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Periodic Trends and Zeff Role of core electron shielding in atomic properties Atomic Radius •Atomic radius is defined as half the distance between neighboring atoms •Covalent Radius is half the distance between two bonded atoms •van der Waals radius is half the distance between neighboring atoms in a gas •We won’t use it in today’s discussion. Atomic Radius As the Zeff increases, the electrons feels more positive ‘pull’ towards the nucleus, so the atomic radius decreases across a period. Atomic Radius As n increases down a group, the atomic radius increases, but then decreases across a group Ionic Radius •The ionic radius of an element is the element’s share of the distance between neighboring ions in an ionic solid. •Generally: •Cations are smaller than their parent atoms •Anions are larger than their parent atoms Ionic Radius Ionic Radius Ionization Energy •Ionization energy is the amount of energy necessary to remove and electron from an element in the gas phase •An element with a low ionization energy will readily form a cation and usually make good conductors. Ionization Energy •Ionization energies increase across a period •Zeff is increasing and therefore the electrons are being held more tightly •The decrease going down a group •Electrons are getting further away from the nucleus First and Second Ionization Energies •The second ionization energy is generally significantly higher than the first ionization energy •Exception: Atoms with valence electrons in the ns shell. •Look at the Group 1 and Group 2 elements. The second ionization energy for the Group 2 elements is nearly equal to the first. Why? Ionization Energies and Metallic Character •Low ionization energies account for metallic character of elements in the s, d and f blocks. •They readily lose electrons and can therefore exist as a metalic solid Electron Affinity •A measure of how much an atom ‘wants’ an electron •A High electron affinity means that energy is released when an element gains an electron •A Low or negative electron affinity implies that energy must be supplied to ‘push’ the electron onto the atom Electron Affinity Electron repulsion effect by adding an electron to an already occupied orbital