1. What is Atomic Radius?
The atomic radius of a chemical element refers to the size of its atoms, typically measured as the average distance from the atomic nucleus to the outermost boundary of the electron cloud. Since this boundary is not a well-defined physical entity, there are multiple non-equivalent definitions of atomic radius. The widely used definitions include van der Waals radius, ionic radius, and covalent radius, each corresponding to different types of chemical bonding.
Depending on the definition, the term can apply to isolated atoms, atoms in tightly bound matter, atoms involved in covalent bonding within molecules, or in excited and ionized states. The value of atomic radius can be obtained through experimental means or calculated using theoretical models. In some definitions, the radius value depends on the state of the atom.
Electrons do not have fixed orbits or well-defined trajectories. Furthermore, their positions must be described as probability distributions that gradually decrease as they move away from the nucleus, without clear boundaries. In addition to being tightly packed in solids and molecules, atomic electron clouds often overlap to some extent, and some electrons can move over a wide region between two or more atoms.
The most common definition sets the radius of isolated neutral atoms in the range of 30 to 300 picometers (pm) or 0.3 to 3 angstroms. However, the atomic radius is around 10,000 times larger than the size of its nucleus (1-10 femtometers) and smaller than 1/1000 of the wavelength of visible light (400-700 nm). For metals, the atomic radius is half the distance between the centers of the nearest metal atoms in a metallic crystal lattice. For covalently bonded atoms, it is half the distance between the nuclei of two atoms of the same element forming a single covalent bond.
For example, a hydrogen molecule consists of two hydrogen atoms bonded together by a single covalent bond, with a distance of 74 pm between the two hydrogen atomic nuclei, giving a covalent radius of 37 pm.
2. Variation of Atomic Radius within the Same Period
a. In the Same Period
In the same period, as we move from left to right with increasing nuclear charge, the atomic radius of s and p-block elements tends to decrease continuously.
This is because in the same period, the number of electron shells for the atoms remains the same, and the shielding effect of inner shells is constant. As we move from left to right, the effective nuclear charge increases. As a result, the outermost electrons experience a stronger attraction from the nucleus, leading to a decrease in atomic radius.
The variation of atomic radius for d-block elements is slow and uneven. The f-block elements exhibit an even slower change in atomic radius.
b. In the Same Group
The variation of atomic radius within a group is as follows:
a. For group A elements, as we move down the group, the atomic radius tends to increase. This is due to the increase in the number of electron shells, which increases the shielding effect provided by the inner electrons.
b. For group B elements, when moving from the first element of the group (period 4) to the second element (period 5), the atomic radius increases. However, when going from the second element to the third element (period 6), the atomic radius either remains unchanged or slightly decreases. This is because the contraction of the f-block elements compensates for the expected increase in atomic radius when transitioning from period 5 to period 6. As a result, the 10 d-block elements have nearly the same covalent radius, similar electron configurations, and many similar properties.
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