Elements, atomic structure and bonding: the major rock-forming elements; atomic structure (protons, neutrons and electrons) and isotopes; ionic, covalent and metallic bonding; how the type of bonding and the arrangement of atoms control the physical properties of minerals such as hardness and cleavage.

What this dot point is asking

Eduqas wants you to name the major rock-forming elements, describe atomic structure (protons, neutrons and electrons) and what an isotope is, distinguish ionic, covalent and metallic bonding, and explain how the bond type and the arrangement of atoms control a mineral's physical properties (especially hardness and cleavage). This is the chemical foundation that the silicate structures, mineral identification and radiometric dating all build on.

The answer

The major rock-forming elements

Almost the whole crust is built from a small number of elements. By mass the eight most abundant are oxygen (about 46 percent), silicon (about 28 percent), then aluminium, iron, calcium, sodium, potassium and magnesium. Oxygen and silicon dominate because they combine to make the silicate minerals, which are the bulk of igneous and metamorphic rocks. Knowing this list explains why silicates are so common and why oxides and carbonates of iron, calcium and magnesium are the next most important groups.

Atomic structure and isotopes

An atom has a tiny dense nucleus of protons (positive) and neutrons (neutral), surrounded by electrons (negative) in shells.

• The atomic number is the number of protons; it defines the element.
• The mass number is the number of protons plus neutrons.
• Atoms are electrically neutral because the number of electrons equals the number of protons.

Isotopes matter twice over in geology: unstable isotopes are the clock behind radiometric dating, and stable isotope ratios (such as $^{18}\mathrm{O}/^{16}\mathrm{O}$) are palaeoclimate proxies.

The three types of bonding

When atoms combine, the outer-shell (valence) electrons are rearranged to reach a stable arrangement. There are three main bond types.

• Ionic bonding. Electrons are transferred from a metal to a non-metal, producing positive and negative ions that attract electrostatically. Example: halite (sodium chloride), where $\mathrm{Na^{+}}$ and $\mathrm{Cl^{-}}$ alternate in a cubic lattice. Ionic minerals are moderately hard, brittle, often soluble, and do not conduct electricity when solid.
• Covalent bonding. Electrons are shared between non-metal atoms. Where this gives a continuous three-dimensional framework (a giant covalent structure) the result is very hard and chemically resistant. Example: diamond (pure carbon), the hardest mineral; the silicon-oxygen bond in quartz is also largely covalent.
• Metallic bonding. Metal atoms release their outer electrons into a shared "sea" of delocalised electrons around positive metal ions. Example: native copper and gold. Metallic minerals are dense, malleable, opaque with a metallic lustre, and conduct electricity.

Many minerals use more than one bond type. Mica and other silicates have strong covalent or ionic bonds within their silicate sheets or frameworks but weaker bonds between them, which is the key to their cleavage.

How bonding controls properties

The single most important idea is that bond strength and the direction of bonding control the physical properties.

• Hardness depends on the strength of the bonds that must be broken to scratch the surface. Strong, uniform bonding (diamond, quartz) gives high hardness; weak bonding (talc, halite) gives low hardness.
• Cleavage depends on the directionality of bonding. If a structure has strong bonds in some directions and weak bonds in others, it splits along the weak planes (mica cleaves in one direction; halite cleaves in three at right angles). A uniform framework with equally strong bonds in all directions has no cleavage and breaks by fracture instead (quartz fractures conchoidally).
• Density depends on how closely the atoms are packed and how heavy they are; metallic and iron-rich minerals (magnetite, galena) are dense, framework silicates less so.

Examples in context

Example 1. Quartz versus mica. Both are silicates, but quartz is a continuous framework (hard, no cleavage, conchoidal fracture) while mica is a sheet silicate (one perfect cleavage, splits into flexible flakes). The contrast is purely a matter of how the silicate units are bonded together.

Example 2. Oxygen-18 in ice cores and shells. The ratio of the stable isotopes oxygen-18 to oxygen-16 in calcite shells and ice records past temperature, which is why isotopes appear again in the palaeoclimate topic.

Try this

Q1. State the two most abundant elements in the Earth's crust by mass. [2 marks]

• Cue. Oxygen (about 46 percent) and silicon (about 28 percent); together they form the silicate minerals.

Q2. Explain why quartz has no cleavage but mica has one perfect cleavage. [3 marks]

• Cue. Quartz is a continuous covalent framework with equally strong bonds in all directions, so it fractures rather than cleaving; mica has strong bonds within its sheets but weak bonds between them, so it splits along those weak planes.

Q3. Native gold is malleable, dense and conducts electricity. State the type of bonding and justify your answer. [2 marks]

• Cue. Metallic bonding: a sea of delocalised electrons around positive metal ions explains the malleability, conductivity and high density.