclei; bond angle 键角 is the angle between adjacent bonds.

Each carbon in ethene has three regions and a planar arrangement, with angles about 120°. Electronegativity 电负性 is attraction for a bonding electron pair. Different electronegativities produce polar bonds. A molecule is polar only if its bond dipoles do not cancel: CO₂ is non-polar; H₂O is polar. Ionic and covalent bonding are ends of a continuum. Polarising power 极化能力 describes a cation’s ability to distort another ion’s electron cloud; polarisability 极化性 describes how readily that cloud is distorted. A small, highly charged cation strongly distorts an anion's electron cloud; large anions are easier to polarise. Do not use one melting point alone as proof of structure.

Small molecular substances usually melt or boil by overcoming intermolecular attractions rather than breaking bonds inside molecules. Diamond has four covalent bonds per atom in a giant network; graphite has three per atom in layers and mobile delocalised electrons; graphene is a single layer. Diamond is hard and insulating; graphite layers can slide and conduct along them; graphene combines strength and conductivity. Metals contain positive ions attracted to delocalised electrons, which carry current in solid and molten metal.

The Period 2/3 melting trend follows changing structures: metallic elements, then giant covalent carbon/silicon, then small molecular non-metals and monatomic noble gases. In Period 2, lithium and beryllium are metallic; boron and carbon form giant covalent structures, then nitrogen, oxygen and fluorine are molecular and neon is monatomic. Molecular electron count and attractions help explain differences among the non-metals. A smooth nuclear-charge explanation alone cannot explain the whole melting trend. Stronger metallic attraction, involving ionic charge, ion size and the number of delocalised electrons, usually raises melting temperature; the detailed metal structure also matters. Heating a metal to melt it does not remove all its electrons.

A functional group 官能团 gives characteristic reactions. A homologous series 同系列 shares a functional group and general formula, with successive members differing by CH₂. Structural isomers 结构异构体 have the same molecular formula but different atom connectivity. In displayed formulae show every atom and bond; in skeletal formulae each unlabelled end or corner is carbon, with enough implied H atoms for four bonds. Show heteroatoms and their attached hydrogen explicitly.

Use meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec- for one to ten carbon atoms. Choose a parent containing the relevant functional group and number to give its required lowest locant, then locate substituents. Acyclic alkanes have formula CₙH₂ₙ₊₂; single-ring cycloalkanes have CₙH₂ₙ. Both are saturated: they contain only carbon–carbon single bonds. CₙH₂ₙ alone does not prove an alkene because a cycloalkane can share it.

Generate structural isomers systematically: keep the formula fixed, change the longest chain or ring size, distribute the remaining carbon atoms, then remove rotated or renumbered duplicates. Pentane has three chain isomers; hexane has five. A different drawing direction alone does not make a new structure.

For saturated single rings, begin with the largest ring and shorten it while moving carbons into substituents. There is one C₃H₆ ring structure, two C₄H₈ and five C₅H₁₀ structures. The C₆H₁₂ set has twelve constitutional structures; distinct cis/trans forms are not counted again as structural isomers.

Fractional distillation separates crude oil into fractions with different boiling ranges. Cracking converts larger hydrocarbons to smaller products, including alkenes; reforming can change straight chains to branched or cyclic structures. Balance atoms in every proposed equation. Complete combustion of a hydrocarbon forms CO₂ and water. Incomplete combustion can form toxic CO or carbon particulates; CO reduces blood oxygen transport by binding haemoglobin. Fuel sulfur and high-temperature nitrogen/oxygen reactions can produce acidic oxides. Carbon particulates harm air quality; unburned hydrocarbons can contribute to photochemical pollution. Removing fuel sulfur does not prevent nitrogen oxides formed from hot air. Carbon neutrality 碳中和 means no net carbon dioxide increase over the stated life-cycle boundary. Bioethanol's growth can take up CO₂, but processing and transport emissions prevent automatic carbon-neutral claims. Hydrogen makes water at use; its production method still matters.

A hazard 危害 is a potential source of harm; risk 风险 considers its likelihood and severity under actual conditions. Use the school's risk assessment, small quantities, ventilation and controls specific to flammability or toxicity. A safer alternative can reduce risk without pretending the original substance has no hazard.

A free radical 自由基 has an unpaired electron. Homolytic fission 均裂 gives one electron to each fragment; heterolytic fission 异裂 gives both to one fragment, forming ions. UV light starts chlorine substitution by breaking Cl₂ homolytically: Cl₂ → 2Cl·. Use single-headed curly arrows for individual electrons.

Propagation: Cl· + CH₄ → HCl + CH₃·, then CH₃· + Cl₂ → CH₃Cl + Cl·. One radical is regenerated, so the chain continues. Termination joins two radicals, for example 2CH₃· → C₂H₆. Further substitution and multiple possible positions produce mixtures, limiting the method's value for making one pure product.

An acyclic alkene with one C=C has formula CₙH₂ₙ. A cycloalkene with one ring and one double bond has CₙH₂ₙ₋₂: the ring removes another two hydrogens. A double bond consists of a sigma bond σ键 from head-on orbital overlap and a pi bond π键 from sideways p-orbital overlap. Rotating one carbon would break pi overlap, so rotation is restricted. E/Z isomerism 顺反异构 requires two different groups on each double-bond carbon. Rank the directly attached atoms by atomic number; if tied, compare the next atoms. Compare priorities separately on the two carbons. Higher-priority groups together give Z, opposite give E. Cis/trans is insufficient where no suitable identical substituents can be compared.

An electrophile 亲电试剂 accepts an electron pair. In HBr addition, a curly arrow begins at the pi bond and ends at H, while another begins at the H–Br bond and ends at Br. This forms a carbocation 碳正离子 and Br⁻; a bromide lone pair then forms the new carbon–bromine bond. Propene mainly gives 2-bromopropane because its route forms a secondary rather than primary carbocation. Tertiary carbocations are generally more stable than secondary, which are more stable than primary, under these conditions.

Bromine is polarised by the electron-rich double bond and adds across it; ethene forms 1,2-dibromoethane. Bromine decolourisation supports unsaturation in an appropriate organic test but does not identify one unique alkene. Avoid unrequired claims about stereochemical product mixtures.

Other additions: hydrogen with nickel gives an alkane; steam with an acid catalyst gives an alcohol; cold, dilute acidified manganate(VII) oxidises C=C to a diol. State reagents and conditions, not only product names. In addition polymerisation 加成聚合, open each C=C into the polymer backbone, retain its side groups, and show brackets with bonds extending through them and $n$ outside. A repeat unit 重复单元 is the smallest backbone segment whose repetition describes the chain, including its side groups. No small molecule is eliminated. Polymer disposal needs evidence: sorting/recycling, persistence, biodegradable 可生物降解的 alternatives and removal of toxic incineration gases have different advantages and costs.