1.4 periodic trends

1.4 periodic trends - Introductory University Chemistry I...

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Introductory University Chemistry I Unit 1 Periodic Table and Atomic Properties
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Introduction • The periodic table is arranged according to the atomic number of the elements • The basis of the periodic table is the electron configuration of elements • The physical and chemical properties of the elements show regular periodic trends
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Core & valence electrons - - ore alence - - + + + + Core electrons Valence electrons Do not take part in bonding Affect the periodic properties Take part in bonding Determine the periodic properties
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Electrostatic attractions between electrons and the nucleus Understanding the nature of electrostatic attractions between electrons and nucleus helps to predict periodic trends Q 1 x Q 2 Energy d
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Effect of nuclear charge (Z) • Increasing nuclear charge is expected to increase the attraction (energy)
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Effect of Shielding (S) Electron in the same shell as the electron of interest shields the nucleus attraction somewhat but not very effective For example: He (g) He + (g) + e (Easier) He + (g) He 2+ (g) + e (Harder)
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Effect of Shielding (S) Stronger attraction (Harder to remove) Electrons in shells with lower quantum numbers than electron of interest shield nucleus better than electrons in the same shell.
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Effect of Distance • The greater the distance an electron from the nucleus, the smaller is the attraction (energy) Closer to the nucleus Harder to remove Further from nucleus Easier to remove
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Moving across a period Moving down a group c What is the periodic trend moving down a group? c What is the periodic trend moving across a period?
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Atomic radius • The radius of an atom is difficult to define due to uncertainty of the atomic oundary. boundary. • The atomic radii are measured by determining the inter-nuclear distance.
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• Covalent radius – ½ distance between two nuclei formed by two identical atoms by a single covalent bond. • Metallic radius - ½ distance between two nuclei formed by two identical atoms by a metallic bond.
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The inter-nuclei distance of Cl – Cl bond is 199 pm Covalent radius of Cl is 199 / 2 100 pm The covalent radius of C - Cl is 177 pm Covalent radius of carbon = 177 pm – 100 pm = 77 pm
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Trends in atomic size
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Trends in atomic size Atomic size increases down a group Atomic size decreases across a period Atomic size of transition elements does not show drastic change
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Trends in atomic size Atomic radius increases going down groups Increase principal quantum number, n, also increases orbital size
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1.4 periodic trends - Introductory University Chemistry I...

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