NDA Chemistry · Formula sheet
Chemical Bonding formulas
4 formulas, 3 reference tables and 11 common traps for NDA Chemistry Chemical Bonding, grouped by subtopic.
Ionic and Covalent Bonding
Learn this subtopic in the notesThe octet rule and the four types of chemical bond
| Bond type | How the octet is reached | Formed between | Example |
|---|---|---|---|
| Ionic (electrovalent) | Electrons transferred (lost / gained) | Metal + non-metal | Na⁺Cl⁻, MgO |
| Covalent | Electrons shared (one pair from each atom) | Non-metal + non-metal | H₂O, CH₄, SiC |
| Coordinate (dative) | Shared pair donated by one atom only | Donor with a lone pair | NH₄⁺, H₃O⁺ |
| Metallic | Valence electrons pooled in a 'sea' | Metal atoms | Na, Fe, Cu Metallic bonding (mobile electron sea) is why metals conduct electricity and are malleable. |
Transfer = ionic; share = covalent; one-sided share = coordinate; pool = metallic.
Properties of ionic versus covalent compounds
| Property | Ionic compounds | Covalent (molecular) compounds |
|---|---|---|
| Melting / boiling point | High (strong lattice) | Low (weak intermolecular forces) Among Na₂O, MgO, Fe₂O₃, CuO the highest melting point is MgO (~2852°C) — the small, doubly-charged Mg²⁺O²⁻ lattice. |
| Electrical conductivity | Conducts when molten or dissolved (not solid) | Does not conduct |
| Solubility | Usually soluble in water | Soluble in organic solvents |
| Physical state | Hard crystalline solids | Gases, liquids or soft solids |
| NaCl lattice coordination number | 6 : 6 (each ion surrounded by 6 of the opposite) | —Q |
Ionic = high MP + conducts when molten; covalent = low MP + does not conduct.
Bond polarity and polar molecules
| Statement about water | True or false |
|---|---|
| Water is a polar molecule | TRUE — it is bent, so the O–H dipoles don't cancel The bank's trap: the FALSE option is 'water is a non-polar molecule'. Water is polar. |
| Water has a bent (V-shaped) geometry | TRUE — bond angle ≈ 104.5° |
| Water is a good solvent for ionic compounds | TRUE — its polarity pulls ions apart |
| Water is a non-polar molecule | FALSE — this is the statement the bank wants flagged |
Common traps
Metal + non-metal is ionic; non-metal + non-metal is covalent
The quick test: a compound of a metal and a non-metal (CaO, Na₃N, ZnS) is ionic; a compound of two non-metals (SiC, CO₂, H₂O) is covalent. Silicon carbide trips students because it 'looks hard like a salt', but Si and C are both non-metals → covalent.
A coordinate bond is still a covalent bond
A coordinate (dative) bond shares a pair of electrons just like an ordinary covalent bond — the only difference is that one atom supplied both electrons. Once formed it is identical to any other covalent bond.
Ionic solids do NOT conduct — only when molten or dissolved
A statement that 'solid sodium chloride conducts electricity' is wrong. In the solid the ions are locked in the lattice; they conduct only when molten or dissolved, when the ions can move.
Higher charge → higher melting point
When ranking oxide melting points, the +2/−2 lattice (MgO) beats the +1/−2 lattice (Na₂O). Don't rank by molecular mass — rank by ionic charge and size.
Water is polar, not non-polar
The single most-tested trap in this subtopic: 'water is a non-polar molecule' is FALSE. Water is polar because its bent shape stops the two O–H bond dipoles cancelling.
Polar bonds don't always make a polar molecule
CO₂ has polar C=O bonds but is non-polar overall because the molecule is linear and the dipoles cancel. Shape decides molecular polarity, not just the bonds.
Valency, Oxidation States and Molecular Formula
Learn this subtopic in the notesValency — combining capacity from the outer shell
Common valencies
Oxidation state — the charge if every bond were ionic
Sum rule for oxidation states
- usually −2
- usually +1
Writing a molecular formula by crossing valencies
Cross-over rule
- valency (charge) of the cation A
- valency (charge) of the anion B
Common traps
Nitrogen's valency is 3, and nitride is N³⁻
Nitrogen has 5 outer electrons but a valency of 3 (it needs 3 to reach 8), and the nitride ion is N³⁻. Don't read 'valency 5' off the group number — count the electrons needed to complete the octet.
Let the known atoms force the unknown
Always fix oxygen at −2 and hydrogen at +1, then set the oxidation states to sum to the net charge and solve for the metal. For V₂O₅ the answer is +5, not +10 — the +10 is split between the two vanadium atoms.
Bracket a polyatomic ion before adding a subscript
Two ammonium ions are written (NH₄)₂, not NH₄₂ or N₂H₈. Wrap the whole polyatomic ion in brackets, then apply the subscript — otherwise the formula is misread.
Bond Counting and Molecular Structure
Learn this subtopic in the notesCounting the covalent bonds in a molecule
Bonds formed per atom
Common traps
Count every link, including the O–H in an alcohol
For methanol CH₃OH the answer is 5, not 4 — don't forget the O–H bond on top of the three C–H bonds and the C–O bond. Tally the whole skeleton, every atom included.
NO₂ dimerizes because it is a radical, not N₂O or N₂O₅
Among the nitrogen oxides only NO₂ has an unpaired electron, so it is the one that dimerizes (to N₂O₄). N₂O, N₂O₃ and N₂O₅ are all even-electron and do not pair up this way.