Same element, different mass
An element is defined by its protons — change that count and you have a different element altogether. But neutrons are more flexible: atoms of the same element with different neutron counts are called isotopes (সমস্থানিক). Hydrogen has three: protium (¹H, no neutron — just a proton), deuterium (²H, one neutron) and tritium (³H, two neutrons, faintly radioactive). All three are hydrogen: one proton, one electron, identical chemistry.
The notation stacks the numbers before the symbol: mass number on the upper left (protons + neutrons), atomic number on the lower left. Carbon-12 and carbon-14 differ only in that upper number — 12 versus 14 — which is why the periodic table lists carbon's mass as 12.011: that is the weighted average of all naturally mixed isotopes, not the mass of any single atom.
Why some atomic masses sit in brackets
Look down the bottom rows of the periodic table and the masses turn into bracketed numbers like [98] for technetium. That is isotope honesty: these elements have no stable isotopes at all, so a "natural average" cannot exist. The bracketed number is the mass of the most stable (or most long-lived) isotope — a courtesy figure, not a measurement of any natural mixture.
The isotope hall of fame
Carbon-14 dating: cosmic rays keep topping up ¹⁴C in living things; at death the clock starts, and the remaining fraction dates everything from mummies to charcoal. Uranium-235 vs uranium-238: chemically identical, physically worlds apart — only the rare ²³⁵U sustains a chain reaction, which is why enrichment exists. Cobalt-60 emits disciplined gamma rays for cancer therapy; iodine-131 homes into the thyroid for both diagnosis and treatment; deuterium moderates some nuclear reactors and traces water through plants.
Bangladesh's only research reactor, at Savar (TRIGA Mark-II), works with low-enriched uranium fuel, and the country's medical isotopes — the cobalt sources in radiotherapy wards and the iodine doses in thyroid clinics — arrive as imports calibrated to the same physics described above. Isotopes are not an exotic footnote; they are working infrastructure.
Isobars are the trap question
Exams love pairing isotopes with their mirror concept: isobars (সমভারিক) — atoms of different elements with the same mass number but different atomic numbers. Calcium-40 and argon-40 are the classic pair: identical upper number, different element. Isotopes share Z and differ in A; isobars share A and differ in Z. Two words, one letter of difference in meaning, plenty of marks lost by students who swap them.
You asked
1Do isotopes behave differently in reactions?
Chemically, almost identically — same protons, same electrons, same bonds. The differences are mass-based: heavier isotopes react a touch slower (kinetic isotope effect) and lighter ones evaporate slightly faster, which is why deuterium-enriched water (heavy water) must be separated physically, not chemically. Nuclear behaviour, however, can be completely different — that is the entire basis of U-235 enrichment.