Chapter 3: Introduction to the Periodic Table Section 3.1: Development of the Periodic Table
Objectives: Summarize the steps in the historical development of the periodic table, Predict similarities in properties of the elements by using the periodic table
Organization Scientists noticed that some elements had similar properties and they wanted to organize the elements into a system that would show similarities and acknowledge difference http://www.ptable.com/
J.W. Dobereiner’s Triads (1829) • Classified some elements in groups of three- called triads • Triads had similar properties and varied in an orderly way according to their atomic masses Ex: p. 85- Cl (35.45u), Br (81.18u), I (126.9u) Actual for Br = 79.9u p. 86 – Ca (40.08u), Sr (88.69u), Ba (137.3u) Actual for Sr = 87.62u
Dmitri Mendeleev (1869) • Developed a periodic table of elements according to increasing atomic mass • Elements were in vertical columns starting with the lightest • Elements in horizontal rows displayed similar properties
Dmitri Mendeleev (1869) • He wrote question marks in places where unknown elements would eventually be placed (Mendeleev successfully predicted properties of most of the undiscovered elements) • Improved version later- patterns of changing properties repeated for elements across horizontal rows and elements in vertical columns showed similar properties • The repeated pattern is an example of periodicity. • PERIODICTY: Tendency to recur at regular intervals.
Modern Periodic Table • Each element has its own block with its name and symbol, atomic number and atomic mass • Modern Periodic Table - p. 90-91 • English Chemist Henry Moseley (1913) Ordered by atomic number, not atomic mass
Modern Periodic Table • Atomic number is equal the number of protons in the nucleus • Atomic number increases by one as you move from element to element across a row • Properties of elements change in an orderly progression from left to right.
PERIODIC LAW Physical and chemical properties of the elements repeat in a regular pattern (periodicity) when they are arranged in order of increasing atomic number is known as the