Chemistry, oriented

Every atom has a nucleus of protons and neutrons with electrons around it. The atomic number, the count of protons, decides the element; isotopes of one element differ only in neutrons, so they share their chemistry but not their mass. The atomic mass on the table is an average weighted by isotope abundance, which is why chlorine reads about 35.45 even though no chlorine atom has that mass.

The periodic table orders elements by atomic number. Elements in a column share the same number of outer electrons, so they behave alike. Electrons fill orbitals in a set order, at most two per orbital with opposite spins, and the table itself shows that order. Across a period the nucleus pulls harder on the outer electrons, so atoms shrink and hold their electrons more tightly; down a group, added shells make atoms larger and their outer electrons easier to remove. Fluorine is the most electronegative element.

Atoms form ions by reaching a noble gas's electron count: group 1 metals lose one electron and group 17 nonmetals gain one. In an ionic formula, the charges must balance, and that sets the subscripts. Name the metal first and the nonmetal second with an -ide ending, and add a Roman numeral for a metal that has more than one common charge.

Rule: when you meet an unfamiliar element, find its group and period first; most of its behavior follows from where it sits.

A mole is 6.022 × 10^23 particles, and a substance's molar mass is the mass of one mole in grams. Molarity is moles of solute per liter of finished solution, so moles equal molarity times volume in liters. Diluting a solution adds solvent but no solute, so the concentration falls as the volume rises.

A balanced equation conserves atoms: change coefficients, never subscripts, because a new subscript describes a different substance. The coefficients give mole ratios, not mass ratios. The standard route is grams to moles, through the mole ratio, then back to grams. When two reactants are given, the one that runs out first, the limiting reactant, decides how much product forms, and the percent yield compares what you actually got with that maximum.

Reactions fall into recognizable patterns. Mixing two salt solutions can produce an insoluble solid, a precipitate. Acids and bases neutralize each other to form a salt and water. In a redox reaction, one species loses electrons, which is oxidation, and another gains them, which is reduction, and the two always happen together. Net ionic equations strip away the spectator ions so the real change stands out.

Rule: convert to moles before you compare any two substances, and check the units at the end; a leftover gram or milliliter is a setup error.

When a metal gives electrons to a nonmetal, the ions form a lattice held by ionic bonds. When nonmetals share electron pairs, they form covalent bonds, polar when one atom pulls harder on the shared pair. In metals, outer electrons move freely through the whole solid, which is why metals conduct.

A Lewis structure accounts for every valence electron, usually giving each atom eight around it. Where more than one structure fits, the real molecule is a blend, as in resonance. Valence shell electron pair repulsion theory then predicts shape: electron domains spread as far apart as possible, and lone pairs push bonds closer together. Water is bent, carbon dioxide is linear, and that difference decides polarity. Carbon dioxide's two polar bonds point in opposite directions and cancel, while water's do not.

Molecules attract each other in several ways. Every molecule has dispersion forces, which grow with size. Polar molecules add dipole attractions, and hydrogen bonded to nitrogen, oxygen or fluorine adds hydrogen bonding, the strongest of the three. Stronger attractions mean higher boiling points, more viscosity and greater surface tension. A liquid boils when its vapor pressure matches the pressure above it, so water boils at a lower temperature at high altitude.

Rule: to compare two substances' boiling points or solubility, identify the strongest attraction each can form, then compare size.

A gas's pressure comes from its molecules hitting the walls. At fixed temperature, halving the volume doubles the pressure; at fixed pressure, volume grows in proportion to absolute temperature. The ideal gas law, PV = nRT, combines these, and it works only with temperature in kelvins. In a mixture, each gas contributes its own partial pressure, and the parts add to the total.

Temperature measures the average kinetic energy of gas molecules, so at the same temperature, lighter molecules move faster. That is why lighter gases effuse more quickly: a gas four times lighter escapes twice as fast. Real gases stop behaving ideally at high pressure and low temperature, when molecular size and attraction start to matter.

In solutions, like dissolves like: polar solvents dissolve polar and ionic substances, and nonpolar solvents dissolve nonpolar ones. Most solids dissolve better in warm water, while gases dissolve worse, which is why warm water holds less oxygen. A gas dissolves in proportion to its pressure above the liquid, which is what keeps a sealed drink fizzy. Dissolved particles lower the freezing point and raise the boiling point in proportion to how many particles there are, so a salt that splits into two ions has twice the effect of the same amount of sugar.

Rule: in any gas law, convert to kelvins first; in any colligative property, count particles, not formula units.

Reactions speed up with higher concentration, higher temperature, more surface area or a catalyst. A catalyst lowers the activation energy without being used up and without changing where equilibrium lies. The orders in a rate law come from experiment, not from the balanced equation.

At equilibrium the forward and reverse reactions run at equal rates, so concentrations hold steady. The equilibrium constant K is products over reactants, each raised to its coefficient; a large K favors products. Comparing the current quotient Q with K tells you which way a mixture will shift. Le Chatelier's principle predicts the response to a disturbance: the system shifts to partly undo it. Only a temperature change alters K itself.

An acid donates a proton and a base accepts one. Water ionizes slightly, and at 25 °C the product of hydronium and hydroxide concentrations is 1.0 × 10^-14. The pH scale is logarithmic, so one unit is a tenfold change in hydronium. Strong acids ionize completely; weak acids only partly. A buffer pairs a weak acid with its conjugate base, so it absorbs added acid or base, and its pH equals the acid's pKa when the two are equal. A slightly soluble salt dissolves until its ion product reaches Ksp, and adding a shared ion makes less of it dissolve.

Rule: when anything is added, removed, heated or squeezed, ask which way the equilibrium moves to oppose it.

An exothermic reaction releases heat and has a negative ΔH; an endothermic reaction absorbs it. Because enthalpy is a state function, you can add, reverse and scale reactions to find an unknown ΔH, which is Hess's law. Entropy measures how widely energy is spread, and it generally rises when solids melt, liquids boil or a reaction makes more gas. The Gibbs free energy, ΔG = ΔH − TΔS, decides spontaneity: a negative value means the process can happen on its own. Spontaneous says nothing about speed.

In a galvanic cell, a spontaneous redox reaction drives electrons through a wire from the anode, where oxidation happens, to the cathode, where reduction happens. A positive cell potential means the reaction is spontaneous, and Faraday's constant, about 96,485 coulombs per mole of electrons, links charge to chemical amount. Electrolysis runs the other way, using electrical energy to force a reaction. Rusting is an electrochemical cell on a metal surface.

Nuclear reactions change the nucleus. Alpha particles are stopped by paper, beta particles by thin metal, and gamma rays need dense shielding. In each half-life, half of a radioactive sample decays. A nucleus has less mass than its parts, and that mass defect is the binding energy, which peaks near iron, so both fission of heavy nuclei and fusion of light ones release energy.

Rule: a negative ΔG or a positive cell potential tells you a reaction can go, never how fast it will.

Carbon forms four bonds and links readily to itself, building chains and rings. What an organic molecule does depends on its functional group, such as the hydroxyl in alcohols or the carbonyl in aldehydes and ketones, more than on its carbon skeleton. Living things build carbohydrates, lipids, proteins and nucleic acids from smaller units, joining them by removing water and breaking them by adding it back. Proteins are chains of amino acids, and enzymes are biological catalysts. Ribonucleic acid uses uracil where deoxyribonucleic acid uses thymine.

In the environment, the same chemistry matters at scale. Carbon dioxide, methane and nitrous oxide trap heat in the atmosphere. Sulfur dioxide and nitrogen oxides form acid rain. Chlorofluorocarbons release chlorine high in the stratosphere, where it destroys the ozone that blocks ultraviolet light, while ozone at ground level is a pollutant formed when nitrogen oxides and volatile organic compounds react in sunlight. Green chemistry aims to prevent waste and hazards at the source rather than clean them up afterward.

In the lab, read a chemical's label and safety data sheet before you use it. Wear eye protection and suitable gloves, use a fume hood for anything volatile or toxic, and know where the eyewash and safety shower are. Add acid to water, never water to acid, and put chemical waste into labeled containers, not down the drain.

Rule: identify the reactive group or the hazard first; both tell you how a substance will behave before you handle it.