Periodic table — পর্যায় সারণি
The periodic table is chemistry's map: all elements arranged by atomic number into periods and groups.
Definition
The periodic table arranges all known elements — 118 of them — in order of atomic number, so that elements with similar outer-electron arrangements line up in the same vertical group and show family resemblances. Mendeleev built the ancestor in 1869 and dared to leave gaps for undiscovered elements; Moseley later showed atomic number, not mass, is the true sorting key. Reading it well is the single most useful skill in chemistry: the table quietly tells you an element's block, shells, valence electrons and likely behaviour.
For example
Your NCTB Class 9–10 chemistry book devotes a full chapter to this table — because once you can read it, oxygen, iron and gold all become addresses rather than names.