The organization of the periodic table reveals predictable trends in six important properties of the elements: atomic radius, ionic radius, electron affinity, electronegativity, ionization energy, and metallic/nonmetallic character.
Periodic Table Trend Summary
| Property | Across a Period (→) | Down a Group (↓) |
|---|---|---|
| Atomic Radius | Decreases | Increases |
| Ionic Radius | Generally decreases | Increases |
| Ionization Energy | Increases | Decreases |
| Electronegativity | Increases | Decreases |
| Electron Affinity | Generally increases | Generally decreases |
| Metallic Character | Decreases | Increases |
| Nonmetallic Character | Increases | Decreases |
Atomic Radius Trend
Atomic radius is half the distance between two identical atoms touching each other.
- Atomic radius increases as you move right to left
- Atomic radius increases as you move down
As you move across the periodic table from left to right, each element contains one more proton and one more electron. Because the additional electrons enter the same principal energy level, shielding changes very little while nuclear charge increases. The stronger attraction between the nucleus and electrons pulls the electron cloud closer to the nucleus, decreasing atomic radius across a period. As you move down a group, additional electron shells increase the size of the atom, so atomic radius increases.
Electron Affinity Trend
Electron Affinity is the ability of an atom to accept an electron. It is measured by the energy change in the atom as an electron is added to the gaseous form of the atom.
- Electron affinity increases as you move left to right
- Electron affinity decreases as you move down (with exceptions)
Atoms with stronger nuclear charge tend to have higher electron affinities as you move across the table. As you move down, the outer electron is both shielded from the nucleus by filled shells and physically further away. Both of these reduce the force of attraction between the nucleus and the added electron.
Electronegativity Trend
Electronegativity is the measure of attraction between the atom’s nucleus and electrons in a chemical bond. In general, the higher the electronegativity, the stronger the force of attraction between bonding electrons and the atom nucleus.
- Electronegativity increases as you move left to right across the table
- Electronegativity decreases as you move down the table
A related term is electropositive. Think of it as the opposite of electronegativity. Atoms with low electronegativity are highly electropositive.
Ionization Energy Trend
Ionization Energy is the energy required to remove an electron from a gaseous atom. The tighter the nucleus holds an electron, the more energy needed to remove it.
- Ionization energy increases as you move left to right across the table
- Ionization energy decreases as you move down the table
Ionic Radius Trend
Ionic radius follows different rules than atomic radius.
- Cations are smaller than their parent atoms.
- Anions are larger than their parent atoms.
Examples:
- Na → Na⁺ becomes smaller.
- Cl → Cl⁻ becomes larger.
When an atom loses electrons, electron-electron repulsion decreases and the remaining electrons are pulled closer to the nucleus. When an atom gains electrons, increased electron repulsion expands the electron cloud.
Metallic and Nonmetallic Character Trend
Metallic and nonmetallic characteristics are a group of physical properties. As you move up and across from the bottom left of the table, the elements begin to take on the characteristics of nonmetals. Moving the opposite way, the elements take on properties of metals. The term for this is metallic character. For a more detailed examination of the difference, check out metals, metalloids and nonmetals.
Easy Way to Remember Periodic Trends
Most periodic trends point toward fluorine.
As you move toward the upper-right corner of the periodic table:
- Electronegativity increases.
- Ionization energy increases.
- Electron affinity generally increases.
- Nonmetallic character increases.
Atomic radius follows the opposite trend and increases toward the lower-left corner.
A simple memory aid is:
“Fluorine pulls, francium pushes.”
Fluorine strongly attracts electrons, while francium most strongly exhibits metallic behavior.
Exceptions to Periodic Trends
Periodic trends describe general patterns, but several important exceptions occur.
Electron Affinity Exceptions
- Noble gases have very low electron affinities because their valence shells are already full.
- Nitrogen has a lower electron affinity than expected because its half-filled p subshell is relatively stable.
Ionization Energy Exceptions
- Boron has a lower first ionization energy than beryllium.
- Oxygen has a lower first ionization energy than nitrogen.
These exceptions occur because electron subshell arrangements sometimes outweigh the general effects of increasing nuclear charge.
Why Do Periodic Trends Exist?
Periodic trends arise from the interplay of three factors:
- Nuclear charge: The number of protons in the nucleus. More protons generally increase attraction between the nucleus and electrons.
- Shielding effect: Inner-shell electrons partially block the attraction between the nucleus and outer electrons.
- Distance from the nucleus: Electrons farther from the nucleus experience weaker attraction.
These factors combine to determine how strongly an atom attracts, loses, or gains electrons. Nearly all periodic trends can be explained by changes in effective nuclear charge and electron shielding.
Effective Nuclear Charge
Effective nuclear charge (Zeff) is the net positive charge experienced by an electron after accounting for shielding by other electrons.
As you move from left to right across a period:
- The number of protons increases.
- Shielding changes very little.
- Effective nuclear charge increases.
As effective nuclear charge increases:
- Atomic radius decreases.
- Ionization energy increases.
- Electronegativity increases.
- Electron affinity generally becomes more negative.
Effective nuclear charge is the underlying reason for most periodic trends.
FAQs
Which element has the highest electronegativity?
Fluorine has the highest electronegativity of any element.
Which element has the largest atomic radius?
Francium is generally considered to have the largest atomic radius, although relativistic effects make measurements difficult.
Why do noble gases often lack electronegativity values?
Electronegativity measures attraction for bonding electrons. Because noble gases rarely form bonds, traditional electronegativity scales often omit them.
What is the difference between electron affinity and electronegativity?
Electron affinity measures the energy change when an atom gains an electron, while electronegativity describes how strongly an atom attracts electrons in a chemical bond.
References and Further Reading
- Allred, A. Louis (2014). Electronegativity. McGraw-Hill Education. ISBN 9780071422895.
- Huggins, Maurice L. (1922). “Atomic Radii. I”. Physical Review. 19 (4): 346–353. doi:10.1103/PhysRev.19.346
- Mendeleev, D. I. (1958). Kedrov, K. M. (ed.). Периодический закон [The Periodic Law] (in Russian). Academy of Sciences of the USSR.
- O’Dwyer, M. F.; Kent, J. E.; Brown, R. D. (1978). “Many-electron Atoms”. Valency. Heidelberg Science Library. doi:10.1007/978-1-4612-6262-6_4. ISBN 978-0-387-90268-5.
- Schrobilgen, Gary J. (2019). “Chemistry at the Edge of the Periodic Table: The Importance of Periodic Trends on the Discovery of the Noble Gases and the Development of Noble-Gas Chemistry”. In Mingos, D. Michael P. (ed.). The Periodic Table I. Structure and Bonding. Vol. 181. pp. 157–196. doi:10.1007/430_2019_49. ISBN 978-3-030-40024-8.