consider H molecule. The distance between the two hydrogen nuclei of the molecule is . Å. So its covalent radius is .
/ = . Å. When you look at the variation of the atomic radii in the periodic table, there are two distinct trends. Along the period, from left to right, the atomic radius of the elements decreases whereas along the groups, from the top to bottom, the atomic radius increases.
The increase, down a group, is due to the increase in the valence shell number down the group. As the shell number increases, the distance between the valence shell and the nucleus increases. In contrast, when you observe along the period, the shell number remains the same but the number of protons (i.e. atomic number) increases.
More and more positive charges impose a strong attraction over the electrons and thus the electron cloud shrinks towards the nucleus, which results in the decrease in the atomic size. Figure . shows how the atomic radius decreases from lithium to boron. Figure .
Variation of atomic radius . . Ionic Radii It is defined as the distance from the centre of the nucleus of the ion upto the point where it exerts its influence on the electron cloud of Figure . Atomic radius H H Atomic radius of Hydrogen the ion.
You know that ions are formed when an atom lose or gain electrons. When a neutral atom loses an electron, it becomes a positively charged ion called cation, whereas the gain of an electron by a neutral atom forms a negatively charged ion called anion. The size of the ions is important to determine their behaviours in solutions and the structure of ionic solids. The size of a cation is always smaller than its corresponding neutral atom.
But, the anion is larger than its neutral atom. Note: As the positive charge increases the size of the cation decreases As the negative charge increases the size of the anion increases For instance, lithium and sodium lose the single electron from their outermost energy level to form cations. The ions so formed are