Covalent radii (in Å) for
some elements of different groups
and periods are listed below.
Plot these values against atomic
number. From the plot, explain
the variation along a period and a
group
2nd
group
elements : Be
Be (0.89),
Mg (1.36), Ca (1.74), Sr (1.91)
Ba(1.98)
17th
group elements : F (0.72), CI
(0.99), Br (1.14), I (1.33)
3rd Period elements : Na(1.57),
Mg(1.36), Al (1.25), Si(1.17),
P(1.10), S(1.04), Cl(0.99)
4th period elements: K(2.03),
Ca(1.74), Sc(1.44), Ti(1.32)
V(1.22), Cr(1.17), Mn(1.17),
Fe(1.17), Co(1.16), Ni(1.15),
Cu(1.17), Zn(1.25), Ga(1.25),
Ge(1.22), As(1.21), Se(1.14),
Br(1.14)
Answers
Answer:
Atomic radius
Atomic radius of an atom is defined as the distance between the centre of its nucleus and the outermost shell containing the valence electron.
It is not possible to measure the radius of an isolated atom directly. Except for noble gases, usually atomic radius is referred to as covalent radius or metallic radius depending upon the nature of bonding between the concerned atoms.
Covalent radius
It is one-half of the internuclear distance between two identical atoms linked together by a single covalent bond. Inter nuclear distance can be determined using x-ray diffraction studies.
Example:
The experimental internuclear distance in Cl2 molecule is 1.98 Å. The covalent radius of chlorine is calculated as below.
The formation of covalent bond involves the overlapping of atomic orbitals and it reduces the expected internuclear distance. Therefore covalent radius is always shorter than the actual atomic radius.
The covalent radius of individual atom can also be calculated using the internuclear distance (dA-B) between two different atoms A and B. The simplest method proposed by Schomaker and Stevenson is as follows.
dA-B = rA + rB - 0.09 (χA-χB)
where χA and χB are the electronegativities of A and B respectively in Pauling units
Here χA χB and radius is in Å.
Let us calculate the covalent radius of hydrogen using the experimental d value is 1.28 Å and the covalent radius of chlorine is 0.99 Å. In pauling scale the electronegativity of chlorine and hydrogen are 3 and 2.1 respectively.
dH-Cl = rH + rCl - 0.09 ( χCl - χH)
1.28 = rH + 0.09 - 0.09 (3 - 2.1)
1.28 = rH + 0.09 - 0.09 (0.9)
1.28 = rH + 0.09 - 0.081
1.28 = rH + 0.909
∴ rH = 1.28 - 0.909 = 0.317 Å
Metallic radius
It is defined as one-half of the distance between two adjacent metal atoms in the closely packed metallic crystal lattice.
For example, the distance between the adjacent copper atoms in solid copper is 2.56 Å and therefore the metallic radius of copper isThe metallic radius can be calculated using the unit cell length of the metallic crystal. You will study the detailed calculation procedure in XII standard solid state unit.
Periodic Trends in Atomic Radius
Variation in Periods
Atomic radius tends to decrease in a period. As we move from left to right along a period, the valence electrons are added to the same shell. The simultaneous addition of protons to the nucleus, increases the nuclear charge, as well as the electrostatic attractive force between the valence electrons and the nucleus. Therefore atomic radius decreases along a period.
Effective nuclear charge
In addition to the electrostatic forces of attraction between the nucleus and the electrons, there exists repulsive forces among the electrons. The repulsive force between the inner shell electrons and the valence electrons leads to a decrease in the electrostatic attractive forces acting on the valence electrons by the nucleus. Thus, the inner shell electrons act as a shield between the nucleus and the valence electrons. This effect is called shielding effect.