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Chemistry words, plainly explained
Chemistry glossary
Eighty terms every NCTB chemistry chapter leans on — each with a one-line quick answer, a full definition and a real-life example, in Bangla and English.
Atoms & structure
AtomপরমাণুThe atom is the smallest particle of an element that still shows that element's chemical identity.Protonপ্রোটনA proton is a positively charged particle in the nucleus; its count is the atomic number.Neutronনিউট্রনA neutron is the uncharged particle in the nucleus; it adds mass but no charge.Electronইলেকট্রনAn electron is the tiny negative particle that races around the nucleus in shells.Atomic numberপারমাণবিক সংখ্যাThe atomic number is the number of protons in an atom's nucleus — the element's identity.Mass numberভর সংখ্যাThe mass number is the total of protons plus neutrons in one nucleus, written as symbol A.Atomic massপারমাণবিক ভরAtomic mass is the average mass of an element's atoms, measured in unified atomic mass units (u).IsotopeআইসোটোপIsotopes are atoms of the same element with equal protons but different neutrons.IsobarআইসোবারIsobars are atoms of different elements that share the same mass number but not the same proton count.Electron shellইলেকট্রন শেলAn electron shell is an energy level around the nucleus where electrons stay, named K, L, M, N and so on.Orbital & subshellঅরবিটাল ও উপশেলShells split into subshells (s, p, d, f), and each subshell holds orbitals — regions where electrons are likely to be found.Electron configurationইলেকট্রন বিন্যাসElectron configuration is the neat listing of how an atom's electrons occupy its shells and subshells.Valence electronযোজ্য ইলেকট্রনValence electrons are the electrons in an atom's outermost shell — the ones that actually form bonds.Diatomic moleculeদ্বি-পরমাণবিক অণুA diatomic molecule is a molecule made of exactly two atoms, of the same or of different elements.
The periodic table
Periodic tableপর্যায় সারণিThe periodic table is chemistry's map: all elements arranged by atomic number into periods and groups.Periodপর্যায়A period is a horizontal row of the table; along it, elements slowly shift from metal to nonmetal.Groupগ্রুপA group is a vertical column of the table; its members share their outer-electron count and family chemistry.Blockব্লকA block is one of the four table regions — s, p, d, f — named after the subshell receiving the last electron.Alkali metalক্ষার ধাতুAlkali metals are the soft Group 1 metals (Li to Fr) with one valence electron and a passion for water.Alkaline earth metalমৃৎক্ষার ধাতুAlkaline earth metals are Group 2 elements (Be to Ra) with two valence electrons; they form +2 ions.Transition metalস্থানান্তর ধাতুTransition metals are the d-block metals of Groups 3–12: hard, useful, colourful and full of variable valency.MetalloidউপধাতুA metalloid is an element straddling the boundary between metals and nonmetals — not quite either.Halogenহ্যালোজেনHalogens are the reactive Group 17 nonmetals that need just one electron to complete their outer shell.Noble gasনিষ্ক্রিয় গ্যাসNoble gases are the Group 18 elements whose outer shells are already full, so they barely react.Lanthanideল্যান্থানাইডLanthanides are the 15 rare-earth metals from lanthanum (Z 57) to lutetium (Z 71), shown as a detached row.Actinideঅ্যাক্টিনাইডActinides are the 15 radioactive elements from actinium (Z 89) to lawrencium (Z 103), filling the 5f subshell.Atomic radiusপারমাণবিক ব্যাসার্ধAtomic radius is the measured size of an atom, half the distance between the nuclei of two bonded identical atoms.Ionization energyআয়নীকরণ শক্তিIonization energy is the energy needed to pull the outermost electron off a gaseous atom.Electronegativityতড়িৎ ঋণাত্মকতাAn atom's power to pull the shared electron pair of a bond toward itself; fluorine tops the scale at 3.98.Metallic characterধাতব ধর্মHow easily an atom loses electrons to become a positive ion; it increases down a group and decreases across a period.
Bonds & ions
Chemical bondরাসায়নিক বন্ধনA chemical bond is the electrical attraction that holds atoms or ions together in a compound or a crystal.Ionic bondআয়নিক বন্ধনAn ionic bond forms when a metal hands electrons to a nonmetal, and the opposite charges of the resulting ions pull the two together.Covalent bondসমযোজী বন্ধনA covalent bond is a shared pair of electrons holding two nonmetal atoms together, as in the O–H bonds of water.Metallic bondধাতব বন্ধনA metallic bond is the attraction between a lattice of positive metal ions and the sea of delocalized electrons flowing around them.Hydrogen bondহাইড্রোজেন বন্ধনA weak attraction in which hydrogen bonded to O, N or F is pulled toward a lone pair on a neighbouring electronegative atom.Ionআয়নAn ion is an atom or molecule that has lost or gained electrons, so it carries a net positive or negative charge.Cationক্যাটায়নA cation is a positively charged ion, formed when an atom or molecule loses one or more electrons.Anionঅ্যানায়নAn anion is a negatively charged ion, formed when an atom or molecule gains one or more extra electrons.MoleculeঅণুA molecule is two or more atoms held together by covalent bonds — the smallest unit of a covalent substance.ValencyযোজনীValency is the number of electrons an atom must lose, gain or share to complete its outer shell — its combining power.Oxidation stateজারণ অবস্থাThe charge an atom would carry if every shared electron pair in its bonds went to the more electronegative partner.PolarityমেরুকতাPolarity arises when shared electrons sit closer to one end of a bond or molecule, giving it positive and negative poles.Octet ruleঅষ্টক নিয়মAtoms bond in whatever way leaves each with eight outer-shell electrons — the settled arrangement of a noble gas.
Reactions
Chemical reactionরাসায়নিক বিক্রিয়াA chemical reaction is a process in which bonds break and new ones form, turning starting substances into different new substances.Reactants & productsবিক্রিয়ক ও উৎপাদReactants are the substances a reaction starts with; products are the new substances formed at the end.CatalystঅনুঘটকA catalyst is a substance that speeds up a chemical reaction yet remains chemically unchanged when the reaction is over.Exothermicতাপোৎসর্মীAn exothermic reaction releases heat to its surroundings, warming the container it happens in.EndothermicতাপাধিশোষীAn endothermic reaction absorbs heat from its surroundings, so the container feels cooler as it proceeds.CombustionদহনCombustion is a fast reaction of a fuel with oxygen that releases heat and light — burning, in one word.Corrosionক্ষয় (মরিচা)Corrosion is the slow eating away of a metal by the oxygen, water or chemicals around it; rusting iron is the classic case.Displacement reactionপ্রতিস্থাপন বিক্রিয়াIn a displacement reaction a more reactive element takes the place of a less reactive one inside its compound.OxidationজারণOxidation is the loss of electrons — in the classical sense, the gain of oxygen or the loss of hydrogen.ReductionবিজারণReduction is the gain of electrons — in the classical sense, the loss of oxygen or the gain of hydrogen.Redox reactionজারণ-বিজারণ বিক্রিয়াA redox reaction is one where oxidation and reduction happen together — electrons pass from one species to another.Reactivity seriesসক্রিয়তা ধারাThe reactivity series ranks metals from most to least reactive — potassium at the top, gold at the very bottom.
Acids, bases & salts
Acidঅ্যাসিডA substance that releases H⁺ ions in water; acids taste sour, turn blue litmus red and have pH below 7.Baseক্ষারকA substance that gives OH⁻ ions in water or accepts H⁺; bases feel soapy, turn red litmus blue and have pH above 7.Alkaliক্ষারA water-soluble base, such as NaOH, that releases OH⁻ ions; every alkali is a base, but not every base is an alkali.pH scaleপিএইচ স্কেলA 0–14 scale for acidity of water solutions: pH 7 neutral, below 7 acidic, above 7 basic; each unit is a tenfold change in H⁺.SaltলবণAn ionic compound formed by neutralizing an acid with a base; table salt NaCl is the most familiar example.Neutralizationপ্রশমন বিক্রিয়াAcid + base → salt + water: H⁺ from the acid and OH⁻ from the base combine and cancel each other.Indicatorনির্দেশকA substance that changes colour in acid versus base — litmus, phenolphthalein and methyl orange are the classics.Electrolysisতড়িৎ বিশ্লেষণUsing electric current to break up an electrolyte; water splits into H₂ at the cathode and O₂ at the anode.
States & mixtures
States of matterপদার্থের অবস্থাThe forms matter takes — solid, liquid, gas (plus plasma); ice, water and steam are one substance in three states.Melting pointগলনাঙ্কThe temperature where a solid becomes a liquid; while melting, temperature holds steady because heat goes into breaking the crystal.Boiling pointস্ফুটনাঙ্কThe temperature at which a liquid boils throughout; water boils at 100 °C at sea level, cooler on a high hill.Densityঘনত্বMass packed into each unit of volume; every pure substance has its own density, so it works like a fingerprint.Solutionদ্রবণA homogeneous mixture where a solute dissolves fully in a solvent — every drop identical, particles too small to filter.Solute and solventদ্রব ও দ্রাবকThe solute dissolves, the solvent dissolves it — in sugar water, sugar is the solute and water is the solvent.Mixtureমিশ্রণTwo or more substances mixed without chemical bonding; each keeps its properties, the ratio can vary, and parts separate physically.
Carbon chemistry
ElementমৌলA pure substance made of just one kind of atom; 118 elements are known, and no chemical reaction can split one further.CompoundযৌগA pure substance of two or more elements chemically bonded in a fixed ratio, like H₂O or NaCl, with its own new properties.Alloyমিশ্র ধাতুA metal mixed with other elements — brass (Cu+Zn), bronze (Cu+Sn), steel (Fe+C) — for extra strength or rust resistance.OreআকরিকA natural rock or mineral from which metal can be extracted profitably, like haematite (Fe₂O₃) for iron.Allotropeঅ্যালোট্রোপDifferent structural forms of one element — diamond, graphite and fullerene for carbon; O₂ and O₃ for oxygen.Conductor and insulatorপরিবাহী ও অপরিবাহীConductors let current flow (metals, graphite); insulators block it (rubber, glass, dry wood) because their electrons cannot move.Physical changeভৌত পরিবর্তনA change of form — state, shape, dissolving — with no new substance made; ice melting or sugar dissolving stay the same chemicals.Chemical changeরাসায়নিক পরিবর্তনBonds break and remake, giving brand-new substances — burning, rusting, souring, ripening; hard to undo, unlike physical changes.