Chemistry Basics Questions for RRB NTPC

Atomic Structure

An atom is the smallest unit of matter. It has a nucleus containing protons (positive charge) and neutrons (no charge), surrounded by electrons (negative charge) moving in shells. The atomic number equals the number of protons, and mass number equals protons plus neutrons. Isotopes are atoms of the same element with different neutron counts — for example, Carbon-12 and Carbon-14 are isotopes of carbon.

• Atomic number = number of protons = number of electrons in a neutral atom
• Mass number = protons + neutrons
• Isotopes: same protons, different neutrons (e.g., Uranium-235 and Uranium-238)
• Isobars: same mass number, different atomic numbers (e.g., Argon-40 and Calcium-40)
• Valence electrons decide chemical behavior — outermost shell electrons
• Electron shells follow 2n squared rule: K=2, L=8, M=18, N=32

Periodic Table Essentials

The modern periodic table has 118 elements arranged in 18 groups (vertical columns) and 7 periods (horizontal rows). Elements in the same group have the same number of valence electrons and similar properties. As you move across a period, atomic size decreases and electronegativity increases. As you move down a group, atomic size increases and metallic character increases. The most electronegative element is Fluorine. The most electropositive (reactive metal) is Cesium (or Francium which is radioactive).

• Groups 1-2 are s-block metals; Groups 13-18 are p-block; Groups 3-12 are d-block (transition metals)
• Group 1 = Alkali metals (Li, Na, K, Rb, Cs, Fr) — very reactive, store in kerosene
• Group 17 = Halogens (F, Cl, Br, I, At) — most reactive non-metals
• Group 18 = Noble gases (He, Ne, Ar, Kr, Xe, Rn) — inert, zero valency
• Metalloids (semiconductors): B, Si, Ge, As, Sb, Te — lie along the staircase line
• Lanthanides and Actinides are f-block elements placed separately at the bottom

Chemical Bonding

Atoms bond to achieve stable electron configuration. Ionic bonds form by transfer of electrons (metal gives to non-metal) — NaCl is the classic example. Covalent bonds form by sharing electrons between non-metals — water (H2O) and carbon dioxide (CO2). Metallic bonds hold metals together through a sea of free electrons, which is why metals conduct electricity.

• Ionic bond: metal + non-metal, high melting point, conducts in solution (NaCl, MgO)
• Covalent bond: non-metal + non-metal, low melting point, usually non-conductor (H2O, CO2, CH4)
• Coordinate bond: both electrons from one atom — found in NH4+ (ammonium ion)
• Hydrogen bond: weak bond responsible for high boiling point of water
• Electronegativity difference greater than 1.7 generally means ionic bond

Acids, Bases and Salts

An acid releases H+ ions in water (pH less than 7), a base releases OH- ions (pH greater than 7), and neutral substances have pH 7. The pH scale runs from 0 to 14. Strong acids like HCl and H2SO4 fully ionize. Weak acids like acetic acid (vinegar) partially ionize. Neutralization reaction: acid + base gives salt + water. Indicators like litmus turn red in acid and blue in base.

• pH less than 7 = acidic, pH 7 = neutral, pH greater than 7 = basic/alkaline
• HCl = hydrochloric acid (stomach acid), H2SO4 = sulphuric acid (battery acid), HNO3 = nitric acid
• NaOH = caustic soda (strong base), Ca(OH)2 = lime water, NH4OH = weak base
• Baking soda = NaHCO3 (sodium bicarbonate) — mild base, used in antacids
• Washing soda = Na2CO3 (sodium carbonate) — stronger base, used in cleaning
• Common salt = NaCl; Plaster of Paris = CaSO4 half water (hemihydrate)

Important Chemical Compounds and Their Uses

RRB NTPC frequently asks about everyday chemicals — their formula, common name, and use. This is pure memory work. Link the chemical to where you encounter it in daily life to remember quickly.

• Water (H2O) — universal solvent; Heavy water (D2O) — used in nuclear reactors
• CO2 — used in fire extinguishers and as dry ice (solid CO2 = -78 degrees C)
• O3 (Ozone) — protects from UV rays in stratosphere; at ground level it is a pollutant
• CaOCl2 — bleaching powder, used for disinfecting water
• Na2SO4.10H2O — Glauber's salt (used as laxative)
• CuSO4.5H2O — Blue vitriol (used in Bordeaux mixture for pesticide)

Types of Chemical Reactions

Chemical reactions are categorized by how reactants and products relate. Combination (synthesis) combines elements into one compound. Decomposition breaks one compound into simpler substances. Displacement is when a more reactive element pushes out a less reactive one. Double displacement involves exchange of ions between two compounds — usually forming a precipitate.

• Combination: A + B gives AB (e.g., 2H2 + O2 gives 2H2O)
• Decomposition: AB gives A + B (e.g., heating CaCO3 gives CaO + CO2)
• Displacement: A + BC gives AC + B (more reactive A replaces B)
• Redox: simultaneous oxidation (loss of electrons) and reduction (gain of electrons)
• Exothermic releases heat (combustion); Endothermic absorbs heat (photosynthesis)
• Corrosion = slow oxidation of metals (rusting of iron: 4Fe + 3O2 + xH2O gives Fe2O3.xH2O)

• Confusing Atomic Number with Mass Number — remember atomic number = protons only, mass number includes neutrons too
• Mixing up Baking Soda (NaHCO3) and Washing Soda (Na2CO3) — exam loves to swap these in options
• Thinking all covalent compounds are non-conductors — graphite is a covalent solid that conducts electricity, making it an exception
• Confusing Isotopes (same element, different mass) with Isobars (different elements, same mass) — these two appear together in options to trick you
• Assuming noble gases never react — Xenon does form compounds like XeF2 and XeF4, so avoid saying noble gases are completely inert in all cases

• PAIN for strong acids: Perchloric, Acetic (weak actually — use as contrast), H-I-N-S for strong: HCl, HI, HNO3, H2SO4 — remember SHIN (Sulphuric, Hydrochloric, Iodic, Nitric)
• LEO the lion says GER: Loss of Electrons = Oxidation; Gain of Electrons = Reduction
• For Group 1 alkali metals order: Li Na K Rb Cs Fr — mnemonic: Little Nancy Keeps Rabbits Called Frankie
• Everyday chemicals hook: Baking soda is in your kitchen BAKING (NaHCO3 is small and mild); Washing soda WASHES hard stains (Na2CO3 is stronger, has more sodium)

• RRB NTPC 2019 and 2020 papers had direct questions on chemical formulas of common substances — always memorize at least 15 everyday compounds with their formula and use
• pH-based questions appear often — know that battery acid has pH near 1, pure water is 7, bleach is around 13, and human blood is slightly basic at 7.4
• Periodic table questions mostly target group properties, not specific element positions — focus on alkali metals, halogens, noble gases, and transition metals as groups
• One question per paper is often on type of chemical reaction — decomposition and displacement are most frequently tested, link to a real example to recall fast
• Timing advice: Chemistry Basics questions are mostly 30-45 second recall questions — do not overthink. If you have memorized formulas and common names, mark and move on