Atomic Structure & the Periodic Table
Atoms have protons (+) and neutrons in the nucleus with electrons (-) around it; the proton count is the atomic number and defines the element, while protons plus neutrons give the mass number.
The periodic table is arranged by atomic number: rows (periods) add electron shells and columns (groups) share valence-electron counts, which is why elements in a group behave alike. Valence electrons — the outermost ones — do all the bonding.