Chapter14: Periodic Trends Modern Chemistry; Holt, Rinehart, & Winston Dmitri Mendeleev (1834-1907) Russian chemist Arranged his periodic table according to atomic mass so that elements with similar properties were in the same group Predicted the properties of elements that had not yet been discovered using his periodic table Henry Moseley (1887-1915) English chemist Proved Mendeleev’s arrangement of the periodic table to be correct – only, the periodic table was arranged according to atomic number, not atomic mass The Periodic Law States that when elements are arranged in order of increasing atomic number, their physical and chemical properties show a periodic pattern Atomic Radii Atomic radius – the distance between the center of the nucleus and the outer most energy level Atomic Radius Distance between nuclei Atomic Radius Decreases across a row Why? Protons are added to the nucleus moving across a period from left to right This increases the charge of the nucleus (effective nuclear charge – Zeff) As a result, it compacts the atom Group Trends Increase down a group Why? The addition of shells increases the electrons’ distance from the nucleus and the size of the atom This makes the atom larger n=3 n=2 n=1 Ionization Energy The energy required to remove one electron from a neutral atom of an element creating an ion Period Trends Increase across a period Why? Zeff increases across the period There is stronger attraction between the positive and negative particles more difficult to remove them. Group Trends Decrease down the group Why? Electrons are far away from positive nucleus, making it easier to remove Electronegativity A measure of the ability of an atom in a chemical compound to attract a bonding pair of electrons A MEASURE OF GREEDINESS! Trends Increase across a period Decrease down a group EXCEPTIONS: NOBLE GAS (GROUP 18). They do not have an electronegativity value because they do not form chemical compounds.