Atomic radius

Atomic radius presentation

1. By- Sanju Sah St. Xavier’s, college, Maitighar, Kathmandu Department of Microbiology Atomic Radius

2. Atomic radius : Atomic radius is the distance from the centre of the nucleus to the outermost shell containing electrons. In other words, it is the distance from the centre of the nucleus to the point up to which the density of the electron cloud is maximum. The units for atomic radii are picometers, equal to 10^-12 meters. As an example, the internuclear distance between the two hydrogen atoms in an H2molecule is measured to be 74pm. Therefore, the atomic radius of a hydrogen atom is 37 pm.

3. Types of Atomic Radii Atomic radii are divided into three types: Covalent radius Van der Waals radius Metallic radius

4. 1) Covalent Radius Covalent radius is one half the distance between the nuclei of two covalently bonded atoms of the same element in a molecule. Therefore, r  covalent  =  ½ (internuclear distance between two bonded atoms). The internuclear distance between two bonded atoms is called the bond length .  Therefore, r  covalent  =  ½( bond length)

5. 2) Van der Waals Radius It is one half the distance between the nuclei of two identical non-bonded isolated atoms or two adjacent identical atoms belonging to two neighbouring molecules of an element in the solid state. The magnitude of the Van der Waals radius is dependent on the packing of the atoms when the element is in the solid state. For example, the internuclear distance between two adjacent chlorine atoms of the two neighbouring molecules in the solid state is 360 pm. Therefore, the Van der Waals radius of the chlorine atom is 180 pm.

6. 3) Metallic Radius A metal lattice or crystal consists of positive kernels or metal ions arranged in a definite pattern in a sea of mobile valence electrons. Each kernel is simultaneously attracted by a number of mobile electrons and each mobile electron is attracted by a number of metal ions. Force of attraction between the mobile electrons and the positive kernels is called the metallic bond. It is one half the internuclear distance between the two adjacent metal ions in the metallic lattice. In a metallic lattice, the valence electrons are mobile, therefore, they are only weakly attracted by the metal ions or kernels.

7. In a covalent bond, a pair of electrons is strongly attracted by the nuclei of two atoms. Thus, a metallic radius is always longer than its covalent radius. For example, the metallic radius of sodium is 186 pm whereas its covalent radius as determined by its vapour which exists as Na 2  is 154 pm. The metallic radius of Potassium is 231 pm while its covalent radius is 203 pm.

8. Variation of Atomic Radii in the Periodic Table Variation Within a Period The  Covalent  and  Van der Waals  radii decrease with increase in atomic number as we move from left to right in a period. The alkali metals at the extreme left of the periodic table have the largest size in a period. The halogens at the extreme right of the periodic table have the smallest size. The atomic size of nitrogen is the smallest. After nitrogen, atomic size increases for Oxygen and then decreases for fluorine. The size of atoms of inert gases is larger than those of the preceding halogens.

9. As we move from left to right in a period,  nuclear charge  increases by 1 unit in each succeeding element while the  number of shells  remains the same. This enhanced nuclear charge pulls the electrons of all the shells closer to the nucleus. This makes each individual shells smaller and smaller. This result in a decrease of the atomic radius as we move from left to right in a period.

10. The  atomic radius  abruptly increases as we move from halogens to the inert gas. This is because inert gases have completely filled orbitals. Hence, the inter-electronic are maximum. We express the atomic size in terms of Van der Waals radius since they do not form covalent bonds. Van der Waals radius is larger than the covalent radius. Therefore, the  atomic size  of an inert gas in a period is much higher than that of preceding halogen

11. Variation Within a Group The atomic radii of elements increase with an increase in atomic number from top to bottom in a group. As we move down the group, the principal quantum number increases. A new energy shell is added at each succeeding element. The valence electrons lie farther and farther away from the nucleus. As a result, the attraction of the nucleus for the electron decreases. Hence, the atomic radius increases.