🎓 Prepared by students from Boğaziçi University

What Are Periodic Table Trends?

Periodic trends are patterns in element properties that repeat across the periodic table. As you move from left to right across a period or down a group, atomic radius, ionization energy, electronegativity, and other properties follow predictable trends, revealing the hidden structure of chemistry.

Short answer

Periodic trends are predictable changes in element properties based on position. Atomic radius decreases left-to-right and increases down; ionization energy and electronegativity increase left-to-right but decrease down; electron affinity shows no single trend but favors halogens.

Atomic Radius Trend (Period 3: Na → Ar)

x: Element (atomic number) · y: Atomic radius (pm)

Step-by-step worked examples

Compare atomic radius: Na vs Cl (both period 3). Which is larger?

Na is at the left of period 3, Cl is at the right.
Atomic radius DECREASES left-to-right.
Na (186 pm) > Cl (99 pm) — Na is much larger.

Compare atomic radius: Li vs Na (both group 1). Which is larger?

Li is in period 2, Na is in period 3.
Atomic radius INCREASES down a group.
Na (186 pm) > Li (152 pm) — Na is larger.

Predict ionization energy trend in period 2 (Li to Ne).

Ionization energy INCREASES left-to-right.
Li (520 kJ/mol) < Be (900) < C (1086) < N (1402) < O (1314) < F (1681) < Ne (2081).
Note: small dip at O due to electron pairing repulsion.

Flashcards

Quick quiz

Q1.Atomic radius increases…

Correct answer: B. Down a group, each element has more electron shells, making it larger.

Q2.Ionization energy is highest for…

Correct answer: B. Noble gases have the highest ionization energy — very stable, hard to remove electrons.

Q3.Which element has the smallest atomic radius?

Correct answer: B. Fluorine is the smallest non-noble gas; highest nuclear charge and lowest shells among reactives.

Q4.Electronegativity increases…

Correct answer: B. Left-to-right: stronger nucleus attracts electrons. Down a group: farther electrons less attracted.

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Common mistakes

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Atomic radius increases left-to-right across a period. — Correct: Atomic radius DECREASES left-to-right (nucleus pulls harder with more protons).

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Ionization energy has no trend. — Correct: It increases left-to-right and decreases down a group (easier to remove outer electrons when farther away).

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All noble gases have the same ionization energy. — Correct: Noble gas ionization energy INCREASES up the group (He is hardest to ionize, smallest nucleus effect).

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Metals are more electronegative than nonmetals. — Correct: Nonmetals (right side) are more electronegative — they attract electrons more strongly.

FAQ

What causes periodic table trends?

Trends arise from two competing factors: nuclear charge (pulls electrons inward) and electron shell distance (shields outer electrons). Position in the table determines the balance.

Why does O have lower ionization energy than N?

N has a stable half-filled 2p³ configuration; O's 2p⁴ has one paired electron, which experiences repulsion and is easier to remove.

How do periodic trends relate to chemical bonding?

Trends predict reactivity: high ionization energy = reluctant to lose electrons; high electronegativity = attracts electrons in bonds. These determine bond type and polarity.

Are periodic trends absolute?

Mostly, but exceptions exist (e.g., O vs N ionization energy, or some transition metals). Trends work best for main-group elements.