Chy102-Lecture 06 - 12/12/2007 D ne o General Chemistry...

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1 General Chemistry Chem102 for Engineering Dr. Raafat 6 th lecture 12/12/2007 Done
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2 Objectives What is meant by the periodic trends in the properties of atoms. Trends in atomic and ionic radii. Ionization energy. Electron affinity Atomic properties and chemical bonds What is a chemical bond? Why do atoms tend to form chemical bonds? Types of chemical bonding Ionic bond Covalent bond Electronegativity and polar covalent bond
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3 One of the most fundamental principles of chemistry is the periodic law : the chemical and physical properties of elements are based essentially on the electronic configurations (atomic numbers ) of their atoms We will discuss how, atomic radius , ionization energy , and electron affinity vary horizontally and vertically in the periodic table . These properties are periodic , they generally increase or decrease in a gradual manner throughout the periodic table . In other words, these properties exhibit certain changes, trends , within a group or a period. Periodic trends in the properties of atoms
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4 I- Trends in atomic radius We can’t speak strictly about the size of an atom as the electron cloud surrounding the nucleus does not have a sharp boundary . However, a quantity, called the atomic radius can be defined and measured assuming a spherical atoms . Metallic radius is one-half the distance between nuclei of adjacent atoms in a crystal of the element . Covalent radius one-half the distance between nuclei of identical covalently bonded atoms . Examples , For aluminum atoms in metallic Aluminum, the atomic or metallic radius is found to be 143 pm, while in case of chlorine (nonmetal), the arrangement of atoms gives a covalent radius of 100 pm.
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5 Trends among the main group elements Atomic radius decreases Atomic radius increases
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6 The atomic radii of the main group elements 1. Decreases across a period from left to right in the periodic table. 2. Increases down a group in the periodic table.
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7 Explanation The variations in atomic size are the result of two opposing influences : 1. Changes in n : As the principle quantum number, n, increases (extra shells are added), the outer electrons spend more time farther from the nucleus, so the atoms are larger. 2. Changes in Z eff : As the effective nuclear charge (Z ), the positive charge actually felt by an electron, increases, outer electrons are pulled closer to the nucleus, so the atoms are smaller.
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8 1. Down a group, n dominates . As we move down a main group, an extra level of inner electrons is added. These inner electrons shield the outer electrons very effectively and as a result atomic radius generally increases in a group from up to down. For example, the alkali metals group . All these elements have a single s electron outside a filled level or filled p sublevel.
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This note was uploaded on 12/13/2010 for the course CHEM 102 taught by Professor Raafat during the Spring '10 term at Ain Shams University.

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Chy102-Lecture 06 - 12/12/2007 D ne o General Chemistry...

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