CDS Chemistry · Formula sheet
Chemical Bonding formulas
2 formulas, 3 reference tables and 12 common traps for CDS Chemistry Chemical Bonding, grouped by subtopic.
Ionic and Covalent Bonds
Learn this subtopic in the notesIonic and covalent bonds
| Property | Ionic | Covalent |
|---|---|---|
| How the bond forms | Electron transfer | Electron sharing |
| Typical partners | Metal + non-metal | Non-metal + non-metal |
| Melting point | High | Usually low |
| Conducts when molten | Yes | No |
| Example | K₂S (highly ionic) | SiO₂, NCl₃, plasticsQ |
Common traps
A big, singly charged cation gives the most ionic compound
Potassium sulphide is highly ionic because K⁺ is large and singly charged. Beryllium compounds lean covalent because Be²⁺ is tiny and highly charged.
Plastics last because of covalent bonds
Plastics are not ionic or metallic. Their long chains are held by strong covalent bonds, which is why they persist in the environment.
SiO₂ is covalent despite its high melting point
Silicon dioxide melts at a very high temperature because it is a giant covalent network, not because it is ionic.
Molecular Shapes and Bond Counting
Learn this subtopic in the notesCounting sigma and pi bonds
Sigma and pi bonds
Hybridisation and molecular shape
| Molecule | Hybridisation | Shape |
|---|---|---|
| HCN | sp | Linear |
| HCHO | sp² | Trigonal planar |
| CH₃F | sp³ | Tetrahedral |
| NH₃ | sp³ (one lone pair) | Trigonal pyramidal |
| PCl₅ | sp³d (dsp³) | Trigonal bipyramidalQ |
| SF₆ | sp³d² | Octahedral |
Common traps
Ammonia is a pyramid, not a tetrahedron
NH₃ has four electron groups (sp³), but one is a lone pair, so the atoms form a trigonal pyramid. CH₄ and CH₃F, with no lone pair, are tetrahedral.
Five groups need a d orbital
PCl₅'s five bonds need five hybrid orbitals: one s, three p and one d, so sp³d (dsp³). sp³ gives only four.
Count the C–H bonds too
A ring of six carbons has 6 C–C bonds, but a hydrocarbon's total also includes every C–H bond. Leaving them out undercounts badly.
Saturated means zero pi bonds
Saturated compounds, cycloalkanes included, have only single bonds. A ring is not a double bond.
Valency and Oxidation Numbers
Learn this subtopic in the notesWorking out oxidation numbers
Sum rule
Elements with more than one valency
| Element | Valencies or oxidation states | Examples |
|---|---|---|
| Phosphorus | 3 and 5 | PCl₃, PCl₅Q |
| Sulphur | 2, 4, 6 | H₂S, SO₂, SO₃ |
| Iron | +2 and +3 | FeSO₄, FeCl₃ |
| Copper | +1 and +2 | Cu₂O, CuO |
| Sodium, lithium | +1 only | NaCl, LiCl |
| Calcium | +2 only | CaCl₂ |
Common traps
Hydrogen is not always +1
Hydrogen is +1 with non-metals, −1 in metal hydrides such as NaH, and 0 in H₂. 'Hydrogen can have more than one oxidation number' is the true statement.
N₂O₅ is +5, the highest for nitrogen
Work each oxide out with oxygen at −2: N₂O is +1, NO +2, NO₂ +4, N₂O₅ +5. Do not judge by the number of oxygen atoms alone.
Peroxide oxygen is −1
In hydrogen peroxide, H₂O₂, each oxygen is −1, not −2, because the two oxygens are bonded to each other.
Only the transition metal varies
Among sodium, calcium, lithium and iron, only iron (a transition metal) shows variable oxidation states. The others have one each.
Phosphorus is 3 and 5, not 3 and 4
Phosphorus shares three electrons (PCl₃) or all five (PCl₅). Pairs such as 2, 3 or 4, 5 are distractors.