Dmitri Mendeleev published the first periodic table in 1869. He showed that when the elements were arranged according to atomic weight, a pattern was produced in which similar properties were periodically repeated for the elements.
Subsequently, based on the work of physicist Henry Moseley, the periodic table was rearranged on the basis of increasing atomic number rather than atomic weight. The revised table could be used to predict the properties of items that had not yet been discovered. Many of these predictions were later verified through experimentation. This led to the formulation of the periodic law, which states that the chemical properties of elements depend on their atomic numbers.
Organization of the periodic table
The periodic table lists the elements by atomic number or, which is the number of protons in each atom of that element. Atoms of one atomic number can have a variable number of neutrons (isotopes) and electrons (ions), but they are still the same chemical element.
The elements of the periodic table are organized into points (rows) and groups (columns). Each of the seven periods is filled sequentially by atomic number. Groups include elements that have the same electronic configuration in their outer shell, resulting in group elements that share similar chemical properties.
The electrons in the outer shell are called valence electrons . Valence electrons determine the properties and chemical reactivity of the element and participate in chemical bonding. The Roman numerals above each group specify the usual number of valence electrons.
One way to categorize elements is according to whether they behave as metals or non-metals. Most of the elements are metals. They are found on the left side of the table. The far right contains the non-metallic elements, plus the hydrogen that exhibits non-metallic characteristics under normal conditions. Elements that have some properties of metals and some of the non-metals are called metalloids or semi-metals. These elements are found along a zigzag line that runs from the upper left of group 13 to the lower right of group 16. Metals are generally good conductors of heat and electricity, are malleable and ductile, and they have a shiny metallic appearance. In contrast, most nonmetals are poor conductors of heat and electricity, tend to be brittle solids, and can take any of several physical forms. While all metals except mercury are solids under normal conditions, nonmetals can be solids, liquids, or gases at room temperature and pressure.
