B.sc 1st year Book
PERIODIC PROPERTIES OF THE ELEMENTS
Size of Atoms and Ions:
The size of an atom decreases successively from left to right in the periodic table. This decrease in size is due to the extra positive charge added to the nucleus whereas an extra electron resides in the same orbit. The increase in the nuclear charge pulls more strongly newly added extra electrons closer to the nucleus. This trend is found in the case of the first, second, and third periods of the periodic table. But as soon as we proceed to the fourth period, there is the inclusion of ten transitional elements and in that case contraction in the atomic size becomes significant. As a result, the atomic size decreases initially with the increase of atomic number as we move from left to right in the first transition series i.θ. from Sc(Z=21) to Cr(Z=24). However, a reverse effect called the screening effect’ operates due to 3 d-electrons on the outermost shell coming into existence (This effect reduces the force of attraction between the nucleus and electrons of the outermost shell and causes an increase in the atomic size). Since both these effects are operating oppositely, there is no further regular change in atomic and ionic size. Consequently, atomic and ionic sizes remain almost constant. In the case of lanthanides, this contraction in the atomic size is even more marked.
From top to bottom in a group of the periodic table, the atomic size of atoms increases because of the fact that extra electrons are added in a new orbit. This addition of electrons in a new orbit diminishes the effect of an increase in nuclear charge.
All the metal atoms and a few of the nonmetals of low electronegativity value-form positive ions. These positive ions are smaller than their corresponding neutral atoms. Similarly, the nonmetals form a negative ion by the addition of an extra electron to the neutral atom. Due to this, the nuclear charge of an atom is reduced by one unit as compared to the number of electrons in the shell. It causes the electrostatic force of attraction acting between the nucleus and outermost shell decreases and the repulsive force between the electrons of the outermost shell increases. Consequently, the outermost shell spread outwards and as result, the negative ions have a bigger size than their corresponding neutral atom.
The size of an atom or an ion is defined by the atomic or ionic radius, such as an atomic radius is defined as the distance between the nucleus and the outermost shell of electrons of an atom: But the position of the outermost electrons in the last shell of an atom is not certain and also the isolated atoms are unstable so it is impossible to isolate an individual atom or an ion. Hence, it is impossible to measure the atomic or ionic radius These quantities are however derived by some indirect methods from the measured distance between the nuclei of two atoms bonded to each other by a normal covalent bond in a gaseous molecule or between the nuclei of two ions in crystals. The internuclear distance between two Ions in crystal or ionic solid is called interionic distance: