Periodic Trends in the Periodic Table
Introduction to Periodic Trends
The periodic table is designed to highlight fundamental patterns in the properties of elements. These periodic trends
describe how atomic characteristics change as you move across a period (left to right) or down a group (top to
bottom). Recognizing these trends allows scientists to predict an element’s behavior, reactivity, and physical
properties.
Atomic Radius
The atomic radius represents the size of an atom, measured from the nucleus to the outermost electron shell. This
trend is influenced by nuclear charge and electron shielding:
• Down a group: Atomic radius increases because additional electron shells make the atom larger. Despite the
increasing nuclear charge, inner electrons shield outer electrons from its full effect, reducing the nucleus’s pull.
• Across a period: Atomic radius decreases from left to right. As the number of protons rises, the nucleus pulls
electrons closer, making the atom smaller. Since these electrons occupy the same energy level, there is no
significant increase in shielding.
For instance, lithium is larger than neon in Period 2 because neon’s higher nuclear charge pulls its electrons inward
more tightly.
Ionic Radius
The ionic radius refers to the size of an ion, which varies depending on whether the ion is a cation (positively
charged) or an anion (negatively charged):
• Cations (positive ions): Smaller than their neutral atoms since losing electrons reduces repulsion, allowing the
nucleus to pull the remaining electrons closer.
• Anions (negative ions): Larger than their neutral atoms due to increased electron repulsion, which expands the
electron cloud.
For example, a sodium ion (Na⁺) is smaller than neutral sodium (Na), whereas a chloride ion (Cl⁻) is larger than a
neutral chlorine atom.
Ionization Energy
Ionization energy is the energy required to remove an electron from an atom. This trend is shaped by atomic
size, nuclear charge, and electron shielding:
• Down a group: Ionization energy decreases because outer electrons are farther from the nucleus and more
shielded, making them easier to remove.
• Across a period: Ionization energy increases from left to right as atomic size decreases and nuclear charge
strengthens, holding electrons more tightly.
Removing multiple electrons (successive ionization energies) requires progressively more energy, with a
significant jump once all valence electrons are removed—since the next electron must come from a stable
inner shell.
For example, the first electron in sodium (Na) is much easier to remove than the second, which requires
breaking into a full inner shell.
Introduction to Periodic Trends
The periodic table is designed to highlight fundamental patterns in the properties of elements. These periodic trends
describe how atomic characteristics change as you move across a period (left to right) or down a group (top to
bottom). Recognizing these trends allows scientists to predict an element’s behavior, reactivity, and physical
properties.
Atomic Radius
The atomic radius represents the size of an atom, measured from the nucleus to the outermost electron shell. This
trend is influenced by nuclear charge and electron shielding:
• Down a group: Atomic radius increases because additional electron shells make the atom larger. Despite the
increasing nuclear charge, inner electrons shield outer electrons from its full effect, reducing the nucleus’s pull.
• Across a period: Atomic radius decreases from left to right. As the number of protons rises, the nucleus pulls
electrons closer, making the atom smaller. Since these electrons occupy the same energy level, there is no
significant increase in shielding.
For instance, lithium is larger than neon in Period 2 because neon’s higher nuclear charge pulls its electrons inward
more tightly.
Ionic Radius
The ionic radius refers to the size of an ion, which varies depending on whether the ion is a cation (positively
charged) or an anion (negatively charged):
• Cations (positive ions): Smaller than their neutral atoms since losing electrons reduces repulsion, allowing the
nucleus to pull the remaining electrons closer.
• Anions (negative ions): Larger than their neutral atoms due to increased electron repulsion, which expands the
electron cloud.
For example, a sodium ion (Na⁺) is smaller than neutral sodium (Na), whereas a chloride ion (Cl⁻) is larger than a
neutral chlorine atom.
Ionization Energy
Ionization energy is the energy required to remove an electron from an atom. This trend is shaped by atomic
size, nuclear charge, and electron shielding:
• Down a group: Ionization energy decreases because outer electrons are farther from the nucleus and more
shielded, making them easier to remove.
• Across a period: Ionization energy increases from left to right as atomic size decreases and nuclear charge
strengthens, holding electrons more tightly.
Removing multiple electrons (successive ionization energies) requires progressively more energy, with a
significant jump once all valence electrons are removed—since the next electron must come from a stable
inner shell.
For example, the first electron in sodium (Na) is much easier to remove than the second, which requires
breaking into a full inner shell.
