Lesson 1 of 7 · Patterns & properties
Periodic trends
Periodic trends are patterns in element properties across the periodic table. Moving left to right across a main-group period, atomic radius usually decreases, while first ionization energy and electronegativity generally increase.
Periodic trends at a glance
Start by checking which property a question asks about. Atomic size, ionization energy and electronegativity follow different patterns. When comparing size, use the same definition of radius for both elements; covalent and van der Waals radii measure different things.
| Property | Across a period → | Down a group ↓ |
|---|---|---|
| Covalent radius | Usually decreases | Usually increases |
| First ionization energy | Generally increases | Generally decreases |
| Electronegativity | Generally increases | Generally decreases |
| Metallic character | Generally decreases | Generally increases |
Why the trends happen
Across a main-group period, each successive element has another proton, while its outer electrons occupy the same principal shell. Inner electrons partly screen the attraction of the nucleus, an effect called shielding. That screening does not fully offset the added protons, so the outer electrons are generally pulled closer. Down a group, additional occupied shells place the outer electrons farther from the nucleus.
First ionization energy is the energy needed to remove one electron from an isolated gaseous atom in its ground state. Tables often report the energy per mole, in kJ/mol. Electronegativity answers a different question: how strongly does an atom attract shared electrons in a chemical bond?
Exceptions to periodic trends
Boron has a lower first ionization energy than beryllium because its first electron is removed from a higher-energy 2p orbital. Oxygen also has a lower first ionization energy than nitrogen. Two of its 2p electrons share an orbital, and their repulsion makes one easier to remove. These exceptions are easier to remember when you look at the electron configurations.
Ions with the same number of electrons are called isoelectronic. O²⁻, F⁻, Na⁺ and Mg²⁺ each have ten electrons. Within this series, more protons generally mean a smaller ion because the nucleus pulls the electrons more strongly. Electron affinity has a less regular pattern and needs a separate explanation; check the sign convention before comparing values.
Work through the examples
Worked example 1
Which has the larger covalent radius: Na or Cl?
- Locate both in period 3.
- Across the period, increasing effective nuclear attraction generally contracts the valence shell.
Answer: Na has the larger covalent radius.
Worked example 2
Order O²⁻, Na⁺ and Mg²⁺ from largest to smallest.
- Count ten electrons in each ion.
- Compare proton counts: 8, 11 and 12. More protons attract the same electron count more strongly.
Answer: O²⁻ > Na⁺ > Mg²⁺.
Avoid these mistakes
- For ions, check the charge and electron count before using a radius trend.
- Do not assume every noble gas has a conventional Pauling electronegativity.
- The radius of an atom is model-dependent; an electron cloud has no hard edge.
Check your understanding
Open printable worksheetTry each question before opening the answer key. When you check your answers, read the explanations too. Your choices stay in this browser.
Read the complete answer key
1. B — Na. Na lies below Li in group 1 and has an additional occupied principal shell.
2. A — N. Nitrogen has the higher value. Repulsion between paired 2p electrons in oxygen makes its first electron easier to remove.
3. C — Mg²⁺. Mg²⁺ has 12 protons, the greatest nuclear charge among these three isoelectronic ions.
4. B — Shared bonding electrons. Electronegativity is a bonding concept. Ionization energy concerns removing an electron from an isolated gaseous species.
Common questions
Why is fluorine so electronegative?
Its small valence shell and strong nuclear attraction give fluorine the highest value on the Pauling scale. This does not mean it has the most exothermic electron affinity.
Are cations always smaller than their parent atoms?
For the usual comparisons, removing electrons reduces electron repulsion and can remove the outer shell, making a cation smaller than its neutral parent. Always use comparable radius definitions.
References & model notes
Use these sources to check the definitions and data, or to read more about the topic. The lesson notes where a model or rounded value simplifies the chemistry.