Foundations of Chemistry
An insight into the fundamental concepts that shape our world.
Atom
An atom is the smallest building block of a chemical element that still retains the element's characteristic properties. It consists of a positively charged nucleus (containing protons and neutrons) and negatively charged electrons that orbit around the nucleus. Atoms of different elements differ in the number of protons they contain.
Molecule
A molecule forms when two or more atoms join together through chemical bonds. This connection can consist of identical atoms (like O₂) or different atoms (like H₂O). Molecules are the smallest particles of a chemical compound that retain the compound's properties.
Ion
An ion is an electrically charged atom or molecule that has formed by gaining or losing electrons. Positive ions (cations) have lost electrons, while negative ions (anions) have gained electrons. Ions are particularly important for salt formation and electrical conductivity in solutions.
Chemical Bond
A chemical bond is the force that holds atoms together in molecules or crystals. The three main types are ionic bonds (between ions), covalent bonds (shared electron pairs), and metallic bonds (electron sea). These bonds determine the properties and behavior of substances.
Metallic Bonding
In metallic bonding, positively charged metal ions are surrounded by delocalized electrons. This electron sea explains typical metal properties such as electrical conductivity, luster, and malleability. The bond strength varies depending on the metal and crystal structure.
Periodic Table
The periodic table organizes all known chemical elements by their atomic number (number of protons) in rows and columns. Elements in the same column (group) have similar chemical properties because they have the same number of outer electrons. It is the most important tool for predicting chemical properties and reactions.
Chemical Reaction
A chemical reaction is a process in which substances transform into other substances by breaking existing chemical bonds and forming new ones. The starting materials are called reactants, and the resulting substances are called products. The total mass is conserved, but the properties of the substances change fundamentally.
Molarity
Molarity indicates how many moles of a substance are contained in one liter of solution (mol/L). It is an important measure of solution concentration in chemistry. Using molarity, chemists can precisely calculate how many particles are involved in a reaction.
Mole
A mole is a unit of quantity in chemistry that contains approximately 6.022 × 10²³ particles (Avogadro's constant). This enormous number allows chemists to work with manageable quantities even though atoms and molecules are extremely small. One mole corresponds to the atomic mass of an element expressed in grams.
pH Value
The pH value measures how acidic or basic a solution is on a scale from 0 to 14. Values below 7 are acidic, 7 is neutral, and values above 7 are basic (alkaline). It indicates the concentration of hydrogen ions (H⁺) in the solution.
Oxidation
Oxidation is a chemical process in which an atom, ion, or molecule loses electrons. Originally, it meant combination with oxygen, but today it encompasses all electron-loss reactions. Oxidation always occurs together with reduction (another substance gains the electrons).
Reduction
Reduction is the opposite of oxidation: an atom, ion, or molecule gains electrons. Formerly, it meant the removal of oxygen, but today it describes all electron-gain reactions. Reduction and oxidation always occur simultaneously and are called redox reactions.
Activation Energy
Activation energy is the minimum energy required for a chemical reaction to occur. Even in energetically favorable reactions, existing bonds must first be broken. Catalysts can lower the activation energy and accelerate reactions.
Catalyst
A catalyst is a substance that accelerates chemical reactions without being consumed in the process. It lowers the activation energy and enables an alternative reaction pathway. After the reaction, the catalyst remains unchanged and can act again.
Chemical Equilibrium
Chemical equilibrium exists when forward and reverse reactions proceed at the same rate. The concentrations of all participating substances then remain constant, even though the reaction continues. The equilibrium can be shifted by changing temperature, pressure, or concentration.
Electrolysis
Electrolysis is the decomposition of substances using electric current. Ions in a solution or melt are moved to oppositely charged electrodes and discharged there. This process is used for metal extraction, electroplating, and hydrogen production.
Crystal Lattice
A crystal lattice is the regular, three-dimensional arrangement of atoms, ions, or molecules in a crystal. This order repeats in all spatial directions and determines the shape and many properties of the crystal. Different lattice types lead to different crystal forms.
Diffusion
Diffusion is the spontaneous spreading of particles from areas of high to low concentration. This process occurs due to particle movement and strives for concentration equilibrium. Diffusion is important for many natural processes like respiration and substance transport in cells.
Electronegativity
Electronegativity describes an atom's ability to attract electrons in a chemical bond toward itself. Fluorine has the highest electronegativity, while metals have low values. Differences in electronegativity determine whether bonds are polar or nonpolar.
Orbital
An orbital is a region around the atomic nucleus where electrons are likely to be found with high probability. Each orbital can contain a maximum of two electrons with opposite spins. The shape and energy of orbitals determine the chemical behavior of elements.
Isotope
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have identical chemical but different physical properties like mass and radioactivity. Examples include the hydrogen isotopes protium, deuterium, and tritium.
Hybridization
Hybridization explains molecular geometry through the mixing of atomic orbitals to form new hybrid orbitals. These all have the same energy and shape and arrange themselves optimally in space to minimize repulsion. Different hybridization types (sp, sp², sp³) lead to different molecular geometries.
Isomerism
Isomers are compounds with the same molecular formula but different structure or spatial arrangement. Constitutional isomers differ in how atoms are connected, while stereoisomers differ in spatial arrangement. Isomers can have completely different properties and biological effects.
Reaction Rate
Reaction rate indicates how fast a chemical reaction proceeds, measured as the change in concentration per unit time. It depends on factors like temperature, concentration, catalysts, and surface area. Higher temperature and concentration usually lead to faster reactions.
Van der Waals Forces
Van der Waals forces are weak intermolecular attractive forces between neutral atoms or molecules. They arise from temporary or permanent dipole moments and are responsible for condensation, evaporation, and many material properties. Although weak individually, they sum to significant forces when many molecules are involved.
Hydrogen Bonding
A hydrogen bond is a particularly strong type of Van der Waals force between a hydrogen atom and an electronegative atom like oxygen or nitrogen. These bonds are crucial for water's properties, protein structure, and DNA structure. They are stronger than normal Van der Waals forces but weaker than covalent bonds.
Enthalpy
Enthalpy is a measure of the heat content of a system at constant pressure. Chemical reactions are classified as exothermic (heat release, negative enthalpy change) or endothermic (heat absorption, positive enthalpy change). The enthalpy change determines whether a reaction proceeds spontaneously or requires external energy.
Entropy
Entropy is a measure of disorder or randomness in a system. According to the second law of thermodynamics, every system strives for maximum entropy. In chemistry, entropy explains why gases expand, why salt dissolves in water, and why some reactions occur spontaneously despite requiring energy input.
Acid
An acid is a substance that can donate protons (H⁺ ions) or accept electron pairs. Strong acids like hydrochloric acid dissociate completely in water, while weak acids only partially dissociate. Acids have a pH below 7 and can dissolve metals, change indicator colors, and be neutralized by bases.
Base
A base is a substance that can accept protons (H⁺ ions) or donate electron pairs. Strong bases like sodium hydroxide dissociate completely, while weak bases only partially dissociate. Bases have a pH above 7, feel slippery, and neutralize acids to form salts.
Polarity
Polarity describes the uneven distribution of electrons in molecules, creating positive and negative partial charges. Polar molecules like water have positive and negative poles, while nonpolar molecules like oil do not. Polarity determines solubility, boiling points, and intermolecular interactions.
State of Matter
States of matter are the different forms in which matter appears: solid, liquid, gaseous, and plasma. They result from different strengths of intermolecular forces at various temperatures and pressures. Transitions between states like melting, vaporization, or sublimation are physical processes without changing chemical composition.
Vapor Pressure
Vapor pressure is the pressure exerted by vapor molecules above a liquid when evaporation and condensation are in equilibrium. It increases with temperature and determines a liquid's boiling point. Liquids with high vapor pressure evaporate more easily and are more volatile.
Solubility
Solubility indicates the maximum amount of a substance that can dissolve in a solvent. It depends on temperature, pressure, and the nature of both substance and solvent. The rule 'like dissolves like' explains why polar substances dissolve in polar solvents and nonpolar substances in nonpolar solvents.
Reaction Mechanism
A reaction mechanism describes the detailed pathway of how a chemical reaction proceeds at the molecular level. It shows all intermediate steps and transition states from reactant to product. Understanding the mechanism helps optimize reaction conditions and minimize side products.
Electron Affinity
Electron affinity is the energy released when a neutral atom gains an electron and becomes a negative ion. It measures how strongly an atom attracts electrons. Nonmetals usually have high electron affinities, while metals have low values.
Ionization Energy
Ionization energy is the minimum energy required to remove an electron from a neutral atom. It increases from left to right across the periodic table and decreases from top to bottom. Low ionization energies explain why metals easily give up electrons and become cations.
Reactivity Series
The reactivity series ranks metals by their tendency to lose electrons and react. Highly reactive metals appear at the top, less reactive and noble metals lower down. This helps predict whether one metal can displace another from a compound.
Allotropy
Allotropy is the occurrence of an element in different structural forms with different properties. Carbon exists as diamond (hard, insulating), graphite (soft, conducting), and fullerenes (spherical). These different arrangements of the same atoms lead to completely different materials.
Redox Potential
The redox potential measures a substance's tendency to gain electrons (be reduced) or lose electrons (be oxidized). It is measured in volts and enables predictions about the course of redox reactions. Substances with higher potential oxidize those with lower potential.
Radioactivity
Radioactivity is the spontaneous decay of unstable atomic nuclei with emission of radiation (alpha, beta, gamma). This transforms one element into another or brings the nucleus to a more stable state. The half-life indicates the time in which half of the radioactive nuclei decay.
Complex Compound
A complex compound consists of a central atom (usually a metal ion) surrounded by ligands (molecules or ions with lone electron pairs). The ligands form coordinate bonds to the central atom. Complexes are important in biology (hemoglobin, chlorophyll) and catalysis.
Buffer
A buffer is a solution that changes its pH only slightly when small amounts of acid or base are added. It consists of a weak acid and its conjugate base or vice versa. Buffer systems are essential for biological processes since enzymes only function within narrow pH ranges.
Spectroscopy
Spectroscopy studies the interaction of matter with electromagnetic radiation and provides information about molecular structure and composition. Different techniques like IR, UV, or NMR spectroscopy use different wavelength ranges. It is indispensable for identifying and characterizing compounds.
Chelate
A chelate is a complex compound where a ligand forms multiple bonds to the same central atom, gripping it like a claw (Greek: chele). These multidentate ligands form more stable complexes than monodentate ones. EDTA is a well-known hexadentate chelating ligand.
Luminescence
Luminescence is the emission of light by matter that is not caused by high temperature. In fluorescence, light emission occurs immediately after excitation, while in phosphorescence it is delayed. These phenomena are based on electron transitions between different energy levels in atoms or molecules.
Half-Life
Half-life is the time required for half of a substance to be broken down by a process. In nuclear chemistry, it describes radioactive decay, while in kinetics it describes the breakdown of reactants. It is characteristic for each process and independent of the initial amount.