Mass spectrometry

Mass spectrometry (MS) is an analytical technique useful for measuring the mass-to-charge ratio of ions of one or more molecules present in a sample. These measurements can often be used to calculate the exact molecular weight of the sample components as well. Mass spectrometry is used in many different fields and is applied to pure […]

Electron affinity

The electron affinity of an element is a measurable physical quantity, namely, the energy released or absorbed when an isolated gas-phase atom acquires an electron, measured in kJ/mol. We must be careful not to confuse electron affinity with electronegativity.

Chemical energy

Chemical energy is the potential energy stored in the bonds of chemical compounds to undergo a transformation through a chemical reaction to transform other chemical substances. It varies due to the formation or breaking of chemical bonds of any kind in the chemical elements involved in chemical reactions. An example of chemical potential energy is the […]


A solid is a state of matter in which atoms or molecules are tightly bound together by powerful forces thereby creating a rigid body (with a defined geometric shape and volume). The formation of a solid occurs with the establishment of bond forces between the atoms, of such intensity as to overcome the energy of thermal agitation. […]


In liquids, the atoms or molecules are not as tightly bound as in solids, and due to that, they have some freedom to move around. The liquid state is a condensed state of matter, because even in it, as in solids, the particles are linked (weakly) to each other. On the other hand, as liquids and […]


Gas is aeriform whose temperature is higher than the critical temperature; as a result, gases cannot be liquefied without first being cooled, unlike vapors. Gas is a fluid that has no volume of its own (tends to occupy all the volume at its disposal) and that is easily compressible. A gas is one of the four […]


Biogas is a mixture of different gases produced by anaerobic decomposition (with methanogen or anaerobic organisms), or fermentation, of biomass – organic material (including animal dung, human sewage, food waste, crop residues, and industrial and municipal wastes). Biogas is a renewable energy source. It is composed primarily of methane (up to 60%), which is the combustible component, carbon dioxide, and […]


A supersolid is a special quantum state of matter where particles form a rigid, spatially ordered structure, but also flow with zero viscosity. A supersolid combines the properties of solids with those of superfluids. At very low temperatures, close to absolute zero, matter behaves strangely: fluids flow without friction (superfluidity), metals float (superconductivity). A supersolid is, in […]


A superglass is a state of matter which is characterized by superfluidity and a frozen amorphous structure at the same time.


The glass is a non-equilibrium, non-crystalline condensed state of matter that exhibits a glass transition. The structure of glasses is similar to that of their parent supercooled liquids (SCL), and they spontaneously relax toward the SCL state. Their ultimate fate, in the limit of infinite time, is to crystallize. Glasses are also known as amorphous solids. The boundary between the […]


Absorption band. Absorption spectroscopy. Absorption spectrum. Spectral line (absorption line).

Antibonding molecular orbital

An antibonding molecular orbital is a type of chemical bond given by the overlap of two half-full molecular orbitals. This kind of bond weakens the chemical bond between two atoms and helps to raise the energy of the molecule relative to the separated atoms. Such an orbital has one or more nodes in the bonding region between […]

Ionic bonding

Ionic bonding is a type of chemical bonding that results from the electrostatic attraction of oppositely charged ions that are typically produced by the transfer of electrons between metallic and nonmetallic atoms. Compounds composed of ions are called ionic compounds (or salts), and their constituent ions are held together by ionic bonds: electrostatic forces of attraction […]

Metallic bonding

Metallic bonding is a type of chemical bonding that rises from the attractive electrostatic force between conduction electrons (in the form of an electron cloud of delocalized electrons) and positively charged metal ions. It may be described as the sharing of free electrons among a structure of positively charged ions (cations).

Covalent bond

Covalent bond is a type of chemical bonding formed between two atoms when both have similar tendencies to attract electrons to themselves (i.e., when both atoms have identical or fairly similar ionization energies and electron affinities). Compounds that contain covalent bonds exhibit different physical properties than ionic compounds. Because the attraction between molecules, which are electrically […]


Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase.


Vaporization (or vaporisation in British English) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomenon.


Ionization (or ionisation in British English), is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes.


Perovskite (discovered in the Ural Mountains of Russia by Gustav Rose in 1839 and is named after Russian mineralogist Lev Perovski 1792–1856) is a calcium titanium oxide mineral composed of calcium titanate (CaTiO3). Its name is also applied to the class of compounds which have the same type of crystal structure as CaTiO3 \((^{\textrm{XII}}\textrm{A}^{2+\textrm{VI}}\textrm{B}^{4+}\textrm{X}^{2-}_3)\), known as the perovskite structure, which includes many synthetic perovskites: many of […]

Chemical reaction

A chemical reaction is a transformation of matter that occurs without measurable variations in mass, in which one or more chemical species (called reactants or reagents) modify their structure and original composition to generate other chemical species (called products). Chemical reactions, in which chemical bonds are broken and formed, require an initial investment of energy. Kinetic energy, […]


Cohesion (or cohesive attraction, or cohesive force) is the tendency of different parts of a substance to hold together (being mutually attractive). Cohesion is due to forces acting between its molecules: a molecule will repel one close to it but attract one that is farther away. This situation results in both cohesion and adhesion. The attractive […]


In chemistry, a suspension is defined as a heterogeneous mixture in which particle size of one or more components is greater than 1000 nm. When mud is dissolved in water and stirred vigorously, particles of mud are distributed evenly in water. After some time, the particles of this solution settle underwater due to the influence of gravity. […]


Chemistry (from kemà, the book of Egyptian art secrets, from the Arabic word al-kīmīā, الكيمياء) is the science that studies the composition of matter and its behavior based on this composition, also called “central science“ because it connects other natural sciences, such as astronomy, physics, materials sciences, biology, and geology.

Van der Waals equation

The van der Waals equation (or van der Waals equation of state; named after Johannes Diderik van der Waals) is an equation of state that generalizes the ideal gas law based on plausible reasons that real gases do not act ideally. \[\left(P+a\dfrac{1}{V_m^2}\right)(V_m-b)=RT\] where \(a\) is a constant whose value depends on the gas, \(b\) is the volume […]

Functional groups

Functional groups are groups of atoms that occur within molecules and confer specific chemical properties to those molecules. We find them along the “carbon backbone” of macromolecules. Chains and/or rings of carbon atoms with the occasional substitution of an element such as nitrogen or oxygen form this carbon backbone. Molecules with other elements in their carbon […]


Homogeneous mixture. Heterogeneous mixture.


The stoichiometric formula (CH2O)n, where n is the number of carbons in the molecule represents carbohydrates. In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. This formula also explains the origin of the term “carbohydrate”: the components are carbon (“carbo”) and the components of water (hence, “hydrate”). Scientists classify carbohydrates into […]


Monosaccharides (mono- = “one”; sacchar- = “sweet”) are simple sugars, the most common of which is glucose. In monosaccharides, the number of carbons usually ranges from three to seven. Most monosaccharide names end with the suffix -ose. If the sugar has an aldehyde group (the functional group with the structure R-CHO), it is an aldose, and […]

Pure substance

Chemical element. Chemical compound. Molecular compound. Ionic compound. Intermetallic compound.


A substance is in the aeriform state when it does not have a defined shape (it is therefore a fluid) or a defined volume, as it tends to expand, completely filling the container that contains it. Any substance above its boiling point can, therefore, be defined as aeriform. An aeriform can also exist below the boiling point, […]

Specific volume

The specific volume is a property of a substance is the ratio of the substance’s volume to its mass. It is the reciprocal of density and an intrinsic property of matter as well. The symbol used to represent specific volume in equations is \(v\) or \(\nu\) with SI units of cubic meters per kilogram. The symbol used to represent volume […]


An electrolyte is a substance that produces an electrically conducting solution when dissolved in a polar solvent, such as water. The dissolved electrolyte separates into cations and anions, which disperse uniformly through the solvent. Electrically, such a solution is neutral. An electrolyte is what is called an ionic conductor. The electrolytes are capable of conducting electric current, […]


If a chemical change is carried out at constant pressure and the only work done is caused by expansion or contraction, the change is called the enthalpy change with the symbol ΔH, or ΔH°298 for reactions occurring under standard state conditions. The value of ΔH for a reaction in one direction is equal in magnitude, but opposite in […]


Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen, such as methane (CH4). The many covalent bonds between the atoms in hydrocarbons store a great amount of energy, which releases when these molecules burn (oxidize). Methane is the simplest hydrocarbon molecule, with a central carbon atom bonded to four different hydrogen atoms. The shape of its electron orbitals […]

Liquid crystal

Liquid crystals (LCs) are a state of matter which has properties between those of conventional liquids and those of solid crystals. The term liquid crystal is used to indicate the intermediate phases (mesophases) between that of crystalline solid and that of isotropic liquid of some substances. A substance can present several mesophases with a decreasing order […]

Standard conditions for temperature and pressure (STP)

Standard conditions for temperature and pressure (STP) are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. STP should not be confused with the standard state of a material (pure substance, mixture, or solution) which is a reference point used to calculate its properties under different […]


Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity. Electronegativity is a measure of the tendency of an atom to attract electrons (or electron density) towards itself. It determines how the shared electrons are distributed between the two atoms in a bond. The more strongly an […]

Radioactivity (radioactive decay)

Radioactive decay (also known as nuclear decay, radioactivity or nuclear radiation) is the process by which an unstable atomic nucleus loses energy (in terms of mass in its rest frame) by emitting radiation spontaneously. Radioactivity is a physical, not a biological, phenomenon. Radioactivity can occur both naturally and through human intervention. An example of artificially induced radioactivity is […]

Amount of substance (mole)

The amount of substance is a basic (or fundamental) quantity that indicates a set of elementary entities and its value is proportional to the number of entities contained in the system. The entities can be atoms, molecules, ions, radicals, or other, and must be specified each time to replace the word “substance“ (for example the chemical quantity […]


Water is essential to life as we know it. Water is one of the more abundant molecules and the one most critical to life on Earth. Water comprises approximately 60-70% of the human body. Without it, life as we know it simply would not exist. The polarity of the water molecule and its resulting hydrogen […]


The neutron is an uncharged, subatomic particle \((\sim 10^{-15}\;\textrm{m})\) consisting of an up quark and two down quarks, with a net electric charge equal to zero, located in the nucleus of an atom. As it consists of quarks, it belongs to the family of hadrons and in particular to the group of baryons. Having half-integer spin is […]


The proton is positively charged, subatomic particle located in the nucleus of an atom. It constitutes the nucleus together with the neutron, which changes continuously by the emission and absorption of pions. As formed by quarks the proton belongs to the family of hadrons and in particular to the group of baryons. Having semi-integer spin is a […]


The electron is a negatively charged, subatomic particle of relatively low mass located outside the nucleus.


The elements that compose the ordinary matter are formed by atoms, the smallest constituent unit with well-defined and specific characteristics of each chemical element, for example, the mass and atomic number. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Although it is the smallest material structure (dimensions of the order […]

Chemical bond

Covalent bond. Delocalized bonds and metallic bonding. Ionic bonding. Antibonding molecular orbital.


The matter is any substance (composed of various types of particles) that has mass, inertia, and occupies physical space by having volume. The atom is the simplest example of matter particles, which represent the smallest unit of matter composed of electrons, the protons, and the neutrons. They retain all of the chemical properties of an […]


Atoms of the same element with the same number of protons but a different number of neutrons (different mass numbers) are known as isotopes. The mass numbers or isotopes of an element are denoted as preceding superscripts. For example, the stable isotopes of the element oxygen are denoted 18O, 17O, and 16O. Oxygen has an atomic number of 8 […]

Law of definite proportions

Joseph Proust (1754-1826) formulated the law of definite proportions (also called the Law of Constant Composition or Proust’s Law). This law states that if a compound is broken down into its constituent elements, the masses of the constituents will always have the same proportions, regardless of the quantity or source of the original substance. Joseph Proust based this […]

Law of multiple proportions

Many combinations of elements can react to form more than one compound. In such cases, this law states that the weights of one element that combine with a fixed weight of another of these elements are integer multiples of one another. The law of multiple proportions states that if two elements form more than one […]

Complex (coordination complex)

A complex (or coordination complex) in Chemistry and Biochemistry is the product of the formation, often reversible, of a bond between a central atom or ion (or coordinating ion) and the atoms, ions, or molecules (called ligands or coordinated ions) that surround the central atom (called the coordination center). A more rigorous definition of a chemical complex can […]