Above each elemental symbol in the periodic table is a number. The number is the element's atomic number and it represents how many protons are contained in the nucleus of that element. The atomic number is unique to each element. Hydrogen is the first element in the periodic table. It has one proton in its nucleus, therefore its atomic number is 1.
Periods are organized as horizontal rows on the periodic table. As elements progress through a period, they gain protons. In the first period: hydrogen has one proton, helium has two protons. In the second period: lithium has three protons, beryllium has four protons and so forth. To balance the change in protons, each element has to also gain electrons.
Electrons are gained systematically throughout a period in an organized structure of shells, subshells and orbitals. Each of these structures represents a compartment where an electron is most likely to be found at any given time. Shells are divided into subshells, which are further divided into orbitals. The number of electrons in the outer shell determines reactivity, or the capacity to form bonds with other atoms. Atoms at the beginning of a period have the fewest electrons in their outer shells. Atoms at the end of a period have a complete outer shell and are nonreactive.
Groups are organized as vertical columns on the periodic table. Groups are also called families because the elements in a group all exhibit similar chemical characteristics. Reactivity, physical state at room temperature--gas, liquid or solid--and types of bonding are all chemical properties that are shared within families.
Groups can be numbered in two ways. The new method simply numbers them one to eighteen from left to right. The older method, still used, numbers them in two sections: Group A and Group B. Group A identifies the representative elements. Group B identifies the transition metals. Some of the groups have special names because of their characteristics. Group IA is named the alkali metals, because they form potent alkaline solutions when mixed with water. Group IIA is the alkaline earth metals; their metal oxides form alkaline solutions in water. Group VIIA are the halogens. Halogens are very reactive because they need just one electron to complete their outer electron shell. Group VIIIA are the noble gases; so named because they do not react with other atoms.
The periodic table also organizes the elements into three groups: metals, nonmetals and metalloids. Metals compose the majority of the elements in the periodic table. Electron affinity, electronegativity and energy of ionization are all properties that can be observed in the elements as they trend toward the upper-right-hand corner of the periodic table. Atomic radius and metallic character all trend in the opposite direction, with the elements in the lower-left corner of the periodic table showing the greatest radii.