Worked example 1
Rank Na, Mg and K in order of increasing atomic radius.
Try it first: Locate all three on the table before comparing anything.
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Periodic TrendsCore periodic trendsContent level: Core 25 min
Atomic Radius, Ionization Energy and Electronegativity
What you'll be able to do: Rank elements by atomic radius, first ionization energy or electronegativity and justify the ranking.
Best after: Shielding and Effective Nuclear Charge
Introduction
Three trends account for most Unit 2 questions, and all three follow from effective nuclear charge and shell distance. Learn one explanation and you can reconstruct all three directions instead of memorizing arrows.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- State and justify the periodic trend in atomic radius
- State and justify the trend in first ionization energy
- State and justify the trend in electronegativity
- Rank a set of elements and defend the order with Zeff and distance
Lesson
Atomic radius
Radius decreases left to right across a period because Zeff pulls the same shell inward, and increases down a group because each period adds a shell. Sodium is larger than magnesium, and potassium is larger than sodium, so potassium is the largest of the three.
Group position beats period position when comparing across both: an extra shell is a bigger change than one extra proton.
First ionization energy
IE₁ is the energy needed to remove the most loosely held electron from a gaseous atom. It increases across a period as Zeff rises, and decreases down a group as valence electrons move further out. Helium has the highest IE₁ of any element, and francium among the lowest.
X(g) → X⁺(g) + e⁻
Electronegativity
Electronegativity measures how strongly a bonded atom attracts shared electrons. It follows ionization energy: increasing across a period, decreasing down a group. Fluorine is the most electronegative element at 3.98 on the Pauling scale, and the noble gases are normally left off the scale.
Electronegativity applies to atoms in a bond. Ionization energy applies to isolated gaseous atoms.
Metallic character runs the other way
Metallic character means the tendency to lose electrons. It increases down a group and decreases across a period, exactly opposite to ionization energy and electronegativity. For group 13, metallic character increases from B to Al to Ga to In to Tl.
One explanation, three trends
Across a period Zeff rises: electrons are held tighter, so atoms shrink and both IE and electronegativity rise. Down a group the valence shell moves away and shielding grows: electrons are held loosely, so atoms grow and both IE and electronegativity fall. Say Zeff and distance in your answers and the reasoning is complete.
Apply trends within their limits
A periodic trend is a general direction, not a guarantee for every adjacent pair. First ionization energy refers specifically to removing one electron from a gaseous neutral atom: X(g) → X⁺(g) + e⁻. The increase across a period has subshell and pairing exceptions, such as Be > B and N > O, which are explained later in this unit. Electronegativity instead describes an atom in a bond, and atomic radius depends on how the radius is defined and measured. On an exam, compare shell number and effective nuclear charge first, then check for a known exception rather than applying a memorized arrow blindly.
Build a causal explanation for a trend
A trend answer is strongest when it names the electron being compared, the change in nuclear attraction or shielding, and the observed property. For example, a smaller radius and a larger first ionization energy can arise together because the nucleus holds the valence electron more tightly. Electronegativity concerns attraction for shared electrons in a bond, so do not use it as a measured property of an isolated noble-gas atom.
Property → electron location → attraction: connect all three in your explanation.
Key ideas
Definition
First ionization energy
Energy required to remove the outermost electron from one mole of gaseous atoms.
Definition
Electronegativity
The relative ability of a bonded atom to attract shared electrons, largest for fluorine.
Rule
Radius trend
Decreases across a period, increases down a group.
Rule
IE and electronegativity trend
Increase across a period, decrease down a group.
Key concept
Same cause
Rising Zeff and changing shell distance explain all three trends at once.
Worked examples
Worked example 2
Which is least electronegative: carbon, oxygen or fluorine, and why?
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Worked example 3
Predict the trend from Mg to Al in atomic radius and first ionization energy, and explain why the IE result is not the simple across-period prediction.
Try it first: Compare the shell, then identify which subshell loses the electron.
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Worked example 4
Predict whether sulfur or chlorine has the smaller atomic radius and higher first ionization energy.
Try it first: Both are in period 3; compare protons while keeping the principal shell fixed.
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Common mistakes
Assuming a heavier element must be a smaller atom.
Why it's wrong: Mass is not the deciding factor. Cesium is much heavier than fluorine but far larger, because it has six occupied shells.
Check instead: Compare shell count first, then Zeff within a period.
Reversing radius and ionization energy directions.
Why it's wrong: They are opposites: a smaller, tightly held atom takes more energy to ionize.
Check instead: Whenever radius decreases, expect ionization energy and electronegativity to increase.
Calling the noble gases the most electronegative because they are on the far right.
Why it's wrong: Noble gases rarely form bonds, so most scales leave them out. Fluorine is the reference maximum.
Check instead: Quote fluorine as the most electronegative element.
After the lesson · Question bank
Independent practice
Work through new problems on this topic on your own. The lesson's quick checks and worked examples stay above if you need to review a step.
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What you should now know
Atomic radius decreases across a period and increases down a group. Ionization energy and electronegativity do the opposite. Every direction follows from rising Zeff across a period and increasing distance down a group.
- Radius decreases across a period and increases down a group
- Ionization energy and electronegativity run opposite to radius
- Fluorine is the most electronegative element
- Metallic character increases down a group
Sources and further reading
- Chemistry 2e, Section 6.5: Periodic Variations in Element PropertiesFlowers, Theopold, Langley, Robinson · OpenStax, Rice University · Chapter 6.5View source
Access for free at openstax.org License
- NIST Atomic Spectra Database: ionization energiesNational Institute of Standards and TechnologyView source
Data from the NIST Atomic Spectra Database