Key concepts in chemistry Revision Notes

Electron Transfer for Stability

Atoms aim for a stable electron configuration, usually a full outer shell (like noble gases). This is achieved by gaining or losing electrons. Metals tend to lose electrons, while non-metals tend to gain them. This electron transfer leads to the formation of charged particles called ions.

Cations: Group 1 and 2 Metals

Group 1 elements (e.g., sodium) have one electron in their outer shell. They lose this electron to form an ion with a 1+ charge (e.g., Na⁺). Group 2 elements (e.g., magnesium) have two outer electrons. They lose both electrons to form an ion with a 2+ charge (e.g., Mg²⁺). These positively charged ions are called cations.

Anions: Group 6 and 7 Non-metals

Group 7 elements (halogens, e.g., chlorine) have seven electrons in their outer shell. They gain one electron to complete their outer shell, forming an ion with a 1- charge (e.g., Cl⁻). Group 6 elements (e.g., oxygen) have six outer electrons. They gain two electrons to form an ion with a 2- charge (e.g., O²⁻). These negatively charged ions are called anions.

Ionic Bonding

The oppositely charged ions (cations and anions) are attracted to each other by strong electrostatic forces. This attraction forms an ionic bond, holding the ions together in a crystal lattice structure to create an ionic compound.

The '-ide' Ending

The suffix '-ide' in a compound's name generally indicates that the compound is made explicitly from two distinct elements. One of these elements is usually a metal, and the other is a non-metal. For example, sodium chloride (NaCl) is formed from sodium and chlorine. Iron sulfide (FeS) is formed from iron and sulfur. Potassium iodide (KI) is formed from potassium and iodine. The '-ide' ending signals the presence of the non-metal's ion in its simplest form.

The '-ate' Ending

The suffix '-ate' is used for compounds that contain a polyatomic ion which includes oxygen. These polyatomic ions are composed of at least one other element covalently bonded to one or more oxygen atoms. Examples include carbonate (CO₃²⁻), sulfate (SO₄²⁻), and nitrate (NO₃⁻). When these ions combine with a metal, they form compounds like calcium carbonate (CaCO₃), copper sulfate (CuSO₄), or sodium nitrate (NaNO₃). The '-ate' ending implies the presence of oxygen within the anionic part of the compound.

Distinguishing -ide and -ate

The key difference is the presence of oxygen as part of a polyatomic ion. If a compound is named with '-ide', it will not typically contain oxygen as a third element within the anionic part (e.g., sodium chloride contains Na and Cl, not O). If a compound is named with '-ate', it will always contain oxygen as part of a polyatomic anion (e.g., calcium carbonate contains Ca, C, and O). This distinction is crucial for understanding the chemical formula and structure of inorganic compounds.

Understanding Ionic Charges

Ionic compounds are made of cations (positive ions) and anions (negative ions) held together by electrostatic forces. The charge an ion carries is crucial for determining the compound's formula. For Group 1 elements, the charge is +1; for Group 2, it's +2. Aluminium forms Al³⁺. For Group 17 (halogens), the charge is -1; for Group 16 (e.g., oxygen), it's -2; and for Group 15 (e.g., nitrogen in nitrides), it's -3. Transition metals can have variable charges, which are usually indicated by a Roman numeral in their name (e.g., iron(II) is Fe²⁺). Polyatomic ions like nitrate (NO₃⁻), sulfate (SO₄²⁻), carbonate (CO₃²⁻), and hydroxide (OH⁻) have fixed charges.

The Criss-Cross Method for Formulae

To write the formula of an ionic compound, the total positive charge must equal the total negative charge. A simple technique is the 'criss-cross' method:

1. Write the symbols of the cation and anion with their charges.
2. Swap the absolute values of the charges, making them the subscripts for the opposite ion.
3. If a subscript is '1', it is usually omitted.
4. If a polyatomic ion requires a subscript greater than '1', enclose the polyatomic ion in parentheses first.
5. Simplify the subscripts to the lowest whole number ratio if possible. For example, for calcium chloride (Ca²⁺ and Cl⁻), you criss-cross: Ca₁Cl₂ which simplifies to CaCl₂. For magnesium oxide (Mg²⁺ and O²⁻), you criss-cross: Mg₂O₂ which simplifies to MgO.

What is an Ionic Lattice?

An ionic lattice is a highly organised, repeating three-dimensional arrangement of positive and negative ions. Imagine a vast, ordered structure where each positive ion is surrounded by negative ions, and each negative ion is surrounded by positive ions. This regular pattern is what defines the 'lattice' part of the structure.

Forces Holding the Lattice Together

The ions within the lattice are held in place by strong electrostatic forces of attraction. These forces exist between the oppositely charged ions (e.g., a positively charged sodium ion and a negatively charged chloride ion). These forces are known as ionic bonds and are responsible for the overall stability and properties of ionic compounds.

Properties Resulting from Lattice Structure

Because of these strong electrostatic forces throughout the entire lattice, a large amount of energy is required to separate the ions. This leads to characteristic properties of ionic compounds, such as very high melting and boiling points. The rigidity of the lattice also makes ionic compounds typically hard and brittle.