PERIODIC TRENDS Atomic size Ionization energy Atomic Size • For the main group elements (s & p blocks), atomic radii increase going down a group and decrease from left to right across a period. 3 Atomic Radius 2.5 K Rb Na 2 Li 1.5 1 Ar Kr Ne 0.5 He 0 0 20 40 Atomic Number 60 Putting Trend to Use • Using the periodic table, arrange the following in order of increasing atomic radius: Br, Te, Se Ionization energy, IE, is the amount of energy required to remove an electron from an isolated, gas phase atom. Li(g) Li1+(g) + e1- • Energy is always required for this process. • IE generally decreases going down a group and generally increases from left to right across a period. Electron Affinity • Energy released when a lone, gaseous atom gains an electron. F + e- F- Electron Affinity, EA, is the amount of energy released when a free, gas phase atom gains an electron. A(g) + e1- A1-(g) EA increases (i.e., more energy is released) as we move to the top and right of the periodic table. Ionic Size •All positive ions are smaller than the atoms from which they were formed •All negative ions are larger than the atoms from which they were formed.