The document summarizes the key developments in the periodic table. It describes how elements are organized in the table with atomic numbers and weights. Early scientists like Dobreiner grouped elements by properties into triads. Mendeleev arranged the table by atomic mass, but Moseley correctly identified atomic number as the fundamental property through his work with X-rays. The modern periodic table arranges elements by increasing atomic number according to the Periodic Law of repeating physical and chemical properties.

1. The Periodic Table

2. Introduction The periodic table is made up of rows of elements and columns. An element is identified by its chemical symbol. The number above the symbol is the atomic number The number below the symbol is the rounded atomic weight of the element. A row is called a period A column is called a group

3. Organizing the Elements Chemists used the properties of elements to sort them into groups. JW. Dobreiner grouped elements into triads. A triad is a set of three elements with similar properties.

4. Mendeleev’s Periodic Table In 1869, a Russian chemist and teacher published a table of the elements. Mendeleev arranged the elements in the periodic table in order of increasing atomic mass.

5. Henry Moseley 1887 - 1915 In 1913, through his work with X-rays, he determined the actual nuclear charge (atomic number) of the elements*. He rearranged the elements in order of increasing atomic number. *“There is in the atom a fundamental quantity which increases by regular steps as we pass from each element to the next. This quantity can only be the charge on the central positive nucleus.”

6. The Periodic Law In the modern periodic table elements are arranged in order of increasing atomic number. Periodic Law states: When elements are arranged in order of increasing atomic number, there is a periodic repetition of their physical and chemical properties.