The Periodic Table Help

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By — McGraw-Hill Professional
Updated on Aug 28, 2011

Periods And Groups

Like a family history, the elements are arranged in family groups such as noble gas, halogen, metal, rare earth, transitional metal, non-metal, alkali metal, and alkaline earth. Just as genetic analysis helps biologists and physicians to determine a person’s make-up, so the grouping of elements into families and groups helps chemists to understand similar properties of different elements.

Elements with similar boiling or melting points usually act the same way when exposed to the same experimental conditions. The same is true of freezing and vaporization points.

The Periodic Table is the most important tool in general chemistry. Probably only the Bunsen burner (laboratory gas flame) rivals it in a distant second place. The amount of information pulled together in one place makes calculations, reactions, and the study of matter a whole lot easier to decipher.

Basically, the Periodic Table is divided into rows and columns, known as periods and groups . Dividing elements into periods and groups helps classify them by their specific characteristics.


Each period ends with an element known as a noble gas. Like kings and queens set apart in impenetrable castles, these gases are chemically unreactive and composed of individual atoms.

A period contains the elements in one horizontal row of the Periodic Table.

The first period contains only hydrogen (H) and helium (He). The second period has 8 elements: lithium (Li) through neon (Ne). The third period also contains 8 elements: sodium (Na) through argon (Ar). The fourth period has 18 elements: potassium (K) through krypton (Kr). The fifth period also has 18 elements: rubidium (Rb) through xenon (Xe). The sixth period has 32 elements; cesium (Cs) through radon (Rn). To make the table less bulky, the sixth and seventh period rows have been divided between 57, 58 and 89, and 90. These rows are shown fully expanded at the bottom of the chart. The seventh period is not complete, but includes gaps just as Meyer did earlier to allow for additional elements: francium (Fr) through lawrencium (Lr). Placeholders are in debate through element 118 and sometimes are not included on older Periodic Tables.


The groups of the Periodic Table are numbered most frequently with Roman numerals. The International Union of Pure and Applied Chemistry (IUPAC), in order to avoid confusion, set up a standard numbering plan in which columns were numbered I-VIII, according to their characteristics.

A group contains the elements in one column of the Periodic Table.

These groups are further divided into A and B sub-groups with the A groups called the main groups or representative elements and the B groups called the transition elements. Numbers 58 (cerium) to 71 (lutetium) are known as the lanthanide series and 90 (thorium) to 118+ (ununoctium) as the actinide series of elements.

The Periodic Table divided into periods and groups is shown in Figure 4.4 .

Elements, Symbols, and the Periodic Table Groups

Fig. 4.4. Groups and periods of elements are found in columns and rows, respectively, on the Periodic Table.

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