Science Double Award Co-Ordinated (0654) international General Certificate of Secondary Education 2025

Element

A pure substance made of only one type of atom.

Compound

A substance formed when two or more different elements are chemically bonded together in fixed ratios.

Mixture

A combination of two or more substances that are not chemically bonded and can be separated by physical methods.

Atom

The smallest particle of an element that can take part in chemical reactions.

Nucleus

The small, dense, positively charged centre of an atom containing protons and neutrons.

Proton

A positively charged subatomic particle found in the nucleus of an atom.

Neutron

A subatomic particle with no charge found in the nucleus of an atom.

Electron

A negatively charged subatomic particle that moves around the nucleus in energy levels (shells).

Proton number (atomic number)

The number of protons in the nucleus of an atom; it identifies the element.

Mass number (nucleon number)

The total number of protons and neutrons in the nucleus of an atom.

Electronic configuration

The arrangement of electrons in the shells (energy levels) of an atom.

Group I

Alkaline metal is in the first column of the Periodic Table; they have one electron in the outer shell, are very reactive, and form +1 ions. with the properties of soft, low-density metals that react vigorously with water.

Group II

Earth-alkaline is in the second column of the Periodic Table; they have two electrons in the outer shell, are reactive metals, and form +2 ions. reactive metals that are harder and less reactive than alkali metals.

Group VI

Chalcogens elements located in the sixth column of the Periodic Table; they have six electrons in the outer shell and tend to form -2 ions.

Group VII

Halogens are the seventh column of the Periodic Table; they have seven electrons in the outer shell, are very reactive non-metals, and form -1 ions. These elements are coloured, toxic non-metals that exist as diatomic molecules.

Group VIII

Noble gas located in the last column of the Periodic Table; they have full outer shells, are very unreactive, and exist as single atoms. Many of its elements are colourless, monatomic gases with very low chemical reactivity.

Isotopes

Isotopes Atoms of the same element that have the same proton number but different numbers of neutrons. Isotopes of the same element have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration.Ionic bond A strong electrostatic attraction between oppositely charged ions, formed after the transfer of electrons from one atom to another, usually between a metal and a non-metal.

Cation

A positively charged ion formed when an atom loses one or more electrons, resulting in more protons than electrons.

Anion

A negatively charged ion formed when an atom gains one or more electrons, resulting in more electrons than protons.

Ionic bond

A strong electrostatic attraction between oppositely charged ions, formed after the transfer of electrons from one atom to another, usually between a metal and a non-metal.

Formation of ionic bonds (metallic and non-metallic elements)

Ionic bonds form when electrons are transferred from a metallic element to a non-metallic element. The metal atom loses one or more electrons from its outer shell to form a positively charged ion, while the non-metal atom gains these electrons to form a negatively charged ion. This electron transfer results in both ions having full outer shells and becoming more stable. The oppositely charged ions are then held together by a strong electrostatic attraction, forming an ionic bond.

Dot-and-cross diagrams (ionic bonding)

Dot-and-cross diagrams are used to represent the transfer of electrons during ionic bonding. Electrons from one atom are shown as dots and electrons from the other atom as crosses. In ionic bonding, the diagram shows electrons being transferred from the metal atom to the non-metal atom. The resulting ions are drawn in square brackets with their charges shown, and each ion has a full outer shell of electrons.

High melting and boiling points of ionic compounds

Ionic compounds have high melting points and boiling points because they consist of a giant ionic lattice made up of many oppositely charged ions arranged in a regular structure. These ions are held together by strong electrostatic forces of attraction between positive and negative charges throughout the lattice. A large amount of energy is required to overcome these strong forces, resulting in high melting and boiling points.

Electrical conductivity of ionic compounds (solid)

Solid ionic compounds do not conduct electricity because the ions are fixed in position within the giant ionic lattice and are unable to move. Since electrical conductivity requires the movement of charged particles, solid ionic compounds cannot carry an electric current.

Solubility and aqueous behaviour of ionic compounds

Ionic compounds are often soluble in water because water molecules are polar and can attract and surround the positive and negative ions. This separates the ions from the lattice, allowing them to move freely in solution.

Giant ionic lattice

A giant ionic lattice is a large, regular, three-dimensional structure made up of alternating positive and negative ions arranged in a repeating pattern. Each ion is surrounded by ions of opposite charge, and the structure is held together by strong electrostatic attractions between the ions.

Giant ionic lattice of sodium chloride

In sodium chloride, each sodium ion (Na⁺) is surrounded by six chloride ions (Cl⁻), and each chloride ion is surrounded by six sodium ions. This regular arrangement of alternating positive and negative ions forms a giant ionic lattice, which is responsible for the strong bonding and characteristic properties of sodium chloride.

Electrical conductivity of ionic compounds (molten or aqueous)

Ionic compounds conduct electricity when molten or dissolved in water because the ionic lattice breaks down and the ions become free to move. These mobile positive and negative ions can carry electrical charge, allowing an electric current to flow.

covalent bond

a covalent bond is formed when a pair of electrons is shared between two atoms, leading to noble gas electronic configurations with low melting points and boiling points due to weak intermolecular forces and poor electrical conductivity.

Giant covalent structures

Giant covalent structures are three-dimensional networks of non-metal atoms all bonded together by very strong covalent bonds, forming a continuous lattice that extends throughout the solid. Because there are so many strong covalent bonds that must be broken to change state, giant covalent substances have very high melting points and boiling points. They generally do not conduct electricity in the solid state as there are no free, mobile charged particles, although some allotropes like graphite have delocalised electrons that allow conductivity. The rigid covalent network makes many giant covalent materials extremely hard and insoluble in water; common examples include diamond, graphite and silicon dioxide.

Metallic bonding

Metallic bonding is the strong electrostatic attraction between positive metal ions in a giant metallic lattice and a 'sea' of delocalised electrons that are free to move throughout the structure. This arrangement explains why metals generally have high melting and boiling points (due to strong attraction between ions and electrons), conduct electricity in solid and molten states (because the delocalised electrons can move to carry charge), and are malleable and ductile (because layers of ions can slide over each other without breaking the metallic bond).:contentReference[oaicite:0]{index=0}

Electrolysis

The process in which an ionic compound, in the molten state or in aqueous solution, is broken down into simpler substances by the passage of an electric current. During electrolysis the electrolyte must contain freely moving ions that can carry charge; positive ions (cations) move to the negative electrode (cathode) where they gain electrons (reduction), and negative ions (anions) move to the positive electrode (anode) where they lose electrons (oxidation). Electrolysis requires an external direct current (DC) supply and results in chemical changes at the electrodes as the ions are discharged to form elements or compounds.

Hydrogen-oxygen fuel cell

A hydrogen-oxygen fuel cell is an electrochemical device that converts the chemical energy of hydrogen and oxygen directly into electrical energy with water as the only chemical product. In the cell, hydrogen gas is supplied to the negative electrode (anode) where it is oxidised to hydrogen ions and electrons; the electrons travel through an external circuit, producing an electric current, while the hydrogen ions move through the electrolyte to the positive electrode (cathode) where they combine with oxygen and the electrons to form water. A continuous supply of hydrogen and oxygen allows the cell to produce electricity for as long as reactants are available, and because the only product is water, the process is clean and efficient.:contentReference[oaicite:0]{index=0}

Exothermic reaction

A reaction that transfers thermal energy to the surroundings so the temperature of the surroundings increases. In an exothermic reaction, more energy is released when new bonds form in the products than is absorbed to break bonds in the reactants, resulting in an overall release of heat energy to the environment.

Endothermic reaction

A reaction that takes in thermal energy from the surroundings so the temperature of the surroundings decreases. In an endothermic reaction more energy is absorbed to break bonds in the reactants than is released when new bonds form in the products, resulting in a net uptake of heat energy from the environment.