[HSA] Buổi 5: EDUCATION & PERSONAL DEVELOPMENT

General Chemistry

Periodic Trends Study Guide 1. Atomic Radius Atomic radius = the size of an atom (distance from the nucleus to the outermost electron area). * Across a period (left → right): atomic radius decreases. * Down a group (top → bottom): atomic radius increases. * Largest atoms are toward the bottom left of the periodic table. * Smallest atoms are toward the top right. Why does atomic radius decrease across a period? * The number of protons increases. * More protons create a stronger pull on the electrons. * Electrons get pulled closer to the nucleus. Why does atomic radius increase down a group? * More electron shells are added. * Outer electrons are farther from the nucleus. * Shielding increases, making the nucleus’s pull weaker on outer electrons. Remember: * Across = smaller. * Down = bigger. * More electron shells = bigger atom. * More protons across a period = stronger pull. 2. Ionization Energy Ionization energy = the energy needed to remove an electron from an atom. * Across a period (left → right): ionization energy increases. * Down a group (top → bottom): ionization energy decreases. * Highest ionization energy: toward the top right. * Lowest ionization energy: toward the bottom left. Why does ionization energy increase across a period? * Nuclear charge increases because more protons are added. * Electrons are held more tightly. * It takes more energy to remove an electron. Why does ionization energy decrease down a group? * More electron shells are added. * Outer electrons are farther from the nucleus. * Shielding increases. * It becomes easier to remove an electron. Remember: * Harder to remove an electron = high ionization energy. * Easier to remove an electron = low ionization energy. * Noble gases generally have very high ionization energies. 3. Electronegativity Electronegativity = how strongly an atom attracts shared electrons in a chemical bond. * Across a period (left → right): electronegativity increases. * Down a group (top → bottom): electronegativity decreases. * Most electronegative element: Fluorine (F). * Least electronegative elements are toward the bottom left. * Noble gases generally aren’t included in basic electronegativity trends. Why does electronegativity decrease down a group? * More electron shells are added. * Bonding electrons are farther from the nucleus. * Shielding increases. * The nucleus has less attraction for shared electrons. Remember: * Fluorine is the most electronegative element. * Across = increases. * Down = decreases. 4. Metallic Character Metallic character = how easily an atom loses electrons and acts like a metal. * Across a period (left → right): metallic character decreases. * Down a group (top → bottom): metallic character increases. * Most metallic elements are toward the bottom left. * Least metallic elements are toward the top right. Why does metallic character increase down a group? * Atomic radius increases. * Outer electrons are farther from the nucleus. * Electrons are easier to lose. Remember: * Metals like to lose electrons. * Nonmetals tend to gain electrons. * Metals are mostly on the left side of the periodic table. 5. Electron Affinity Electron affinity = the energy change when an atom gains an electron. * Across a period: generally increases (becomes more negative/more favorable). * Down a group: generally decreases (becomes less favorable). * Halogens have high electron affinities. * Chlorine has the highest electron affinity in the periodic table in the usual textbook trend. * Noble gases generally have very low or unfavorable electron affinities. Why do nonmetals tend to gain electrons? * They have high electronegativity. * They generally have high electron affinity. * They are close to having a full outer electron shell. Remember: * Electron affinity = gaining electrons. * Ionization energy = removing electrons. * Don’t mix these two up! 6. Periodic Table Groups and Elements Periods * Horizontal rows (go across). * There are 7 periods. Groups * Vertical columns (go up and down). * There are 18 groups. * Elements in the same group often have similar properties. Important groups: Group Name Important information 1 Alkali metals Very reactive metals 2 Alkaline earth metals Reactive metals 3–12 Transition metals Metals 13–16 Other groups Contain different types of elements 17 Halogens Very reactive nonmetals 18 Noble gases Generally unreactive gases Other important facts: * Metals: left side of periodic table. * Nonmetals: right side of periodic table. * Metalloids: along the staircase line. * Hydrogen is a nonmetal. * Mercury (Hg) is the only metal that is liquid at room temperature. 7. Important Element Families Alkali metals * Group 1. * Very reactive metals. * Examples: Lithium (Li), Sodium (Na), Potassium (K). Alkaline earth metals * Group 2. * Reactive metals. * Examples: Beryllium (Be), Magnesium (Mg), Calcium (Ca). Transition metals * Groups 3–12. * Mostly hard, shiny, malleable, and ductile. * Examples: Iron, Copper, Gold. Halogens * Group 17. * Very reactive nonmetals. * Examples: Fluorine, Chlorine, Bromine, Iodine, Astatine. Noble gases * Group 18. * Generally unreactive gases. * Examples: Helium, Neon, Argon, Krypton, Xenon, Radon. 8. Important Definitions These definitions are especially important for the matching and short-answer questions. Term Simple definition Atomic radius Size of an atom Ionization energy Energy needed to remove an electron Electronegativity Attraction for shared electrons in a bond Electron affinity Energy change when an atom gains an electron Metallic character How easily an atom loses electrons Nuclear charge Positive charge from protons in the nucleus Shielding Inner electrons block some of the nucleus’s pull on outer electrons Valence electrons Electrons in the outermost shell Period Horizontal row Group Vertical column Metals Usually shiny, malleable, and good conductors Nonmetals Usually poor conductors Metalloids Have properties of both metals and nonmetals Malleable Can be hammered into thin sheets Ductile Can be pulled into wires 9. Periodic Table Trends – Memorize This! This is probably the MOST important section of your study guide. Property Across → Down ↓ Atomic radius Decreases Increases Ionization energy Increases Decreases Electronegativity Increases Decreases Metallic character Decreases Increases Electron affinity Generally increases Generally decreases Easy way to memorize: * Atomic radius and metallic character increase toward the bottom left. * Ionization energy and electronegativity increase toward the top right. * Fluorine is the most electronegative element. * Neon has a very high ionization energy. * Cesium is among the elements with the lowest ionization energies. 10. Other Facts From Your Test * The modern periodic table is arranged by increasing atomic number. * Atomic number = number of protons. * Atomic number increases by one as you move from one element to the next. * The modern periodic table has 118 elements. * Group 3–12 elements are called transition metals. * The staircase separates metals and nonmetals. * Elements along the staircase are metalloids. * Horizontal rows are called periods. * Vertical columns are called groups. * Oxygen combines with other elements to form compounds called oxides. * The chemical symbol for sodium is Na. * The chemical symbol for sulfur is S. Quick Memorization Section These are facts worth memorizing before your test: Most important elements * Most electronegative: Fluorine (F) * Highest ionization energy: Helium (He) * Highest electron affinity (usual textbook comparison): Chlorine (Cl) * Lowest ionization energy: Cesium (Cs), among the lowest * Liquid metal at room temperature: Mercury (Hg) Important numbers * 7 periods * 18 groups * 118 elements * Groups 3–12 = transition metals * Group 17 = halogens * Group 18 = noble gases Four definitions to never mix up * Atomic radius = size * Ionization energy = removing electrons * Electron affinity = gaining electrons * Electronegativity = attracting shared electrons

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