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 radii 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.

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.

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

In spite of these limitations, There are three operational concepts:

(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?
A. It is not possible to isolate a single atom.
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.It is likely to change due to environmental effect. and many more reasons

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 >Metallic Radius>Covalent

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 Internuclear distance does not represent the covalent radius.

 It 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.

• The value of van der Waal 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).

 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

• 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 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.

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 Radii.
• Principal quantum number(n) Atomic radii.
• The number of bond 1 / Atomic Radii.

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.

 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