Chemistry · 26 pages
Chemistry explainers
Clear, accurate explainers for the core ideas of chemistry — atomic structure, chemical bonding, reactions, the periodic table, the mole and molar mass, the pH scale and the states of matter. Each page leads with a concise, IUPAC-grounded definition and links across to related concepts and the wider CASRAI standards.
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Covalent vs ionic bond
The difference is how electrons are handled. A covalent bond forms when two atoms share one or more pairs of electrons, typically between non-metals. An ionic bond forms when one atom transfers electrons to another, creating positive and negative ions held together by electrostatic attraction, typically between a metal and a non-metal. Sharing makes covalent; transfer makes ionic.
ComparisonEndothermic vs exothermic
The difference is the direction of energy flow. An exothermic reaction releases energy to the surroundings, usually as heat, so the surroundings warm up and the products hold less energy than the reactants. An endothermic reaction absorbs energy from the surroundings, so they cool down and the products hold more energy than the reactants. Burning is exothermic; melting ice is endothermic.
ComparisonPhysical vs chemical change
The difference is whether a new substance forms. In a physical change, a substance changes form, state or appearance, but its chemical identity stays the same — melting ice still gives water. In a chemical change, atoms are rearranged into one or more new substances with different properties, as when iron rusts. Physical changes are often reversible; chemical changes usually are not.
ComparisonElement vs compound
The difference is composition. An element is a pure substance made of only one type of atom and cannot be broken down into simpler substances by chemical means — oxygen and gold are elements. A compound is two or more different elements chemically bonded in a fixed ratio, with new properties — water and carbon dioxide are compounds. Compounds can be split into their elements; elements cannot.
ComparisonMixture vs compound
The difference is bonding and proportion. A mixture combines two or more substances physically, in any proportion, without chemical bonding, so each part keeps its properties and can be separated physically. A compound joins elements by chemical bonds in a fixed ratio, creating a new substance with new properties that can only be separated chemically. Air is a mixture; water is a compound.
ComparisonAtom vs molecule
The difference is one of scale and structure. An atom is the smallest particle of an element that retains its chemical identity, built from protons, neutrons and electrons. A molecule is two or more atoms held together by chemical bonds — they may be the same element, like O₂, or different elements, like H₂O. Atoms are the building blocks; molecules are what many of them build.
ComparisonAcid vs base
The difference is what they do with hydrogen ions. An acid donates hydrogen ions (H⁺, protons) in solution and has a pH below 7. A base accepts hydrogen ions, and an alkali is a base that dissolves to release hydroxide ions (OH⁻), giving a pH above 7. When an acid and a base react, they neutralise each other to form a salt and water.
DefinitionCatalyst
A catalyst is a substance that speeds up a chemical reaction without being consumed by it. It works by lowering the activation energy — the barrier reactants must overcome — so more collisions succeed and the reaction proceeds faster. Because it is regenerated, a small amount can process large quantities. Enzymes are biological catalysts. A catalyst changes only the rate, not how much product forms.
DefinitionCovalent bond
A covalent bond is a chemical bond in which two atoms share one or more pairs of electrons. By sharing, each atom can complete a stable outer electron shell. Covalent bonds usually form between non-metal atoms and hold molecules together. A shared pair makes a single bond; two shared pairs make a double bond, and three make a triple bond. Water and methane are held by covalent bonds.
DefinitionElectron configuration
Electron configuration is the arrangement of electrons within an atom’s energy levels, subshells and orbitals. Electrons fill from the lowest energy level upward, following set rules, and the configuration is written in notation such as 1s² 2s² 2p⁶. Because the outermost (valence) electrons drive bonding, an atom’s electron configuration largely determines its chemical behaviour and its place in the periodic table.
DefinitionIonic bond
An ionic bond is the electrostatic attraction that holds together oppositely charged ions. It forms when one atom transfers one or more electrons to another, creating a positive cation and a negative anion that attract each other. Ionic bonds typically form between metals and non-metals and build extended crystal lattices rather than molecules. Sodium chloride, common table salt, is the classic example.
DefinitionBalancing chemical equations
Balancing a chemical equation means adjusting the numbers in front of each substance, the coefficients, so that every element has the same number of atoms on the reactant and product sides. This satisfies the law of conservation of mass: atoms are rearranged in a reaction, never created or destroyed. Only coefficients may be changed — never the subscripts within a formula, which would alter the substances.
DefinitionMolecule
A molecule is two or more atoms held together by chemical bonds, usually covalent bonds. It is the smallest particle of many substances that still keeps that substance’s chemical properties. The atoms may be the same element, as in oxygen gas (O₂), or different elements, as in water (H₂O). A molecule made of different elements is also a compound. Ionic substances form lattices, not molecules.
DefinitionMolar mass
Molar mass is the mass of one mole of a substance, measured in grams per mole (g/mol). For an element it equals the relative atomic mass in grams; for a compound it is the sum of the atomic masses in its formula. Water (H₂O) is about 18 g/mol. Molar mass is the bridge between mass you can weigh and the number of particles.
DefinitionAtomic structure
Atomic structure describes the way an atom is put together: a small, dense central nucleus containing positively charged protons and neutral neutrons, surrounded by negatively charged electrons. The number of protons (the atomic number) defines the element. Atoms are mostly empty space, and they are electrically neutral overall because the number of protons equals the number of electrons.
DefinitionTypes of chemical reactions
Chemical reactions are commonly grouped into a few main types by how atoms rearrange. Synthesis combines substances into one; decomposition breaks one into several; single displacement swaps one element for another; double displacement exchanges parts between two compounds; and combustion burns a fuel with oxygen, releasing energy. Many reactions also count as oxidation–reduction (redox), where electrons transfer between substances.
DefinitionThe pH scale
The pH scale is a measure of how acidic or alkaline a solution is, based on its concentration of hydrogen ions. It usually runs from 0 to 14: values below 7 are acidic, 7 is neutral, and above 7 is alkaline (basic). The scale is logarithmic, so each whole-number step represents a tenfold change in hydrogen ion concentration. Pure water at 25 °C has a pH of 7.
DefinitionStates of matter
The states of matter are the physical forms a substance can take, distinguished by how its particles are arranged and how freely they move. The three everyday states are solid, liquid and gas; plasma is a fourth, common in stars. A substance changes state — by melting, freezing, boiling or condensing — when heating or cooling alters its particles’ energy, without changing its chemical identity.
DefinitionThe mole
The mole is the SI unit for amount of substance. One mole contains exactly 6.022 × 10²³ particles — atoms, molecules or ions — a value known as Avogadro’s number. The mole lets chemists count particles by weighing, because one mole of a substance has a mass in grams equal to its molar mass. It is to chemists what a dozen is to eggs: a fixed counting unit.
DefinitionOxidation and reduction
Oxidation is the loss of electrons by a substance, and reduction is the gain of electrons. The two always occur together — electrons lost by one substance are gained by another — so the combined process is called a redox (reduction–oxidation) reaction. A common memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain. Rusting, burning and respiration are all redox reactions.
DefinitionIsotope
An isotope is one of two or more atoms of the same element that have the same number of protons but different numbers of neutrons. Because they share the proton count (atomic number), isotopes are the same element with the same chemistry, but their differing neutron counts give them different masses. Carbon-12 and carbon-14 are isotopes of carbon. Some isotopes are stable; others are radioactive.
DefinitionValence electrons
Valence electrons are the electrons in an atom’s outermost energy level, or shell. They are the electrons involved in forming chemical bonds, so they largely determine how an atom reacts and what compounds it forms. For main-group elements, the number of valence electrons matches the group number in the periodic table. Atoms tend to gain, lose or share valence electrons to reach a stable, full outer shell.
DefinitionElectronegativity
Electronegativity is a measure of the tendency of an atom to attract a shared pair of electrons in a chemical bond. The more electronegative an atom, the more strongly it pulls bonding electrons toward itself. It generally increases across a period and decreases down a group of the periodic table, making fluorine the most electronegative element. The difference in electronegativity between two atoms determines a bond’s polarity.
DefinitionThe periodic table
The periodic table is a chart that arranges the known chemical elements in order of increasing atomic number — the number of protons — into rows and columns. Its layout groups elements with similar properties together, so chemical behaviour recurs in a regular, periodic pattern. The rows are periods and the columns are groups. It is chemistry’s central organising tool, devised by Dmitri Mendeleev.
DefinitionChemical bond
A chemical bond is a lasting attraction between atoms, ions or molecules that holds them together in chemical substances. Bonds form because the bonded arrangement is more stable — lower in energy — than the separate atoms, usually achieved by atoms gaining, losing or sharing electrons to complete their outer shells. The three main types are ionic, covalent and metallic bonds, each defined by how the electrons are handled.
DefinitionChemical reaction
A chemical reaction is a process in which one or more substances, called reactants, are transformed into one or more different substances, called products. Existing chemical bonds are broken and new bonds are formed, rearranging the atoms — but never creating or destroying them, so mass is conserved. Signs of a reaction include colour change, a gas, a precipitate, or a temperature change. Burning, rusting and respiration are examples.
