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86 chemistry small. Secondly, since the electron cloud Theatomicradiiofafewelementsarelisted surrounding the atom does not have a sharp in Table 3.6. Two trends are obvious. We can boundary, the determination of the atomic explainthesetrendsintermsofnuclearcharge size cannot be precise. In other words, there and energy level. The atomic size generally is no practical way by which the size of an decreases across a period as illustrated in individual atom can be measured. However, Fig. 3.4(a) for the elements of the second an estimate of the atomic size can be made by period. It is because within the period the knowing the distance between the atoms in outer electrons are in the same valence shell the combined state. One practical approach to and t he effective nuclear charge increases estimate the size of an atom of a non-metallic as the atomic number increases resulting in element is to measure the distance between the increased attraction of electrons to the two atoms when they are bound together nucleus. Within a family or vertical column by a single bond in a covalent molecule and from this value, the “Covalentadius” of the of the periodic table, the atomic radius increases regularly with atomic number as element can be calculated. For example, the illustrated in Fig. 3.4(b). For alkali metals bond distance in the chlorine molecule (Cl2 and halogens, as we descend the groups, is 198 pm and half this distance (99 pm), is the principal quantum number (n) increases taken as the atomic radius of chlorine. For and the valence electrons are farther from metals, we define the term “Metallic Radius” the nucleus. This happens because the inner which is taken as half the internuclear distance separating the metal cores in the energy levels are filled with electrons, which serve to shield the outer electrons from the metallic crystal. For example, the distance pull of the nucleus. Consequently the size of between two adjacent copper atoms in solid copper is 256 pm; hence the metallic radius the atom increases as reflected in the atomic of copper is assigned a value of 128 pm. For radii. Note that the atomic radii of noble gases simplicity, in this book, we use the term Atomic Radius to refer to both covalent or are not considered here. Being monoatomic, metallic radius depending on whether the their (non-bonded radii) values are very element is a non-metal or a metal. Atomic large. In fact radii of noble gases should be radii can be measured by X-ray or other compared not with the covalent radii but with spectroscopic methods. the van der Waals radii of other elements. Table 3.6(a) Atomic Radii/pm Across the Periods Atom (Period II ) Li Be B C N O F Atomic radius 152 111 88 77 74 66 64 Atom (Period III ) Na Mg Al Si P S Cl Atomic radius 186 160 143 117 110 104 99 Table 3.6(b) Atomic Radii/pm Down a Family Atom Atomic Atom Atomic (Group I) Radius (Group 17) Radius Li 152 F 64 Na 186 Cl 99 K 231 Br 114 Rb 244 I 133 Cs 262 At 140 Reprint 2026-27