1.4.5Physical Chemistry
Dot and Cross Diagrams — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 21 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Drawing Conventions
Outer shell only
- Outer shell only Only the outer electrons are drawn because these are the ones involved in bonding;
- inner shells are left out of the diagram.
Dots and crosses
- Dots and crosses Electrons from one atom are shown as dots and from the other as crosses, which keeps track of where each electron originated.
Overlap
- Overlap In covalent molecules the shared pair is drawn in the region where the outer circles of the two atoms overlap.
Lone pairs
- Lone pairs Non-bonding pairs are drawn on the atom outside the overlap and must be included, because they affect the molecular shape.
Total electrons
- Total electrons Count every electron before and after to confirm that none has been lost or added in a covalent molecule.
Section 2 of 4Ionic and Covalent Cases
Ionic and Covalent Cases
Ionic compounds
- Ionic compounds Show the cation with its emptied outer shell removed and the anion with eight outer electrons, each enclosed in square brackets with its charge.
Lithium nitride
- Lithium nitride Three lithium atoms each give one electron to nitrogen, producing 3Li⁺ and N³⁻, so the formula is Li₃N.
Methane
- Methane Carbon shares one pair with each of four hydrogen atoms, giving four bonding pairs and no lone pairs around carbon.
Carbon dioxide
- Carbon dioxide Carbon makes two double bonds with oxygen, so two electron pairs are shared in each C=O bond.
Section 3 of 4Exceptions to the Octet
1
Some molecules have fewer than eight electrons around the central atom, such as BeCl₂ with four and BF₃ with six.
2
These electron-deficient molecules can accept a lone pair, so BF₃ readily forms a dative bond with ammonia.
3
Other molecules have more than eight electrons around the central atom, called an expanded octet.
4
Phosphorus pentachloride PCl₅ has ten electrons around phosphorus and sulfur hexafluoride SF₆ has twelve around sulfur.
5
Expansion is possible only for atoms in period 3 or lower, because they have available d orbitals in their outer shell.
| Molecule | Electrons around central atom | Description |
|---|---|---|
| BeCl₂ | 4 | Electron-deficient |
| BF₃ | 6 | Electron-deficient |
| PCl₅ | 10 | Expanded octet |
| SF₆ | 12 | Expanded octet |
Section 4 of 4Exam Checklist & Tips
1.4.5 – Key Points for the Exam
- Dot and cross diagram A drawing of outer-shell electrons that uses different symbols for electrons from different atoms.
- Lone pair A non-bonding pair of outer electrons on an atom.
- Octet A stable outer shell of eight electrons.
- Expanded octet An outer shell holding more than eight electrons, possible from period 3 downwards.
Common Mistakes
- Drawing inner-shell electrons as well, which makes the diagram cluttered and wrong in an exam.
- Leaving out lone pairs on the central atom, then predicting the wrong shape.
- Omitting square brackets and charges around ions in ionic dot and cross diagrams.
- Applying the octet rule to period 2 elements with more than eight electrons, which is impossible for them.
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