Such as for the ions with the same charge the ionic radius increases down the column. Ionic radii also follow the same vertical trend as the atomic radii. While the size of negative ions or anions is greater than the parent atoms as they have gained the electrons. The size of a positive ion or cation is smaller than the original atom as they have lost their electron or electrons. The size of the ions is not the same as their parent atom. If the effective nuclear charge is greater the outer most electrons are more strongly attracted by the nucleus and atomic radii become smaller. Irregularities in the atomic radii of both group I and group II elements can be explained by the variations in the effective nuclear charge which is experienced by the electrons in the outer most orbitals of the elements. However, it is a dramatical fact that radius is dependent on the nuclear charge. It may seem reasonable to say that it is due to the successive addition of the electrons. The increase in atomic radii is due to the electron shielding but the situation is quite complex as the principal quantum number n is not constant. Atomic radius increases from top to bottom of the elements and an additional electron shell or energy level are being added to each successive element. By moving down in the group, the number of occupied energy levels are increased from 2 to 6 and radium of an atom of the element is increased from 134 pm to 225 pm.
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