An atom is assumed to be spherical in shape and thus, the size of atom is generally in term of radius of the sphere and is called atomic radius. It is usually defined as the distance between the centre of the nucleus and outermost shell where electron or electrons are present. The exact measure of atomic radius is not easy due to following reasons :
(a) The atom does not have well defined boundary. The propability of finding the electron is never zero even at large distance from the nucleus.
(b) It is not distances from to get an isolated atom. The electron density around an atom is affected by the presence of neighbouring atoms, i.e., the size of the atom changes in going from one set of environment to another.
(c ) The size of an atom is very small, of the order of abouit `1.2 Å`, i.e., `1.2xx10^(-10)m`.
An estimate of the size of the atom can, however, be made by knowing the distance between the atoms in the combined state. The distance between the atoms, i.e., bond lengths are generally measured by the application of techniques such as X-ray diffraction, electron diffraction, infrared spectroscpy, nuclear magnetic resonance spectroscopy, etc. However, bond length change with different type of bonding. Three types of radius are commonly used, i.e.,
(a) Covalent radius (b) Crystal radius
(c ) van der Waal's radius
Which of the following set of ions have the same value of screening constant for the valence electron, calculated from Slater's rule?

Options

A. `Li^(+),Na^(+),K^(+)`
B. `Na^(+),Mg^(2+),AI^(3+)`
C. `F^(-),CI^(-),Br^(-)`
D. `F^(-),O^(2-),S^(2-)`

Answer

B