Trends in Atomic and Ionic Radius
Introduction to Periodic Properties
Periodic properties are characteristics of elements that exhibit a repeating pattern across the periodic table.
This guide focuses on trends in atomic and ionic radius.
Atomic Radius
Definition of Atomic Radius
Atomic radius is a measure of the size of an atom. Since atoms do not have a definite edge (electrons exist
in regions of probability), atomic radius is practically defined as half the distance between the nuclei of two
similar atoms bonded together in a diatomic molecule. This distance between nuclei is known as the bond
length.
Trend Across a Period (Left to Right)
Trend: Atomic radius decreases from left to right across a period.
Explanation:
Atoms in the same period have the same number of electron shells (energy levels).
As you move from left to right, the number of protons in the nucleus increases.
This increase in positive nuclear charge leads to a stronger electrostatic attraction between the
nucleus and the outermost electrons.
This stronger attraction pulls the electron shells closer to the nucleus, resulting in a smaller
atomic radius.
Example: Comparing sodium (Na) and argon (Ar) in the third period:
Sodium has 11 protons and 3 electron shells.
Argon has 18 protons and 3 electron shells.
The 18 protons in Argon exert a stronger pull on the same 3 electron shells compared to the 11
protons in Sodium, making Argon atoms smaller than Sodium atoms.
Trend Down a Group (Top to Bottom)
1/6
, Trend: Atomic radius increases from top to bottom down a group.
Explanation:
As you move down a group, new electron shells (energy levels) are added.
These additional shells are located further from the nucleus.
The inner electron shells exert a "shielding effect" on the outermost electrons, reducing the
attractive force from the nucleus.
The combination of increased distance from the nucleus and the shielding effect leads to a
larger atomic radius.
Example: Comparing lithium (Li) and potassium (K):
Lithium is in the second period and has 2 electron shells.
Potassium is in the fourth period and has 4 electron shells.
Potassium's outermost electron is in the fourth shell, further from the nucleus than lithium's
outermost electron in the second shell. The inner shells in potassium also provide more shielding.
Therefore, potassium atoms are larger than lithium atoms.
Example Problem: Smallest Atomic Radius
Question: Which of the elements Arsenic (As), Selenium (Se), Bromine (Br), and Krypton (Kr) is likely to have
the smallest atomic radius?
Solution:
These elements are all in the same period (Period 4). As we move from left to right across a period, the
atomic radius decreases. Krypton (Kr) is the furthest to the right among these elements. Therefore, Krypton
will have the smallest atomic radius. Rubidium (Rb) is in the next period below Potassium (K), meaning it has
an additional electron shell, making its atoms larger than those of As, Se, Br, and Kr.
Ionic Radius
Definition of Ionic Radius
Ionic radius refers to the size of an ion. It is the radius of an atom after it has gained or lost electrons to
form an ion.
Cations (Positive Ions)
Formation: Cations are formed when atoms lose electrons, typically metals.
2/6
Introduction to Periodic Properties
Periodic properties are characteristics of elements that exhibit a repeating pattern across the periodic table.
This guide focuses on trends in atomic and ionic radius.
Atomic Radius
Definition of Atomic Radius
Atomic radius is a measure of the size of an atom. Since atoms do not have a definite edge (electrons exist
in regions of probability), atomic radius is practically defined as half the distance between the nuclei of two
similar atoms bonded together in a diatomic molecule. This distance between nuclei is known as the bond
length.
Trend Across a Period (Left to Right)
Trend: Atomic radius decreases from left to right across a period.
Explanation:
Atoms in the same period have the same number of electron shells (energy levels).
As you move from left to right, the number of protons in the nucleus increases.
This increase in positive nuclear charge leads to a stronger electrostatic attraction between the
nucleus and the outermost electrons.
This stronger attraction pulls the electron shells closer to the nucleus, resulting in a smaller
atomic radius.
Example: Comparing sodium (Na) and argon (Ar) in the third period:
Sodium has 11 protons and 3 electron shells.
Argon has 18 protons and 3 electron shells.
The 18 protons in Argon exert a stronger pull on the same 3 electron shells compared to the 11
protons in Sodium, making Argon atoms smaller than Sodium atoms.
Trend Down a Group (Top to Bottom)
1/6
, Trend: Atomic radius increases from top to bottom down a group.
Explanation:
As you move down a group, new electron shells (energy levels) are added.
These additional shells are located further from the nucleus.
The inner electron shells exert a "shielding effect" on the outermost electrons, reducing the
attractive force from the nucleus.
The combination of increased distance from the nucleus and the shielding effect leads to a
larger atomic radius.
Example: Comparing lithium (Li) and potassium (K):
Lithium is in the second period and has 2 electron shells.
Potassium is in the fourth period and has 4 electron shells.
Potassium's outermost electron is in the fourth shell, further from the nucleus than lithium's
outermost electron in the second shell. The inner shells in potassium also provide more shielding.
Therefore, potassium atoms are larger than lithium atoms.
Example Problem: Smallest Atomic Radius
Question: Which of the elements Arsenic (As), Selenium (Se), Bromine (Br), and Krypton (Kr) is likely to have
the smallest atomic radius?
Solution:
These elements are all in the same period (Period 4). As we move from left to right across a period, the
atomic radius decreases. Krypton (Kr) is the furthest to the right among these elements. Therefore, Krypton
will have the smallest atomic radius. Rubidium (Rb) is in the next period below Potassium (K), meaning it has
an additional electron shell, making its atoms larger than those of As, Se, Br, and Kr.
Ionic Radius
Definition of Ionic Radius
Ionic radius refers to the size of an ion. It is the radius of an atom after it has gained or lost electrons to
form an ion.
Cations (Positive Ions)
Formation: Cations are formed when atoms lose electrons, typically metals.
2/6
