Atomic size increases as you move left to right across a period. The periodic table of the elements provides information on atomic size in the form of atomic number and atomic weight.
The atomic number of an element is located in the upper left-hand corner of the element’s square on the periodic table. Atomic number indicates the number of protons in the nucleus, or center, of an atom. The number of protons is the factor that makes each element what it is. The nucleus of an atom may also contain neutrons. Atoms with the same number of protons but different numbers of neutrons are isotopes of the same element. The atomic weight, found at the bottom of each square on the table, indicates the average weight of an atom and its isotopes.
The periodic table is organized both by periods and groups, the former being the horizontal rows and the latter the vertical columns. Periods indicate the highest unexcited energy level that electrons of the included elements can occupy. Imagine that tracks, occupied by electrons, surround the nucleus of an atom. Each track, or shell, holds a specific number of electrons; the more electrons an atom possesses the more shells contain electrons. The periods of the periodic table show how many shells contain electrons. For example, hydrogen and helium occupy the first period of the periodic table. Both of these elements contain electrons in only one shell.
Groups deal with the atoms’ valence electrons. These electrons are in the outermost shell. Hydrogen and helium both possess one shell that contains electrons. Hydrogen possesses a single electron while helium contains two, so they occupy different groups. The configuration of valence electrons is important because it determines how an element bonds with other elements. Thus, elements in the same group tend to react in the same way when bonding.