As a result, the distance between the centre of the nucleus and the outermost shell containing the valence electron increases. As we move down a group, new shells are opened to accommodate the newly added valence electrons. In the periodic table, the atomic radius of elements increases down the group. Z eff = Z – S i.e.= 21- 18 ∴ Z eff = 3 Calculation of effective nuclear charge on 3d electron The electronic configuration of scandium is 1s 2, 2s 2, 2p 6, 3s 2, 3p 6, 4s 2, 3d 1. Summation of the shielding effect of all the electrons gives the shielding constant 'S'Įxample: Let us explain the calculation of effective nuclear charge on 4s electron and 3d electron in scandium. If the electron of interest belongs to d or f orbital, then each of electron left of the group of electron of interest shields to an extent of 1.00 unit of nuclear charge. to an extent of 1.00 unit of nuclear charge. Ii) each electron within the (n-2) group (or) even lesser group (n-3, (n-4) etc.completely shields i.e. I) each electron within the (n-1) group shields to an extent of 0.85 unit of nuclear charge, and If the electron of interest belongs to either s or p orbital, However, it is 0.30 unit for 1s electron. The electron present right to this group does not contribute to the shielding effect.Įach of the electrons within the identified group (denoted by 'n') shields to an extent of 0.35 unit of nuclear charge. Identify the group in which the electron of interest is present. Write the electronic configuration of the atom and rearrange it by grouping ns and np orbitals together and others separately in the following form.
Where Z is the atomic number and 'S' is the screening constant which can be calculated usi ng Slater's rules as described below. It is approximated by the below mentioned equation. The net nuclear charge experienced by valence electrons in the outermost shell is called the effective nuclear charge. Thus, the inner shell electrons act as a shield between the nucleus and the valence electrons.
The repulsive force between the inner shell electrons and the valence electrons leads to a decrease in the electrostatic attractive forces acting on the valence electrons by the nucleus. In addition to the electrostatic forces of attraction between the nucleus and the electrons, there exists repulsive forces among the electrons. Therefore atomic radius decreases along a period. The simultaneous addition of protons to the nucleus, increases the nuclear charge, as well as the electrostatic attractive force between the valence electrons and the nucleus. As we move from left to right along a period, the valence electrons are added to the same shell. You will study the detailed calculation procedure in XII standard solid state unit.Ītomic radius tends to decrease in a period. The metallic radius can be calculated using the unit cell length of the metallic crystal. It is defined as one-half of the distance between two adjacent metal atoms in the closely packed metallic crystal lattice.įor example, the distance between the adjacent copper atoms in solid copper is 2.56 Ã… and therefore the metallic radius of copper is In pauling scale the electronegativity of chlorine and hydrogen are 3 and 2.1 respectively.ĭ H-Cl = r H + r Cl - 0.09 ( χ Cl - χ H) Let us calculate the covalent radius of hydrogen using the experimental d value is 1.28 Ã… and the covalent radius of chlorine is 0.99 Ã…. Where χ A and χ B are the electronegativities of A and B respectively in Pauling units The simplest method proposed by Schomaker and Stevenson is as follows. The covalent radius of individual atom can also be calculated using the internuclear distance (d A-B) between two different atoms A and B. Therefore covalent radius is always shorter than the actual atomic radius. The formation of covalent bond involves the overlapping of atomic orbitals and it reduces the expected internuclear distance. The covalent radius of chlorine is calculated as below. The experimental internuclear distance in Cl 2 molecule is 1.98 Ã…. Inter nuclear distance can be determined using x-ray diffraction studies.
It is one-half of the internuclear distance between two identical atoms linked together by a single covalent bond. Except for noble gases, usually atomic radius is referred to as covalent radius or metallic radius depending upon the nature of bonding between the concerned atoms. It is not possible to measure the radius of an isolated atom directly. Atomic radius of an atom is defined as the distance between the centre of its nucleus and the outermost shell containing the valence electron.