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Periodic Trends Section 6.3 Periodic Trends The size of an atom is determined by x-ray diffraction X-rays bend around atoms in molecules and create a pattern Computers generate an image from x-rays X-ray diffraction of NaCl 2 Trends to explain all other trends Electron Shielding- the reduction of the attractive force of the nucleus for the outer electrons caused by electrons in energy levels between the nucleus and the outer electrons Nuclear charge- the number of protons in the nucleus. More protons = increased nuclear charge and increased attraction between the nucleus and electrons. Atomic Radius Atomic Radius- ½ of the distance between the nuclei of two atoms of the same element in a diatomic molecule Atomic Radius Periodic Trends Trend for atomic size Down a group, size increases Occurs because # of E levels increases & Electrons shielding reduces amount of attraction between nucleus and outer electrons Across a period, size decreases # of protons increases (nuclear charge increases), pulling electrons closer Electron shielding doesn’t change because electrons are added to the same energy level Ionization Energy Ion- atom that gains or loses electrons Ionization Energy- energy required to remove an electron. Easiest to remove 2 electrons from 2A Because there are 2 valence Easiest to remove 3 electrons from 3A E must be added to overcome the attraction of the positive charge of the nucleus X(g) X+(g) + e- 1st ionization X+(g) X2+(g) + e- 2nd ionization Because there are 3 valence Outer shell electrons are easier to remove than other electrons! Ionization Energy Periodic Trends Ionization energy Down a group- decreases because electrons are held more loosely due to increased electron shielding Across a period- increases because electrons are held more tightly due to increased nuclear charge (increased # of protons in the nucleus) Periodic Trends Metals form positive ions- Cations More likely to lose electrons (lower ionization energy) Nonmetals form negative ions- Anions More likely to gain electrons (higher ionization energy) Periodic Trends Ionic Radii Trends Cations- smaller than neutral atom because fewer electrons result in greater attraction by nuclei Anions- larger than neutral atom because more electrons result in less attraction by nuclei Within period- size decreases Down a group – size increases Forming cations Forming anions Atom versus Ion Periodic Trends Electronegativity- tendency for the atoms of the element to attract electrons when the atoms are part of a compound F is most electronegative Noble gases- no electronegativity valuesdon’t form compounds Periodic Trends Electronegativity TrendsDown a group – decreases- electron shielding results in less attraction for electrons by the nucleus Across a period- increases- higher atomic number and consistent electron shielding result in more attraction for electrons Electronegativity allows you to predict bond type: covalent- polar vs. nonpolar and ionic Electron Affinity Electron affinity of an element is the energy given off when a neutral atom in the gas phase gains an extra electron to form a negatively charged ion Example: F(g) + e- F-(g) Ho (ENERGY) = -328.0 kJ/mol Think of it like electronegativity without the need to bond… It still has to do with attraction for electrons. Trends in Electron Affinity Down a group, it decreases because electron shielding blocks some of the attraction from the nucleus Across a period, it increases because nuclear charge increases, attracting electrons more strongly. Electron Affinity http://chemed.chem.purdue.edu/genchem/ topicreview/bp/ch7/ie_ea.html#ea Periodic Trends Knowledge of trends in electron shielding and nuclear charge explain all other trends http://www.teachersdomain.org/resource/ls ps07.sci.phys.matter.graphperiodic/ 6.3 Summary of Trends Increases Increases Decreases Ionization Ionicof size energy Size Size Electronegativity Atomic Nuclear Shielding of anions cations Size Charge DecreasesConstant 6.3 Section Quiz 1. Which of the following sequences is correct for atomic size? Mg > Al > S Li > Na > K F > N > B F > Cl > Br 6.3 Section Quiz 2. gain Metals tend to electrons to form cations. gain electrons to form anions. lose electrons to form anions. lose electrons to form cations. 6.3 Section Quiz 3. Which of the following is the most electronegative? Cl Se Na I