CC5 · Ionic bonding · Topic 1 — Key concepts in chemistry | GCSE Edexcel Combined Science

Revise the key ideas

An ion is an atom or group of atoms with an overall charge because its numbers of protons and electrons do not balance. Ordinary ion formation changes electrons, not protons.
Losing negatively charged electrons produces a positive ion, a cation. Gaining electrons produces a negative ion, an anion.
Metals commonly lose outer electrons. Sodium, 2,8,1, loses one to form Na⁺ with configuration 2,8; magnesium, 2,8,2, loses two to form Mg²⁺ with 2,8.
Non-metals commonly gain electrons. Chlorine, 2,8,7, gains one to form Cl⁻ with 2,8,8; oxygen, 2,6, gains two to form O²⁻ with 2,8.
Common simple ion charges follow main-group patterns: group 1 +1, group 2 +2, group 3 +3, group 6 −2 and group 7 −1. Transition metals may have different charges, specified when needed.
Common main-group patterns; transition-metal charges can vary.
These ions often have a full outer shell, a more stable electronic arrangement. The proton number and element identity remain unchanged.

An ionic bond is the strong attraction between positively and negatively charged ions. This attraction between opposite charges is called electrostatic attraction. Ionic compounds commonly form between metals and non-metals.
In sodium chloride formation, a sodium atom transfers one outer electron to a chlorine atom. Na⁺ and Cl⁻ form, and their opposite charges attract.
A transferred electron forms ions; attraction between their charges is the ionic bond.
In magnesium oxide, magnesium transfers two electrons to oxygen. Mg²⁺ and O²⁻ attract; each ion has a complete outer shell.
In magnesium chloride, one magnesium atom loses two electrons, one to each of two chlorine atoms. The ions are Mg²⁺ and two Cl⁻.
Dot-and-cross diagrams distinguish electrons originally from different atoms. Draw brackets and charges for final ions and full outer shells; dots and crosses are not different kinds of electron.
Outer electrons involved in transfer are shown; Na⁺ also has a full inner shell (2,8).
Electron transfer explains formation of ions. The bond itself is the attraction between ions, not an electron travelling continuously between two atoms.

An ionic compound is electrically neutral overall. Its formula gives the simplest whole-number ratio of ions, not a separate molecule.
Na⁺ and Cl⁻ balance 1:1, giving NaCl. Mg²⁺ and O²⁻ also balance 1:1, giving MgO.
Mg²⁺ needs two Cl⁻ ions to balance charge, giving MgCl₂. Two Na⁺ balance one O²⁻, giving Na₂O.
Two Al³⁺ give +6 and three O²⁻ give −6, so aluminium oxide is Al₂O₃. Always simplify a ratio to its lowest whole numbers.
Balance total positive and negative charge, then simplify the ratio.
For groups of atoms, preserve the group when using brackets: calcium nitrate from Ca²⁺ and NO₃⁻ is Ca(NO₃)₂. Ammonium is NH₄⁺ and sulfate is SO₄²⁻.
A small number below the line (a subscript) counts atoms or groups. A charge above the line (a superscript) gives an ion’s charge. NaCl is neutral overall, so its formula has no overall + or − charge.

Ionic solids form giant regular three-dimensional lattices of alternating positive and negative ions. Each ion attracts oppositely charged neighbours in many directions.
Ions attract opposite charges throughout the lattice; this is not a collection of separate molecules.
Strong electrostatic attractions need much energy to overcome, so ionic compounds generally have high melting and boiling points.
A solid ionic compound does not conduct electricity because its ions are fixed in position, even though they are charged.
When molten, ions can move and carry charge. When an ionic compound dissolves in water, mobile dissolved ions can also conduct.
Mobile charged ions carry current in molten and aqueous ionic compounds.
Not all ionic compounds dissolve in water. An insoluble compound cannot make a useful conducting aqueous solution simply by adding water.
Ionic crystals are brittle: they can break when layers shift and bring ions with the same charge next to each other. These ions repel, splitting the crystal. Metals are different: their layers can slide without this splitting.

A high-melting solid that conducts when molten but not solid is consistent with an ionic structure. Combine evidence rather than deciding from appearance alone.
The charge carriers in molten or aqueous ionic compounds are ions. They are not a sea of delocalised electrons as in a metal.
In electrolysis, cations move to the negative cathode and anions to the positive anode. The electrode signs describe an electrolytic cell connected to a power supply.
A flat lattice diagram shows only a slice of a three-dimensional arrangement and is not to scale. Ion sizes and spacing are simplified.
Use particle-level explanations: strong attractions explain high melting point, while ability of ions to move explains conduction. “It has bonds” alone is not sufficient.