Ordering elements by atomic number
Modern tables order elements by atomic number, the number of protons in the nucleus. Horizontal rows are periods, while vertical groups organize recurring similarities. Position therefore provides clues for comparing chemical behaviour rather than serving only as a sequence to memorize.
References: [1]
From atomic weights to periodicity
Nineteenth-century classifications commonly used atomic weights. Mendeleev, Meyer and others recognized recurring properties and revised their arrangements. Predictions for some gaps allowed newly discovered elements to test the organizing system rather than merely being added to a list.
Similarities with qualifications
Related valence-electron structures help explain similarities within many groups, but their members are not chemically identical. Categories such as metals, nonmetals and metalloids summarize trends; actual behaviour still depends on the element and the conditions of a reaction.
References: [1]
Reading a periodic table
Tables commonly show symbols, names, atomic numbers and atomic weights, which describe different quantities. Isotopic mass numbers differ from average atomic weights. Historical tables also need to be read with attention to contemporary names, ordering assumptions and unresolved gaps.