Atomic Radius

During the research, scientists found the smallest particle of matter and named it as an atom. The different atoms of different elements show different chemical and physical properties. This can be seen when the atomic radius change in periodic table trend. The change in the atomic radius has a great impact on the behavior of atoms during the chemical reaction. It is because it influences the ionization energy, chemical reactivity, electronegativity and many other factors.

Atomic Radii Definition

The general picture of an atom in our mind is that of a sphere. If it is regarded as correct, then it is defined as:

The distance between the center of its nucleus and electrons in the last orbit.

However, there is no certainty about the exact position of electrons at any time. Theoretically, an electron, at one time, may be very close to the nucleus while at other time it may be far away from the nucleus. Also, It is not possible to measure the exact value of the atomic radius of an atom of the element because an atom is very much smaller in size.Atomic Radius

However, we can express the different forms of atom depending upon the nature of bonding of atoms.

What is Atomic Radius

In spite of these limitations, we need to have some operational definition of the term atomic radius. There are three operational concepts of atomic radius.

(a) If the bonding is covalent, the radius is called a covalent radius.

(b) If the bonding is ionic, the radius is called ionic radius.

(c) If the two atoms are not bonded by a chemical bond (as in noble gases) the radius is called van der Waal’s radius.

Why not possible to exact determination of atomic radius is not as Simple?
A. It is not possible to isolate a single atom in order to determine its radius.
B. It is not possible to measure the exact distance of the atom does not have well-defined shape or boundary and the probability of electron is level zero even at a large distance from the nucleus.
C.The atomic radius is likely to change due to environmental effect. and many more reasons

Atomic Radius Periodic Table

It is to be noted that the atomic radius of the last element in each period which is a noble gas element is quite large.It is because noble gases are considered van der Waal’s radius which always has the higher value than a covalent radius.When we compare the three atomic radii the order of forces is

  • Van der Waal radius>Metallic Radius>Covalent radius

Atomic Radius

Covalent Radius

In homoatomic molecules (containing the same type of atoms) covalent radius is defined as
The one-half distance between the centers of two bonded atoms joins each other by a single covalent bond.
  •  If the two atoms linked to each other by a double bond or triple Bond, then the half of the Inter nuclear distance does not represent the covalent radius.

Van der waal radius 

Van der Waal radius is the half the distance between two similar atoms belongs to the neighboring molecule of the same substance in the solid state.

Actually, the van der Waal forces are weak forces their magnitude(power)  of attraction is smaller in gaseous as well as in the liquid state of the substance. Therefore radius is determined in the solid state when the magnitude of the force is expected to a maximum.

Atomic Radius

  • The value of van der Waal radius is more than that of the covalent radius.
  • Example, the van der Waal force of chlorine is 180 pm while covalent radius is 99 pm(picometer).

Metallic radius 

Half the distance between the center of nuclei of two adjacent atoms in a metallic crystal.

Since metallic bond is weaker than covalent bond the internuclear molecular distance between the two atoms in metallic bond is more than covalent bond

  • The metallic bond is more than the covalent bond.

 How to find Atomic Radius

  • The distance can be measured by the X-ray diffraction and spectroscopic method.Thus

 

Atomic Radius

  • Stevenson’s formulaIn Heteroatomic atoms, covalent radius shifted towards the higher electronegative element.Then, the covalent radius is calculated by the relation which is called Stevenson’s formula as follows:

dAB = rA + rB – C|xA – xB|

Where rA and rB = radius of an atom ‘A’ and a radius of an atom ‘B’

 dAB =  predicted bond length(distance between two atoms)

|xA – xB| = difference in the electronegativities of elements A and B.

C is a constant.

Atomic Radius Trend

In a period, the number of shells remains the same but the nuclear charge increases. This resulting increase in the force of attraction towards the nucleus, which causes contraction of size.

  • Nuclear attraction  1 / Atomic Radius.
  • Principal quantum number(n) Atomic radius.
  • Screening effect atomic radius.
  • The number of bond 1 / Atomic Radius.

In a group, as we move from top to bottom in a group the atomic radius increases with the increase of atomic number, this is due to the fact that the number of energy shells increases.

Atomic Radius Chart

Period

Group

IA

IIA

IIIA

IVA

VA

VIA

VIIA

Zero

1.

H

0.37

He

0.93

2.

Li

1.34

Be

0.90

B

0.82

C

0.77

N

0.73

O

0.74

F

0.72

Ne

1.31

3.

Na

1.54

Mg

1.30

Al

1.18

Si

1.11

P

1.06

S

1.02

Cl

0.99

Ar

1.74

 Largest Atomic Radius

  • The size of Hydrogen is the smallest.
  • Francium having atomic number 87 has a larger covalent and Vander Waals radius than Cesium.
  • Since Francium is an extremely unstable element. So, Caesium has the largest atomic number.

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