Atomic radius is the distance from the nucleus of an atom to its outermost shell or electron. It has different values for different atoms depending on their sizes. However, atomic radius trends vary with periods and groups in the periodic table. It decreases across the periods from left to right while increasing down the groups from top to bottom.
These are the following widely used definitions of atomic radius:
Van der Waal’s radius
It is a type of atomic radius defined as the minimum distance between the nuclei of two atoms that are not bounded by covalent or metallic bonds. In metals, van der Waal’s radius is mostly defined for metals. This is because metals have van der Waal’s interactions that are generated by the quantum fluctuations of atomic polarisation.
This is the radius between the two covalent bonded atoms and the length of the covalent bond must be equal to the sum of their covalent radii.
Ionic radius is the distance or space between the two adjacent oppositely charged ions and the length of the ionic bond should be equivalent to the sum of their ionic radii.
Metallic radius is the distance between the atoms that are bonded by metallic atoms.
It is the radius of the lowest energy electron orbit and can be applied to the atoms or ions that have single electrons like hydrogen, ionized helium, etc.
Explanation of Atomic radius Trends
Periodic table has seven periods and eighteen groups. The columns are represented by groups and the rows are periods. Elements are arranged on the basis of their size and nuclear charge.
In the periods, when moving from the left side to the right side, the atomic number increases. Protons have positive charges while electrons have negative charges. More number of protons means a more nuclear charge that attracts electrons toward the nucleus of the atom. However, the incoming electrons enter the same outermost shell. This results in the contraction of the atomic size.
Atomic radius decreases across the periods due to the increasing number of protons. However, there is a greater attraction between the oppositely charged particles protons and electrons. This results, draw the electrons toward the protons due to greater attraction and decreases the atomic radius.
In the groups, the outermost shell of an atom is completely filled that’s why the incoming electrons enter in the new shell that gradually increases the atomic radius. An increased nuclear charge can be balanced by the increasing number of electrons and shells which results in an increase in the size of atoms moving from top to bottom in the group known as the shielding effect. However, lanthanide contraction is an exception.
Atomic radius increases down the groups due to more energy levels. There is also greater space between the electrons and protons which increases the atomic radius.
Trend of atomic radius and Ionization energy
Ionization energy is the minimum amount of energy that is required to remove an electron from the gaseous atom. The trend of atomic radius and ionization energy is opposite to each other. Ionization energy increases from moving left to right and decreases from going top to bottom in the periodic table.
Trend of atomic radius and Electronegativity
Trends of atomic radius and electronegativity are different from each other. Electronegativity is the ability of an atom to attract shared pair of electrons toward itself during the formation of a chemical bond. It increases when moving in the periods from the left side to the right side and decreases moving from top to bottom in the group.
The atomic radius of the lanthanide series is smaller than expected. Lutetium is smaller than yttrium, hafnium has the same size as zirconium, niobium, and tantalum have similar sizes. Lanthanide contraction can be noticed up to platinum after that it is masked by the inert pair effect.
These are five observations due to lanthanide contraction:
- Size of lanthanide ions decreases with increasing atomic number.
- There is a decrease in the ionic radii.
- Their action as reducing agents decreases as increasing the atomic number.
- D-block second and third rows of elements in the transition series have similar properties.
- These types of elements occur in the mineral that is not easy to separate.
What are the atomic radius trends and what causes them?
Left to right in the periodic table, the atomic radius decreases due to more nuclear charge while down a group, the atomic radius increases due to the shielding effect.
How big are atoms?
The size of atoms depends on the atomic number and the position of atoms in the periodic table. For example, the atoms (elements) that are placed in the first period have smaller sizes while atoms in the last period have larger sizes.
why does atomic radius increase down a group?
Atomic radius increase down a group due to increasing the number of shells.
why does atomic radius decrease across a period?
Atomic radius decreases across a period due to an increase in the nuclear charge.
What are the exceptions in atomic radii trends of elements in the periodic table?
Lanthanide contractions are the exceptions in the atomic radius trend.