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WHAT IS A PERIODIC TREND? A change that is predictable and repeats ex: seasons, months, days of the week, school day Atomic size: Because atoms have no true boundary, atomic radius is used to measure size Periodic Trends ATOMIC RADIUS: one-half of the distance from the nucleus of two like atoms TREND ACROSS a period (left to right): DECREASES EXPLANATION: electrons enter the SAME energy level, as protons and electrons are added the force between the protons and electrons increase. The electrons are pulled closer to the nucleus making the atom smaller TREND DOWN a family: INCREASES EXPLANATION: as electrons are added to more energy levels, the inner layers shield the outer electrons from the pull of the nucleus. The added electrons have less attraction to the nucleus due to this shielding effect, increasing the atom‛s size Ionic Radius: • Positive ions are smaller than their neutral atom • Negative ions are larger than their neutral atom Oct 109:43 AM Atomic Radius Ionization Energy Sep 247:50 AM Electronegativity decrease IONIZATION ENERGY: the energy needed to remove the outermost electron from an atom TREND ACROSS a period: INCREASES increase Pull Atomic Radius EXPLANATION: the atom‛s radius decreases, the electrons are held tighter by the nucleus, so it will take more energy to remove an electron TREND DOWN a family: DECREASES EXPLANATION: the atom‛s radius increases, the electrons are held less tightly by the nucleus, so it will take less energy to remove an electron Oct 109:32 AM Sep 249:24 AM 1 ELECTRONEGATIVITY: the ability of an atom to attract an electron from another atom Electron affinity: the energy released when adding an electron Trends Summarized TREND ACROSS a period(left to right): INCREASES Atomic Radius Ionization Energy Electronegativity EXPLANATION: the atom‛s radius decreases, the nucleus has a stronger pull on an extra electron decrease increase increase increase decrease decrease TREND DOWN a family: DECREASES EXPLANATION: the atom‛s radius increases, the nucleus has a weaker hold on an extra electron Sep 249:31 AM Oct 109:21 AM 2