3.4(a) for the elements of the second period. The atomic size generally decreases across a period as illustrated in Fig. We can explain these trends in terms of nuclear charge and energy level. The atomic radii of a few elements are listed in Table 3.6. Atomic radii can be measured by X -ray or other spectroscopic methods. For simplicity, in this book, we use the term Atomic Radius to refer to both covalent or metallic radius depending on whether the element is a non-metal or a metal. For example, the distance between two adjacent copper atoms in solid copper is 256 pm hence the metallic radius of copper is assigned a value of 128 pm. For metals, we define the term “Metallic Radius” which is taken as half the internuclear distance separating the metal cores in the metallic crystal. For example, the bond distance in the chlorine molecule (Cl 2) is 198 pm and half this distance (99 pm), is taken as the atomic radius of chlorine. One practical approach to estimate the size of an atom of a non-metallic element is to measure the distance between two atoms when they are bound together by a single bond in a covalent molecule and from this value, the “Covalent Radius” of the element can be calculated. However, an estimate of the atomic size can be made by knowing the distance between the atoms in the combined state. Practical way by which the size of an individual atom can be measured. Secondly, since the electron cloud surrounding the atom does not have a sharp boundary, the determination of the atomic size cannot be precise. Do you know why? Firstly, because the size of an atom (~ 1.2 Å i.e., 1.2 × 10 –10 m in radius) is very small. You can very well imagine that finding the size of an atom is a lot more complicated than measuring the radius of a ball. which show periodic variations.However, we shall discuss the periodic trends with respect to atomic and ionic radii,ionization enthalpy, electron gain enthalpy and electronegativity. ![]() There are numerous physical properties of elements such as melting and boiling points,heats of fusion and vaporization, energy of atomization, etc. In this section we shall discuss the periodic trends in certain physical and chemical properties and try to explain them in terms of number of electrons and energy levels. ![]() But why do the properties of elements follow these trends? And how can we explain periodicity? To answer these questions, we must look into the theories of atomic structure and properties of the atom. Likewise within a group of representative metals (say alkali metals) reactivity increases on moving down the group, whereas within a group of non-metals (say halogens), reactivity decreases down the group. For example, within a period, chemical reactivity tends to be high in Group 1 metals, lower in elements towards the middle of the table, and increases to a maximum in the Group 17 non-metals. There are many observable patterns in the physical and chemical properties of elements as we descend in a group or move across a period in the Periodic Table. ![]() Introduction to Trends in Physical Properties Introduction to Trends in Physical Properties.
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