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Trends in Atomic Properties

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Trends in Atomic Properties Trends in Atomic Properties When the elements are arranged in order of increasing atomic number, certain chemical and physical properties repeat periodically. The periodicity of a physical property for a series of elements can be shown by plotting the experimental value of the property against increasing atomic number. Some of the important physical properties include: Electronegativity: the degree to which an atom in a molecule attracts electrons in a chemical bond Atomic radius: the radius of an atom of the element Electron affinity: the energy released or absorbed when a neutral gaseous atom accepts an electron Ionization energy: the energy required to remove an electron from a gaseous atom Melting point: the temperature at which the solid and liquid phases of a substance coexist Boiling point: the temperature at which the vapor pressure of the liquid equals one atmosphere Molar volume: the volume of a unit cell times Avogadro,s Number divided by the number of atoms in the unit cell Atomic Radius Assumption: atoms which are closest to each other in an elementary substance are actually touching. The atomic radius of solid elements is taken to be one half of the distance between the nuclei of adjacent atoms. For gaseous elements which form molecules, the atomic radius is taken to be one half of the distance between centers of atoms in the molecule. In general, atomic radius decreases as one moves across the periodic table from left to right and increases as one moves down within a given group. The increase down the group can be explained in terms of electronic configuration. For example, elements in 1A have a single s electron outside a filled level or filled p sublevel. The electrons in the inner shells are much closer to the nucleus and effectively shield the outer s electrons from the positive charge of the nucleus. 1 The decrease in atomic radius along the period can be explained in similar manner. All the electrons are nearly at the same distance from the nucleus. They are poor shields for each other. The effective nuclear charge increases steadily as we move across the period. As the effective nuclear charge increases, outer electrons are pulled in more tightly and atomic radius decreases. First Ionization Energy: The first ionization energy of an element is the energy change, E, for the removal of the outermost electron from a gaseous atom to form a +1 ion: M(g) + E M+(g) + e- Energy must be absorbed to remove an electron. Therefore, ionization energy is always a positive quantity. The magnitude of E measures the ease with which an atom gives up an electron. The smaller the E, the more readily the electron is removed. Ionization energy increases as one moves across the Periodic Table and decreases as one moves down in any group. There is inverse relationship between ionization energy and atomic radius. The smaller the atom, the more tightly its electrons are held to the nucleus and the more difficult they are to remove. Large atoms such as those of the alkali metals have relatively low I.E. because the outer electrons are far away from the positively charged nucleus. Electronegativity The tendency of an atom to attract electrons Cs (0.7) F (4.0) Electronegativity increases moving across the periodic Table and decreases down the table. Electron Affinity The energy change that occurs when an electron is added to a gaseous atom or ion is called the electron affinity. M(g) + e- M-(g) + E For most neutral atoms and for all cations, energy is evolved when an electron is added; E is therefore negative in sign. The electron affinities of anions and some neutral atoms are positive since work must be done to force them to accept electron. In general, the electron affinities become more negative as we move across a period. Electron affinities do not change greatly going down in any group. 2 Metallic Character Metals have high tendency to lose electrons readily. This means that chemically, they tend to form positive ions Na+, Mg2+, Al3+ .., when they react with nonmetals. Metallic character can be associated with low ionization energy. Therefore, metallic character should decrease as we move across the Periodic Table and increases down the group. Properties of Metals 1. High electrical conductivity 2. High thermal conductivity 3. Luster - ability to reflect light 4. Ductibility - can be drawn into wire 5. Malleability - can be hammered into sheets 3 .
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