• Western University

• CHEMISRTY 101

Introduction to Periodic Trends As you have read, the periodic table was constructed largely without an understanding of the subatomic structure of the atom. Elements were placed in columns due to similar physical and chemical properties. It was quickly observed that certain trends in other properties can be found when moving across a period or down a group. The reason for these regular patterns can be found in the structure of the atom. Key Terminology ∙ Outermost electrons: the electrons in the outer layer of an atom which are responsible for atomic reactivity (often called valence electrons) ∙ Cation: has less electrons than protons and thus is a positively charged ion ∙ Anion: has more electrons than protons and thus is a negatively charged ion ∙ Atomic Radius: an estimation of the distance from the nuclei to the “edge” of the outermost orbital ∙ Covalent Radius: half the distance between like-atoms that are bonded together in a molecule ∙ Ionic Radius: The effective radius of ions in solids such as NaCl ∙ Ionization Energy: The energy required to remove an electron from an atom by overcoming the electrostatic forces between the nucleus and the electron. ∙ Electron Affinity: the energy released (or absorbed) when an electron is incorporated by an atom, forming an anion. ∙ Effective Nuclear Charge (Zeff): the sum of all attractions and repulsions of the nucleus upon a specific electron ∙ Shielding: the effect of the inner orbitals have on decreasing the effect of Zeffon the outermost electrons ∙ Core Charge: (# protons) – (# inner electrons) Questions 1. Which element would you predict to have the largest atomic radii? List reason(s) for your prediction. a. B, Li, or F b. K, Li, or Na c. Cl, Br, or I Graphing Assignment Part 1: Atomic Radii of the Elements i. Using a pen for the axes and labels and pencil for the data points and lines, use the data in the chart provided to graph atomic radius versus atomic number. ii. Choose your scales so that the graph occupies the majority of the graph paper and all of the data can be plotted. Using a ruler, connect the points. iii. Divide the horizontal axis into 36 intervals using as much of the axis as possible. iv. Join consecutive points with straight solid lines. If the data for an element is missing, interpolate using a broken straight line to join the points for adjacent elements. i. Circle the points corresponding to the peaks on the graph. Write the symbol for the elements at each peak.