Chemical Bonding and Stoichiometry Questions for NDA

Types of Chemical Bonds

Atoms bond to achieve stability, usually by completing their outermost shell (octet rule). The three main bond types tested in NDA are ionic, covalent, and coordinate (dative) bonds. Ionic bonds form between metals and non-metals via electron transfer. Covalent bonds form between non-metals via electron sharing. Coordinate bonds are a special type of covalent bond where both electrons in the shared pair come from only one atom (the donor).

• Ionic bond: metal + non-metal, large electronegativity difference (greater than 1.7), forms crystal lattice, high melting point.
• Covalent bond: non-metal + non-metal, electrons shared equally or unequally, can be single, double, or triple.
• Coordinate/Dative bond: one atom donates both electrons, shown by an arrow. Example: NH4+ (ammonium ion), H3O+ (hydronium ion).
• Hydrogen bond: weak attraction between H (bonded to F, O, or N) and another F, O, or N. Explains high boiling point of water.
• Van der Waals forces: weakest intermolecular forces, present in all molecules, responsible for noble gas liquefaction.
• Metallic bond: sea of electrons shared by all metal atoms, explains conductivity and malleability.

Lewis Structures and VSEPR Theory

A Lewis structure shows valence electrons as dots around atoms. Shared pairs form bonds; unshared pairs are lone pairs. VSEPR (Valence Shell Electron Pair Repulsion) theory predicts molecular shape: electron pairs around a central atom repel each other and arrange to stay as far apart as possible. Lone pairs cause more repulsion than bonding pairs, compressing bond angles.

• 2 bond pairs, 0 lone pairs: Linear, 180 degrees. Example: CO2, BeCl2.
• 3 bond pairs, 0 lone pairs: Trigonal planar, 120 degrees. Example: BF3.
• 4 bond pairs, 0 lone pairs: Tetrahedral, 109.5 degrees. Example: CH4.
• 4 pairs, 1 lone pair: Trigonal pyramidal, ~107 degrees. Example: NH3.
• 4 pairs, 2 lone pairs: Bent/V-shape, ~104.5 degrees. Example: H2O.
• Lone pairs compress bond angles: tetrahedral > pyramidal > bent (109.5 > 107 > 104.5).

Polarity and Dipole Moment

A bond is polar if the two atoms have different electronegativities. But a molecule can be non-polar even if it has polar bonds, if the bond dipoles cancel out due to symmetry. NDA frequently asks: which molecule has zero dipole moment despite having polar bonds?

• CO2 is linear and symmetric, bond dipoles cancel, net dipole = 0 despite C=O being polar.
• H2O is bent, dipoles do NOT cancel, so it is polar with a net dipole moment.
• BF3 is trigonal planar and symmetric, net dipole = 0.
• NH3 is pyramidal, dipoles do not fully cancel, so it is polar.
• CCl4 is tetrahedral and symmetric, net dipole = 0. CHCl3 is not symmetric, so it is polar.
• Rule: Symmetric shape = non-polar molecule (even with polar bonds).

Mole Concept and Avogadro's Number

The mole is the bridge between atomic scale and lab scale. One mole of any substance contains 6.022 x 10^23 particles (Avogadro's number). The molar mass (in grams) of a substance equals its molecular weight numerically. This concept is the foundation of all stoichiometry calculations.

• 1 mole = 6.022 x 10^23 particles (atoms, molecules, ions).
• Molar mass = mass of 1 mole in grams = numerically equal to molecular weight in amu.
• Moles = given mass / molar mass.
• At STP (0 degrees C, 1 atm), 1 mole of any ideal gas occupies 22.4 litres.
• Number of particles = moles x 6.022 x 10^23.
• Percentage composition: (mass of element / molar mass of compound) x 100.

Stoichiometry and Limiting Reagent

Stoichiometry uses the balanced chemical equation to relate amounts of reactants and products. The mole ratio from the equation is the key. The limiting reagent is the reactant that runs out first and decides how much product forms. The excess reagent is left over.

• Always balance the equation first before doing any calculation.
• Convert all given masses to moles using molar mass.
• Use mole ratio from balanced equation to find moles of product.
• Limiting reagent: divide moles of each reactant by its coefficient; whichever gives the smaller value is the limiting reagent.
• Theoretical yield: moles of product x molar mass of product.
• Percentage yield = (actual yield / theoretical yield) x 100.

• Confusing molecular shape with electron geometry: NH3 has 4 electron pairs (tetrahedral electron geometry) but its molecular shape is trigonal pyramidal. NDA asks about molecular shape, not electron geometry.
• Assuming a molecule with polar bonds is always polar: CO2 and CCl4 have polar bonds but are non-polar molecules due to symmetry. This is a classic NDA trap.
• Using 22.4 L/mol for gases NOT at STP: this value applies only at 0 degrees C and 1 atm. At room temperature (25 degrees C), molar volume is approximately 24.5 L/mol.
• Forgetting to balance the equation before stoichiometry: an unbalanced equation gives wrong mole ratios and completely wrong answers.
• Picking the reactant in excess as the limiting reagent: the limiting reagent is the one that produces LESS product, not the one with fewer grams given.

• IONIC = I OWN them (electron transfer, metal gives, non-metal takes). COVALENT = CO-OWN (sharing). COORDINATE = CHARITY (one atom donates both electrons, like a donation).
• For VSEPR shapes, remember LTPVB in order of increasing lone pairs: Linear, Trigonal planar, Pyramidal (tetrahedral base), V-shape (Bent). Lone pairs compress angles like an unwanted guest pushing people apart.
• Mole formula triangle: write M at top, m at bottom-left, n at bottom-right. Cover what you want: cover n = m/M, cover m = nM, cover M = m/n.
• Limiting Reagent: think of making sandwiches. If you have 4 bread slices and 1 cheese slice, cheese is limiting (you can only make 1 sandwich). The ingredient that runs out first limits the product.

• NDA typically has 2-3 conceptual bonding questions per paper (shape of molecule, type of bond, which has zero dipole moment) and 1-2 numerical stoichiometry questions. Conceptual ones are faster to solve.
• High frequency PYQ themes: identify shape of H2O vs CO2, which molecule is polar, moles of gas at STP, and percentage composition. Practice these specific question types.
• Stoichiometry numericals in NDA are straightforward one-step or two-step calculations. If your working is taking more than 3 steps, recheck the equation balance.
• For bond type questions, remember the electronegativity order: F > O > N > Cl > Br > C > H. You do not need exact EN values, just the trend to decide polarity direction.
• Hydrogen bonding is tested in context of physical properties: why water has a higher boiling point than H2S, why HF is a weaker acid than HCl despite being more electronegative. These are direct one-liner GAT questions.