1. Organization of elements
The History of the Periodic Table

2. Patterns in the elements
Several scientists proposed systems to organize elements based on their properties in the early 1800s, but none of the suggestions worked well
In the 1860s, Dmitri Mendeleev decided to work on the problem
Dmitri Mendeleev discovered a set of patterns that applied to all elements
Mendeleev knew that some elements have similar chemical and physical properties
In order to find the pattern, he wrote each element’s melting point, density, and color on a card; also included atomic mass and the number of chemical bonds formed
Arranged the elements in order of increasing atomic mass
Found that the properties of elements repeated so he began to place those specific elements into groups

3. However, arranging elements strictly by atomic mass did not always group similar elements together
In 1869, Mendeleev published his first periodic table in which elements were grouped by similar properties
There were 63 total elements that had been discovered when the first periodic table was produced so Mendeleev left empty spaces so that new elements that would be discovered later could be placed in the table
Mendeleev even described some of the properties of the unknown elements and 6 years later the first new element that he predicted would be discovered in France (Gallium)

5. The Modern Periodic Table
In 1913, Henry Moseley, a British scientist, discovered a way to measure the positive charge, which became the atomic number
After this discovery, the table was rearranged based on atomic number
In the periodic table, the properties repeat in each period, or row of the table
The properties of an element can be predicted from its location in the periodic table
Atomic numbers increase as you move left to right on the table
The table is organized in horizontal rows called periods
The table also falls into 18 vertical columns called groups or families

6. An element’s position in the table can give information about the element
The highly reactive elements are located on the left side of the table and become less reactive as you move right across the table
Atoms on the left side of the table form positive ions, while atoms on the right side, except group 18 form negative ions
Densities generally increase from the top of a group to the bottom

8. Reading an Element’s Square