The modern periodic table is arranged in ascending order according to atomic number. An element's atomic number is equal to the number of protons in each atom. Within this order, elements are arranged into distinct groups that share properties.
Around 80 percent of the periodic table consists of metals, and 15 percent of the table consists of nonmetals. The remaining 5 percent of elements are metalloids, or elements that share qualities with both metals and nonmetals. The metals include alkali metals, alkaline earth metals, transition metals, lathanides and actinides, and metals not otherwise grouped. The nonmetals include the noble gases and halogens.
Their similar electron configurations give elements in the same group physical similarities. Alkali metals and halogens are both highly reactive groups that readily form compounds with each other; alkali metals have one valence electron to donate, while halogens have the capacity to accept one valence electron. A classic example is the reaction between sodium and chlorine, which forms table salt: NaCl.
The noble gases are the least reactive elements on the periodic table, and they get their name from their inability to form compounds in nature. Noble gases have a complete set of valence electrons, rendering them generally chemically inert.