Atomic Radius
By: Kelly Sun, David Zeng, George Xu
Definition of Atomic Radius
Atomic Radius is a measure of the size of an atom
Total distance from atoms nucleus to the outermost electron orbital
Usually found be finding half the distance between two nuclei.
No standard definition
There are four types of radii: covalent atomic radii, ionic radii, metallic radii, and van
der Waals radii
Measured in picometers or angstroms
Covalent Atomic Radii
For atoms that are bonded by covalent bonds and share electrons, the radius of the
atom is less than if the atoms were just touching each other or tangent to each other.
Covalent atomic radii are measured for nonmetallic atoms
Ionic Radius
The radius of an atom forming an ionic bond or ion. Element’s share of the distance between neighboring ions
in an ionic solid.
Generally:
Cations are smaller than their parent ions
Anions are larger than their parent ions
Metallic Radii
Metallic Radii are the radii of atoms joined by a metallic bond (similar to covalent
bonds)
van der Waals Radii
van der Waals radii apply for the most part to only noble gases
The two atoms are just touching because the attractive forces bonding them are much
less, leaving the two atoms “unsquashed”
Description of Trend
The atomic radius decreases across a period: moving from left to right along the periodic table
The atomic radius increases down a group
Atomic Radius is not a fixed property as electrons do not move in a fixed orbit within their orbitals
Reasons for Trends
The atomic radius increases down a group:
Why: As the number of energy levels increase moving down a group, so does the number of electrons present
Each subsequent energy level is further from the nucleus than the last. Therefore, the atomic radius
increases as the group and energy levels increase. The shielding effect also increases the distance electrons
are to the nucleus as inner electrons have a repulsive charge on the outer electrons.
The atomic radius decreases across a period:
Why: Moving across a period, electrons are added to the same energy level. At the same time, protons are
being added to the nucleus. The concentration of more protons in the nucleus creates a "higher effective
nuclear charge” meaning there is a stronger force pulling the electrons closer to the nucleus resulting in a
small atomic radius.
How it’s measured (Determination)
Large degree of uncertainty since they do not have definite orbits
No fixed radius
Covalent Radii are found by measuring bond lengths between pairs of covalently bonded atoms and dividing
the length by two
Metallic Radii are found in a way similar to covalent radii but between pairs of atoms bonded by metallic
bonds
Van-der-Waals radii are determined from the contact distances between unbonded atoms in touching
molecules or atoms.
Anomalies
Some Noble Gases, although the farthest atoms to the right in the group 8A column, are larger than atoms in
the group 7A column. This is because atomic radius can only be measured by taking the distance between
bonded atoms and Noble Gases do not form covalent and ionic bonds due to their full electron shell. As a
result, atomic radius for these can be only estimated using Van Der Waals forces which is not as strong.
Example: Neon, Radon
In the Lanthanide Series, the atomic radius decreases at a greater rate from left to right than expected.
Normally, as nucleus becomes more charged across a period, the decrease in atomic radius is partially
counteracted by a shielding effect. However, in the lanthanide series, the f shell electrons is not effective at
shielding the outer shells. As a result, atomic radius decreases more quickly across the series.
Anomalies 2
Transition Metals do not exhibit the general trend as the rest of the periodic table. Moving across the periods
on the periodic table for transition metals shows an increase in atomic radius rather than a decrease.
This due to to the ability of transitional metals to accept unpaired electrons from other atoms to fill vacancies
in their d orbitals, which would therefore increase the atomic radius.
Example Problems
Order elements from smallest to largest:
Ne, F, Ba, Na, Mg, H, Al