Edexcel GCSE Chemistry (1CH0) — Key Concepts in Chemistry
To accurately "sketch" a 2D representation from a 3D ball-and-stick or space-filling model, follow these sequential steps:
Different models highlight distinct features, but every model has limitations. You must evaluate these explicitly in the exam.
| Model Type | What it shows clearly | Limitations (Negative features) |
|---|---|---|
| Displayed formula (2D) | Connectivity of atoms and all bonds as solid lines. | Does not show 3D shape, bond angles, or relative sizes of atoms. |
| Dot-and-cross diagram (2D) | Outer-shell electron arrangement and the source of shared electrons. | Does not show the 3D physical arrangement or relative sizes. |
| Ball-and-stick model (3D) | 3D geometry, exact bond angles, and spatial arrangement. | Misleadingly depicts bonds as physical sticks; shows non-existent empty gaps. |
| Space-filling model (3D) | Relative atomic radii and how tightly atoms are packed. | Internal bonds and the underlying structure are completely hidden. |
A 3D ball-and-stick model of a metallic halide shows a repeating unit. Counting the spheres reveals for every two metal ions (), there are four halide ions (). Deduce the empirical formula.
A cubic nanoparticle has a side length of . Calculate its surface area to volume ratio.
Students often describe the structure of diamond as "strong", but the specific chemical term examiners want for a substance resisting scratching is "hard".
In 6-mark evaluation questions, you MUST state at least one specific limitation for each model (e.g., "the ball-and-stick model falsely shows empty gaps between atoms").
When sketching a 2D dot-and-cross diagram from a 3D model, always check the Group number of the central atom. 3D ball-and-stick models do NOT show lone pairs, but you must include them in your 2D sketch if the atom is in Group 5, 6, or 7.
Ionic lattices only conduct electricity when molten or aqueous because their ions (not electrons) are free to move. In contrast, graphite conducts because of delocalised electrons.
Molecular geometry
The three-dimensional arrangement of atoms within a molecule, which dictates its shape and bond angles.
Displayed formula
A 2D graphic representation of a molecule showing all atoms as symbols and all covalent bonds as solid lines.
Dot-and-cross diagram
A 2D diagram that illustrates the arrangement of outer-shell electrons and indicates which atom each shared electron originally came from.
Ball-and-stick model
A 3D physical or digital model where atoms are represented by spheres and bonds by rods, clearly showing geometry.
Space-filling model
A 3D representation using spheres with radii proportional to the actual atomic radii, showing how closely atoms pack together.
Giant ionic lattice
A three-dimensional structure of alternating positive and negative ions held together by strong electrostatic forces.
Empirical formula
The simplest whole-number ratio of atoms or ions present in a compound or lattice structure.
Put your knowledge into practice — try past paper questions for Chemistry
Molecular geometry
The three-dimensional arrangement of atoms within a molecule, which dictates its shape and bond angles.
Displayed formula
A 2D graphic representation of a molecule showing all atoms as symbols and all covalent bonds as solid lines.
Dot-and-cross diagram
A 2D diagram that illustrates the arrangement of outer-shell electrons and indicates which atom each shared electron originally came from.
Ball-and-stick model
A 3D physical or digital model where atoms are represented by spheres and bonds by rods, clearly showing geometry.
Space-filling model
A 3D representation using spheres with radii proportional to the actual atomic radii, showing how closely atoms pack together.
Giant ionic lattice
A three-dimensional structure of alternating positive and negative ions held together by strong electrostatic forces.
Empirical formula
The simplest whole-number ratio of atoms or ions present in a compound or lattice structure.
