Early Attempts at Classification
As the number of discovered elements grew, scientists sought patterns to classify them based on their properties.
- Dobereiner's Triads (1817): Johann Wolfgang Dobereiner arranged elements with similar properties into groups of three (triads). The atomic mass of the middle element was roughly the average of the atomic masses of the other two elements (e.g., Lithium, Sodium, Potassium).
- Newlands' Law of Octaves (1866): John Newlands arranged elements in increasing order of atomic masses and observed that every eighth element had properties similar to the first, comparing it to the octaves of music. However, this only worked well up to Calcium.
Mendeleev's Periodic Table
Dmitri Mendeleev formulated a periodic law stating that "the properties of elements are the periodic function of their atomic masses." He arranged elements with similar properties into vertical columns (groups) and horizontal rows (periods). Mendeleev left gaps for elements that were yet to be discovered (like Scandium, Gallium, and Germanium) and boldly predicted their properties, which turned out to be remarkably accurate. However, anomalies regarding atomic mass order and the position of hydrogen and isotopes remained.
The Modern Periodic Table
In 1913, Henry Moseley discovered that atomic number is a more fundamental property than atomic mass. This led to the Modern Periodic Law: "Properties of elements are a periodic function of their atomic number." This resolved the anomalies of Mendeleev's table.
Structure and Trends in the Modern Periodic Table
The modern table has 18 vertical columns called Groups and 7 horizontal rows called Periods. Elements in the same group have the same number of valence electrons and thus similar chemical properties.
- Valency: Across a period, valency increases from 1 to 4 and then decreases to 0. Down a group, valency remains the same.
- Atomic Size: Atomic radius decreases from left to right across a period due to increasing nuclear charge. It increases down a group as new electron shells are added.
- Metallic Character: The tendency to lose electrons (metallic character) decreases across a period and increases down a group. Non-metallic character increases across a period and decreases down a group.