Beneath the molecule is the label, “B r radius equals 228 p m divided by 2 equals 114 pm. We can see from the electron configuration of a carbon atom1 s2 2 s2 2 p2 that it has 4 valence electrons (2 s2 2 p2) and 2 core electrons (1 s2 ). The distance between the radii is 228 p m. Valence shell electrons (or, more simply, the valence electrons) are the electrons in the highest-numbered shell, or valence shell, while core electrons are the electrons in lower-numbered shells. The periodic table shows the atomic mass and atomic number of each element. Take a closer look at the some of the elements in the periodic table’s far right column in Figure 3. 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 periodic table is arranged in columns and rows based on the number of electrons and where these electrons are located. 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. As we go across a period from left to right, we add a proton to the nucleus and an electron to the valence shell with each successive element. 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. For example, the elements in the first column (sometimes labeled IA), all have one valence electron. Table 2.6.2 below summarizes the number of valence electrons for each main group column of elements. The first model, in light green, is used to find the F atom radius. The number of valence electrons for each main group element can be determined by the column, or group, it occupies on the periodic table. In figure a, 4 diatomic molecules are shown to illustrate the method of determining the atomic radius of an atom. The general trend is that radii increase down a group and decrease across a period. (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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