ATOMIC RADIUS
The distance between the centre of the nucleus and electron cloud of the outermost energy level is called the atomic radius.
It cannot be measured directly.
It can be measured from the inter nuclear distance of bonded atoms using x-ray diffraction techniques.
The atomic radius depends on factors like
1) the number of bonds formed by atom
2) nature of bonding
CRYSTAL RADIUS OR ATOMIC RADIUS:
It is applicable for metal atoms.
Half of the distance between the nuclei of two adjacent atoms in metallic crystal is called crystal radius.
It is measured in angstrom units
(1 Å = 10–8 cm = 10–10 m) or nanometres. (1 nm = 10–9 mm = 10–7 cm)
COVALENT RADIUS:
This is used for non metals which form covalent bonds.
It is half of the distance between the nuclei of two atoms connected by a covalent bond in a homodiatomic molecule.
Crystal radius is slightly greater than the covalent radius.
VANDERWAAL’S RADIUS:
It is used for molecular substances in the solid state only.
It is half of the distance between the nuclei of two adjacent nonbonded atoms in neighbouring molecules.
Vanderwaal's radius is greater than the covalent radius as the Vanderwaal's forces are weak. Vanderwaal's radius is approximately 40% greater than the covalent radius.
TREND :
The atomic radius increases down the group due to the addition of new shells and increase in screening effect.
The atomic radius decreases from left to right in a period due to the increase in nuclear positive charge.
In a given period alkali metal is the largest atom and halogen is the smallest atom.
Since the inert gas atoms are non bonded their atomic radius should be taken as the Vanderwaal's radius.
Thus in every period inert gas atom is larger than halogen.
In 2ndperiod inert gas atom is the largest and in other periods alkali metal atom is the largest.
The decrease in atomic radius of transition elements is less than expected due to the screening effect of (n – 1)d electrons.
Thus transition metals have similar atomic radii.
In transition families, atomic radius increases normally from 3d to 4d series. But there after it doesn't charge much due to lanthanide contraction.
LANTHANIDE CONTRACTION:
In inner transition elements the differentiating electron enters into ‘f’ orbitals of the antepenultimate shell.
As the atomic number increases in lanthanides due to the dispersed shape of f-orbitals and their poor shielding effect the atomic and ionic radii steadily decrease. This is called lanthanide contraction.
Lanthanide contraction is also observed in 5d transition series.
The atomic radius of 5d transition elements are very close to those of 4d transition elements due to Lanthanide contraction.
As a result 4d and 5d transition elements are more similar in properties when compared to 3d and 4d transition elements e.g. Zr and Hf resemble most closed to each other than other elements.
IONIC RADIUS:
The distance between the nucleus and the outermost e–in an ion. When an atom loses one or more electrons a positive Ion is formed. The cation is smaller in size then the neutral atom.
The ionic radius is smaller than the atomic radius.
As the number of electrons removed from the atom increases, the ionic radius is further decreased.
ATOMIC RADIUS IONIC RADIUS
i) Na | 1.86 Å | Na+ 1.02 Å |
If an atom gains electrons negative ion is formed (anion). The negative ion is bigger in size than the neutral atom.
The size of anion increases with increase in negative charge. With increase in z/e ratio ionic radius decreases.
In cations, the z/e ratio is greater than 1 and in anions, the z/e ratio is less than 1. (charge per electron = z/e)
The atomic radius of chlorine atom (0.99 Å) is much smaller than that of chloride ion (1.81 Å).
ISOELECTRONIC SPECIES:
Ionic species having the same number of electrons is called isoelectronic species.
In isoelectronic series the size decreases with the increases in nuclear positive charge.
Ion | C4 – | N3– | O2– | F– | Na+ | Mg2+ | Al3+ | Si4+ |
Ion | ||||||||
Radius | 2.60 | 1.71 | 1.40 | 1.36 | 0.95 | 0.65 | 0.50 | 0.41 |
(Å) | ||||||||
In isoelectronic series, size decreases with increase in number of protons
i) Smallest atom is 'H' iii) Smallest cation is H + v) Largest cation is Cs+
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