We know that as we scan down a group, the principal quantum number, n, increases by one for each element. General trends noted are increasing circle size moving from top to bottom in a group, with a general tendency toward increasing atomic radii toward the lower left corner of the periodic table. No spheres are provided for the noble or inert gas, group 18 elements. Beneath the molecule is the label, “I radius equals 266 p m divided by 2 equals 133 p m.” In figure b, a periodic table layout is used to compare relative sizes of atoms using green spheres. The distance between the radii is 266 p m. Beneath the molecule is the label, “B r radius equals 228 p m divided by 2 equals 114 pm.” The fourth diatomic molecule is in purple. The distance between the radii is 228 p m. Beneath the molecule is the label, “C l radius equals 198 p m divided by 2 equals 99 pm.” The third diatomic molecule is in red. The distance between the radii is 198 p m. The second diatomic molecule is in a darker shade of green. Beneath the molecule is the label, “F radius equals 128 p m divided by 2 equals 64 p m.” The next three models are similarly used to show the atomic radii of additional atoms. The distance between the centers of the two atoms is indicated above the diagram with a double headed arrow labeled, “128 p m.” The endpoints of this arrow connect to line segments that extend to the atomic radii below. Two spheres are pushed very tightly together. The first model, in light green, is used to find the F atom radius. The number of valence electrons of an element can be determined by the periodic table group (vertical column) in which the element is categorized. In figure a, 4 diatomic molecules are shown to illustrate the method of determining the atomic radius of an atom. In the second period elements, the two electrons in the 1s 1 s sublevel are called inner-shell electrons and are not involved directly in the elements reactivity, or in the formation of compounds. The general trend is that radii increase down a group and decrease across a period. Valence electrons are the electrons in the highest occupied principal energy level of an atom. (b) Covalent radii of the elements are shown to scale. The atomic radius for the halogens increases down the group as n increases. \): (a) The radius of an atom is defined as one-half the distance between the nuclei in a molecule consisting of two identical atoms joined by a covalent bond.
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