## Friday, 6 December 2019

Atomic Radius - The general picture of an atom in our mind is that of a sphere. If it is regarded as correct, then its definition is given below. https://www.universetoday.com/

The distance between the center of its nucleus and electrons in the last orbit.
-OR -
It is defined as the distance between the nucleus and the outermost orbit.

However, remember according to quantum theory, there is no certainty about the exact position of electrons at any time. Theoretically, an electron, at one time, maybe very close to the nucleus while at other times 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.
So, on certain facts, it is difficult to explain the exact atomic radius definition. Let’s explain it more clearly.

### Why not possible to exact determination of Atomic Radius?

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 a 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.

However, we can express the different forms of atom depending upon the nature of the bonding of atoms. In spite of the above limitations, There are three operational concepts:

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

### Three Types of Atomic Radius

The are three types of atomic radius.

Covalent radius: If bonding present in between atoms in a molecule is covalent, the radius is called covalent radius. It is half of the distance between the nucleus of two like atoms bonded together by a single bond or by a 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.

Metallic radius: It is defined as one half of the inter nuclear distance between the nucleus of two adjacent atoms in the metallic lattice. The metallic radius of an element is greater than the covalent radius.
Since the metallic bond is weaker than the covalent bond the internuclear molecular distance between the two atoms in the metallic bond is more than the covalent bond.
• The metallic bond is more than the covalent bond

Van der Waal’s radius: It is one half of the distance between the nucleus of two adjacent atoms belonging to two neighboring molecules of an element 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 In Periodic Table

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. https://www.ibchem.com

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

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.
• Screening effect ∝ 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.

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

#### Atomic Radius Trend in Period and Groups

##### Period
Along a period, the atomic radius trend (or radii) of the elements generally decrease from left to right. The atomic radii of the elements present in the second period are given.
 Element Li Be B C N O Nuclear Charge +3 +4 +5 +6 +7 +8 Atomic Radius (pm) 152 111 88 77 75 74

From the values, it is clear that the alkali metal (Li) placed on the extreme left has the maximum atomic radius while the halogen (F) on the extreme right has the minimum value.

Explanation

In moving from left to right in a period,
The nuclear charge gradually increases by one unit and at the same time, one electron is also being added to the electron shell. Due to increased nuclear charge from left to the right, the electrons are also getting attracted more and more towards the nucleus. Consequently, the atomic size is expected to decrease as shown in the case of the elements of the second period.
It may be noted that the atomic radius of the last element in each period which is a noble gas element is quite large. For example, in the second period, the atomic radius of neon (Ne) is 160 pm (not shown). Actually, it is van der Waals radius while the rest of the elements have covalent radii. Therefore, no comparison can be made. The van der Waals radii also decrease in a period from left to the right.

##### Group
The atomic radii of the elements in every group of the periodic table increase as we move downwards. Covalent Radii of the alkali metals present in group 1 are given. Since all of them are metallic in nature, their metallic radii have also been given for reference.
 Element Li Na K Rb Cs Fr Nuclear Charge +3 +11 +19 +37 +55 +87 Covalent Radius (pm) 123 157 202 216 235 – Metallic Radius (pm) 125 186 231 244 262 –
From the values, it is quite clear that the atomic radius of lithium (Li) is the minimum while that of caesium (Cs) is the maximum. The value of the last element francium (Fr) is not known since being as radioactive in nature, it is unstable.

Explanation

On moving down a group, there is an increase in the principal quantum number and thus, an increase in the number of electron shells. Therefore, the atomic size is expected to increase. But at the same time, there is an increase in the atomic number or nuclear charge also. As a result. the atomic size must decrease. However, the effect of an increase in the electron shells is more pronounced than the effect of an increase in nuclear charge. Consequently, the atomic size or atomic radius increases down a group. This is well supported by alkali metals of group 1 the values given in Table for the alkali metals.  