Science, Tech, Math › Science Periodic Law Definition in Chemistry Share Flipboard Email Print MEHAU KULYK/SCIENCE PHOTO LIBRARY / Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated June 26, 2019 The Periodic Law states that the physical and chemical properties of the elements recur in a systematic and predictable way when the elements are arranged in order of increasing atomic number. Many of the properties recur at intervals. When the elements are arranged correctly, the trends in element properties become apparent and can be used to make predictions about unknown or unfamiliar elements, simply based on their placement on the table. Importance of Periodic Law Periodic Law is considered to be one of the most important concepts in chemistry. Every chemist makes use of Periodic Law, whether consciously or not, when dealing with the chemical elements, their properties, and their chemical reactions. Periodic Law led to the development of the modern periodic table. Discovery of Periodic Law Periodic Law was formulated based on observations made by scientists in the 19th century. In particular, contributions made by Lothar Meyer and Dmitri Mendeleev made trends in element properties apparent. They independently proposed Periodic Law in 1869. The periodic table arranged the elements to reflect Periodic Law, even though scientists at the time had no explanation for why properties followed a trend. Once the electronic structure of atoms was discovered and understood, it became apparent the reason characteristics occurred in intervals was because of the behavior of electron shells. Properties Affected by Periodic Law The key properties that follow trends according to Periodic Law are atomic radius, ionic radius, ionization energy, electronegativity, and electron affinity. Atomic and ionic radius are a measure of the size of a single atom or ion. While atomic and ionic radius are different from each other, they follow the same general trend. The radius increases moving down an element group and generally decreases moving left to right across a period or row. Ionization energy is a measure of how easy it is to remove an electron from an atom or ion. This value decreases moving down a group and increases moving left to right across a period. Electron affinity is how easily an atom accepts an electron. Using Periodic Law, it becomes apparent the alkaline earth elements have a low electron affinity. In contrast, the halogens readily accept electrons to fill their electron subshells and have high electron affinities. The noble gas elements have practically zero electron affinity because they have full valence electron subshells. Electronegativity is related to electron affinity. It reflects how easily an atom of an element attracts electrons to form a chemical bond. Both electron affinity and electronegativity tend to decrease moving down a group and increase moving across a period. Electropositivity is another trend governed by Periodic Law. Electropositive elements have low electronegativities (e.g., cesium, francium). In addition to these properties, there are other characteristics associated with Periodic Law, which may be considered properties of element groups. For example, all of the elements in group I (alkali metals) are shiny, carry a +1 oxidation state, react with water, and occur in compounds rather than as free elements.