Key terms: Chemical Bond

Chemical Bond rewards precise definitions, and a definition question is one of the easiest marks to secure if you have learned the exact wording. Below are the terms you must be able to define for this chapter. Learn the short version of each and practise saying it without looking.

Stability and the octet idea

• Octet rule. The tendency of atoms to lose, gain or share electrons so that each atom achieves a stable outer shell of eight electrons, like a noble gas. It is the reason atoms form bonds.
• Duplet. A stable outer shell of two electrons, the arrangement of helium. Small atoms such as hydrogen reach stability with a duplet rather than an octet.
• Stable electron arrangement. A full outer shell, which makes an atom or ion unreactive. Atoms bond to reach the stable arrangement of the nearest noble gas.
• Noble gas. An element in Group 18 with a full outer shell, so it is chemically unreactive and exists as single atoms.

Ions and ionic bonding

• Ion. A charged particle formed when an atom loses or gains electrons, so that its numbers of protons and electrons are no longer equal.
• Cation. A positively charged ion, formed when an atom (usually a metal) loses one or more electrons, for example Na⁺ or Mg²⁺.
• Anion. A negatively charged ion, formed when an atom (usually a non-metal) gains one or more electrons, for example Cl⁻ or O²⁻.
• Ionic bond. The strong electrostatic force of attraction between oppositely charged ions, formed by the transfer of electrons from a metal to a non-metal.
• Electron transfer. The complete movement of one or more electrons from one atom to another, producing ions. It is the process behind ionic bonding.
• Electrostatic force of attraction. The attractive force between particles of opposite charge. In an ionic compound it holds the cations and anions together.
• Giant ionic lattice. A regular three-dimensional arrangement of a very large number of oppositely charged ions, held together by strong electrostatic forces throughout the structure.

Covalent bonding

• Covalent bond. A bond formed when two non-metal atoms share one or more pairs of electrons, so that each atom achieves a stable electron arrangement.
• Bonding pair. A pair of electrons that is shared between two atoms and forms a covalent bond.
• Lone pair. A pair of outer electrons that belongs to one atom and is not shared in a bond, for example the two lone pairs on the oxygen atom in water.
• Single, double and triple bond. A covalent bond made of one, two or three shared pairs of electrons respectively, as in H₂ (single), O₂ (double) and N₂ (triple).
• Simple molecular structure. A structure made of separate small molecules held to each other by weak intermolecular forces, giving low melting and boiling points.
• Electronegativity. The tendency of an atom to attract the shared electrons in a bond. Fluorine, oxygen and nitrogen are highly electronegative.

Dative and hydrogen bonds

• Dative (coordinate) bond. A covalent bond in which both of the shared electrons are supplied by the same atom, as in the ammonium ion NH₄⁺ and the hydronium ion H₃O⁺.
• Hydrogen bond. A weak attraction between molecules, formed when a hydrogen atom bonded to fluorine, oxygen or nitrogen is attracted to a lone pair on a neighbouring molecule. It raises melting and boiling points.
• Intermolecular forces. The forces of attraction between separate molecules, much weaker than the covalent bonds within a molecule; they must be overcome for a molecular substance to melt or boil.

Diagrams and properties

• Dot-and-cross diagram. A drawing that shows the outer electrons of atoms as dots and crosses, so you can see which electrons are transferred or shared and where each came from.
• Electrical conductivity. The ability of a substance to carry an electric current. It requires mobile charge carriers, free ions or free electrons.
• Solubility. The ability of a substance to dissolve in a given solvent. Ionic compounds generally dissolve in water, while many simple covalent compounds dissolve in organic solvents.

More terms to know

• Valence (outer) electrons. The electrons in the outermost shell of an atom. These are the electrons that are transferred or shared when a bond forms, so only they appear in a dot-and-cross diagram.
• Compound. A substance made of two or more different elements chemically combined in a fixed ratio, such as sodium chloride or water. It can be broken down only by chemical means.
• Molecule. A group of two or more atoms held together by covalent bonds, such as O₂, H₂O or CH₄. It is the smallest particle of a covalent substance that can exist on its own.
• Molten. The liquid state of a substance after it has melted. When an ionic compound is molten, its ions become free to move, which is why it can then conduct electricity.
• Aqueous solution. A solution in which the solvent is water, shown by the state symbol (aq). When an ionic compound dissolves in water, its ions separate and become free to move.
• Polar molecule. A molecule with an uneven distribution of electric charge, such as water. Polar solvents like water dissolve ionic compounds well.
• Charge balance. The rule that in an ionic compound the total positive charge equals the total negative charge, which fixes the formula, for example one Mg²⁺ to two Cl⁻ in MgCl₂.
• Melting point. The temperature at which a solid becomes a liquid. A high melting point signals strong forces throughout the structure, as in a giant ionic lattice.
• Non-metal. An element that tends to gain or share electrons when bonding, such as chlorine, oxygen or carbon. Non-metals bond covalently with one another and ionically with metals.
• Metal. An element that tends to lose its outer electrons when bonding, such as sodium, magnesium or calcium. Metals form cations and bond ionically with non-metals.
• Chemical formula. A representation showing the kinds and ratio of atoms or ions in a substance, such as NaCl, MgCl₂ or CO₂. For an ionic compound the ratio is fixed by charge balance.

Using this glossary

Test yourself by covering the definition and giving it from the term alone, then check your wording against the version here. In a definition question, the exact phrase, “strong electrostatic force of attraction”, “share one or more pairs of electrons”, “both electrons from the same atom”, is what earns the mark. A one-to-one teacher can check your definitions are complete and precise, which is where the quick marks in SPM Chemistry are won or lost.