Periodic Trends Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Periodic Trends.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

Periodic trends are the predictable patterns in element properties across the periodic table, especially atomic radius, ionization energy, electronegativity, and metallic character.

As you move across or down the periodic table, element behavior changes in regular, trackable ways.

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How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Periodic Trends asks which group, period, and trend justify the prediction.

Common stuck point: Students often know a formula related to periodic trends but skip the recognition step: Am I using an element position, group, period, or trend to predict a chemical property or behavior? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I using an element position, group, period, or trend to predict a chemical property or behavior?

Worked Examples

Example 1

medium

Atom X has IE1 = 580 kJ/mol and IE2 = 1820 kJ/mol; atom Y has IE1 = 740 and IE2 = 1450 kJ/mol. Which is more likely a group 1 element?

Answer

\text{Neither — both look group 2; the larger jump should appear after losing all valence electrons}

First step

Group 1 elements show a giant jump between IE1 and IE2 (e.g., Na: 496 → 4562).

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Example 2

hard

Element Q has successive IEs (kJ/mol): 738, 1451, 7733, 10540. Which group is Q in?

Example 3

challenge

Use Slater-style reasoning: estimate

Z_{eff}

for a 2p electron in oxygen (Z = 8) assuming each inner-shell electron shields 0.85 and same-shell electrons shield 0.35.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy

How does atomic radius change across a period from left to right?

Example 2

easy

How does atomic radius change down a group?

Example 3

easy

How does first ionization energy change across a period left to right?

Example 4

easy

How does first ionization energy change down a group?

Example 5

easy

In which direction on the periodic table does electronegativity increase most?

Example 6

easy

Which has the larger atomic radius: Li or Cs (both group 1)?

Example 7

easy

Which has higher ionization energy: Na or Ar (same period)?

Example 8

easy

Do atomic radius and ionization energy trend in the same or opposite directions?

Example 9

medium

Rank Na, Mg, and Cl by increasing atomic radius (same period 3).

Example 10

medium

Rank F, Cl, Br by decreasing electronegativity.

Example 11

medium

Which has higher ionization energy: Mg or Al, and why is this an exception?

Example 12

medium

Which is larger: a neutral Na atom or its cation

Na^+

Example 13

medium

Which is larger: a neutral Cl atom or its anion

Cl^-

Example 14

medium

Among period 3 elements, where is metallic character greatest?

Example 15

medium

Two elements are isoelectronic:

Na^+

and

Mg^{2+}

(both 10 electrons). Which has the smaller radius?

Example 16

medium

Predict which has higher ionization energy: P (

3p^3

) or S (

3p^4

), noting the half-filled exception.

Example 17

medium

Rank Li, Na, K by increasing first ionization energy.

Example 18

challenge

Explain why fluorine has a smaller atomic radius than oxygen, and why both are smaller than nitrogen.

Example 19

challenge

Sodium's first ionization energy is 496 kJ/mol but its SECOND is 4562 kJ/mol. Explain the huge jump.

Example 20

challenge

Using periodic trends, predict which element is the strongest reducing agent (most easily loses electrons): Li, Na, K, or Cs.

Example 21

easy

Which element has higher electronegativity: O or S?

Example 22

easy

Which has the larger atomic radius: Mg or Sr?

Example 23

easy

Which element has the highest electronegativity on the periodic table?

Example 24

easy

Which group contains the most reactive metals?

Example 25

easy

Which has lower first ionization energy: Be or Ba?

Example 26

medium

Rank by increasing atomic radius: O, S, Se.

Example 27

medium

Rank by increasing first ionization energy: B, C, N.

Example 28

medium

Which isoelectronic species is smallest:

O^{2-}

F^-

Na^+

Mg^{2+}

(all 10 electrons)?

Example 29

medium

Why does the second ionization energy of an atom always exceed the first?

Example 30

medium

Predict which atom is more electronegative: Si or Cl.

Example 31

medium

Rank by decreasing atomic radius: Na, Mg, Al, Si.

Example 32

medium

Which is larger:

Fe^{2+}

Fe^{3+}

Example 33

medium

Predict the most electronegative atom in

HOCl

Example 34

medium

Why are noble gases generally omitted from electronegativity rankings?

Example 35

hard

Explain the anomaly: O has lower IE1 than N despite N having fewer protons.

Example 36

hard

Among Ne, Na,

Na^+

, F,

F^-

, which has the smallest radius?

Example 37

hard

Explain why electron affinity becomes more negative across a period but is essentially zero (or positive) for noble gases.

Example 38

hard

Predict the most likely ionic charge for the following: Ca, Al, S, Br.

Example 39

hard

Using trends only, predict whether the bond in

MgO

is more or less ionic than the bond in

MgS

Example 40

hard

Rank these atoms by decreasing electron affinity magnitude (most exothermic gain first): Cl, F, Br.

Example 41

hard

Why does atomic radius barely change across the d-block (transition metals)?

Example 42

challenge

Predict which atom has the smaller covalent radius: Fr or H. Justify using trends.

Example 43

challenge

Using periodic trends only, predict whether

CaF_2

CaI_2

has the higher melting point.

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Related Concepts

Periodic Table Electron Configuration Electronegativity Chemical Bond

Background Knowledge

These ideas may be useful before you work through the harder examples.

periodic tableelectron configuration