The periodic table is not just a random grid of elements — it is organized so that elements in the same vertical column, called a group, share remarkably similar chemical personalities. This happens because elements in the same group have the same number of valence electrons (electrons in the outermost shell), and it is these valence electrons that determine how an atom reacts. Group 1 elements all have one valence electron, Group 2 elements have two, Group 17 elements have seven, and Group 18 (the noble gases) have a complete outer shell of eight.
Some groups are so distinctive that they have earned their own family names. Group 1 is the alkali metals — lithium, sodium, potassium, and their cousins — all soft, shiny metals that react violently with water. Drop a chunk of sodium in water and it fizzes, melts, and skitters across the surface; drop cesium and it literally explodes. Group 17 is the halogens — fluorine, chlorine, bromine, iodine — all reactive nonmetals that love to grab an extra electron. Group 18 is the noble gases — helium, neon, argon — famous for being almost completely unreactive because their electron shells are already full. Even their nicknames tell a story: "noble" gases were named because they were thought too "aristocratic" to mingle with common elements.
The power of group classification is prediction. If you know how one element in a group behaves, you can reasonably predict how the others will act. Lithium reacts with water gently, sodium more vigorously, potassium even more so — the reactivity increases as you go down Group 1 because the valence electron is farther from the nucleus and easier to remove. This pattern holds across the table: properties trend predictably within each group, making the periodic table not just a catalog but a genuine predictive tool. When Mendeleev arranged his original table, these group similarities were the key pattern that led him to the periodic law.