4.0 Size and type of bonding in atoms.

  Periodic Table

Atomic Radius

• The distance of the outermost orbit from the center of the nucleus of an atom is called atomic radius.
• Single Bond Covalent Radius, SBCR –
• For homo-atomic molecules$$\begin{equation} \begin{aligned} {d_{A - A}} = {r_A} + {r_A} = 2{r_A} \\ {r_A} = \frac{{{d_{A - A}}}}{2} \\\end{aligned} \end{equation} $$
• For hetero-diatomic molecules in which electronegativity remains approximately same.$${d_{A - B}} = {r_A} + {r_B}$$
• For hetero-nuclear diatomic molecule A-B, while difference between their electronegativity values of atom A and atom B is relatively large,$${d_{A - B}} = {r_A} + {r_B} - 0.09({X_A} - {X_B})$$
• where $X_A$ and $X_B$ are electronegativity values of high electronegative element A and less electronegative element B respectively.
• This formula is given by Stevenson & Schomaker.
• Note: Covalent radius is slightly smaller than actual radius.

• Vanderwaals Radius
• Half of the distance between the nuclei of two non-bonded atoms belonging to two different molecules closest to one another is called vanderwaals radius.
• The values of atomic radii in noble gases are always determined as vanderwaal radii. Therefore, the values of vanderwaals radius of a noble gas is always greater than that of the halogen coming before it in the same period.
• Note: Vanderwaals radius is slightly larger than the actual radius.

• Periodicity in Atomic Radius
• The atomic radius depends upon the following factors.

a. Effective Nuclear Charge: The effect of increase in the number of protons increases the effective nuclear charge. These results in decrease in the value of atomic radius because protons attract the electronic orbits with greater force.

b. Number of Orbits: The effect of increase in the number of orbit in an atom increases the atomic size.

c. Shielding effect / screen effect: The electrons of inner shell repel the electrons of valence shell from coming closer to the nucleus. Due to this the atomic radius increases. This is called shielding. When an atom has more number of shells, the shielding will also be greater.

Within a shell the shielding provided by different types of orbitals follows the order $s>p>d>f$. (As $‘s’$ orbital experience greater penetration towards the nucleus, they provide greater shielding to the electrons of $p, d$ and $f$ orbitals). In the elements of $‘d’$ and $‘f$’ blocks where number of shells remain the same in a particular series, so along with shielding effect, the nuclear charge is also compared.

• In a period: The number of orbit remain same on going from left to right in a period while there is a unit increase in the atomic number. Thus, the electron experiences more force of attraction towards nucleus. Hence atomic radius decreases from left to right in a period.
• In a group: The atomic radii increases on going downward in a group because the number of orbits also increase on going from top to bottom in a group.
• In a period, the size of an alkali metal (Group $1 A$) is second largest because it has minimum number of proton, while the size of halogen is smallest.
• In the periodic table, $Cs$ is the biggest atom because $Fr$ is a radioactive element, while $H$ is the smallest atom.

• Ionic Radius

• When a neutral atom loses one or more electrons it forms a cation having one or more number of positive charge. Similarly when a neutral atom acquires one or more electrons it forms an anion having one or more number of negative charge.
• Ionic radius is the distance between the nucleus and the limit of the electron cloud scattered around the nucleus in an ion.
• Cationic Radius

a. An atom forms a cation on loss of one or more electrons. The cationic radius can be defined as the distance between the nucleus and the limit of the electron cloud scattered around the nucleus.

b. The size of a cation is smaller in comparision to the size of its corresponding atom. Usually a cation has one shell less than the neutral atom hence it has smaller size than the atom. This is because of the fact that an atom on loosing electron froms cation, which has lesser number of electron than the number of proton. This results in increase of effective nuclear charge.
Examples :$ Mn >M{n^{ + 2}}>M{n^{ + 3}}>M{n^{ + 4}}>M{n^{ + 6}}>M{n^{ + 7}}>P{b^{ + 2}} >P{b^{ + 4}}$

Anionic Radius
a. When a neutral atom gains one or more electrons, it becomes negatively charged ion known as anion. The distance between the nucleus of an anion and the limit of the electron cloud scattered around the nucleus is called its anionic radius.

b.The size of an anion is greater than the size of its corresponding atom, because the number of electrons present in the anion gets larger than the number of protons due to gain of electron. This results in decrease of effective nuclear charge.
Examples : ${O^{ - 2}}>{O^{ - 1}}> O > M{n^{ + 6}} >M{n^{ + 7}}$

• Size of Electronic Series
• The species, which have same number of electrons nut different nuclear charges, constitute an isoelectronic series. In the isoelectronic species, with the increase in effective nuclear charge, the size of ion goes on decreasing.