S- and p-block element chemistry

IB Chemistry SL· 6 min read

1. Classification of s- and p-block Elements★★☆☆☆⏱ 10 min

📘 Definition

s-block elements

Groups 1 and 2

Elements with their outermost (valence) electron occupying an s-orbital. All s-block elements are highly reactive metals that readily lose valence electrons to form cations.

Example:

Sodium (Na) has electron configuration $[N e] 3 s^{1}$ , a reactive group 1 s-block metal.

📘 Definition

p-block elements

Groups 13 to 18

Elements with their outermost valence electrons occupying p-orbitals, after the valence s-orbital is filled. This block contains metals, non-metals and metalloids with diverse properties.

Example:

Chlorine (Cl) has electron configuration $[N e] 3 s^{2} 3 p^{5}$ , a reactive non-metal p-block element.

An element's block is determined exclusively by the subshell of its highest energy valence electron, not its atomic number or position. This classification allows us to predict properties based on periodic position.

📐 Worked Example

Classify magnesium (Z=12) and arsenic (Z=33) as s-block or p-block.

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Step 1: Write the electron configuration for each element
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$Mg\ (Z=12): 1s^2 2s^2 2p^6 3s^2$
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$As\ (Z=33): 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^3$
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Step 2: Identify the highest energy valence subshell
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For Mg: valence electrons are in 3s → s-block. For As: highest energy valence electrons are in 4p → p-block.

2. Group Trends in s-block Elements★★☆☆☆⏱ 15 min

Group 1 (alkali metals) and group 2 (alkaline earth metals) show consistent trends moving down the group. Atomic radius increases due to additional electron shells, ionization energy decreases due to increased shielding and larger distance from the nucleus, and reactivity increases as a result.

📐 Worked Example

Explain why potassium is more reactive than lithium in water.

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Step 1: Compare atomic structure: K has 4 electron shells, Li has 2. K has larger atomic radius and more inner electron shielding.
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Step 2: Nuclear attraction for the valence electron is weaker in K than Li, so K has a lower first ionization energy.
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Step 3: Since s-block reactivity relies on losing the valence electron, K loses its electron more easily, so it is more reactive than Li.

✓ Quick check

Test your understanding of group 1 trends

Which property decreases down group 1?
- Atomic radius
- First ionization energy
- Reactivity
- Relative atomic mass
Reveal answer
First ionization energy —
Correct! Ionization energy decreases down the group. All other options listed increase moving down group 1.

3. Group Trends in Selected p-block Elements★★★☆☆⏱ 15 min

For IB SL, the most commonly tested p-block groups are 17 (halogens) and 18 (noble gases). Group 17 are reactive non-metals that gain one electron to form -1 anions. Unlike s-block metals, reactivity decreases down group 17.

📘 Definition

Halogen displacement reaction

A reaction where a more reactive halogen (higher up group 17) displaces the halide ion of a less reactive halogen from aqueous solution.

Example:

Chlorine disolves bromide ions from solution to form bromine.

📐 Worked Example

Predict if a reaction occurs when chlorine water is added to aqueous potassium iodide, write the balanced equation.

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Step 1: Order halogen reactivity: reactivity decreases down group 17, so Cl > I, chlorine is more reactive than iodine.
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Step 2: A more reactive halogen displaces a less reactive halide ion, so reaction occurs.
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$Cl_2(aq) + 2KI(aq) \rightarrow 2KCl(aq) + I_2(aq)$
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The solution turns brown due to formation of elemental iodine.

4. Acid-Base Trends Across Period 3 Oxides★★★☆☆⏱ 15 min

Across period 3, elements change from metallic to non-metallic, so their oxides change from ionic (basic) to covalent (acidic), with one amphoteric oxide in the middle. This is a very common exam question.

📐 Worked Example

Classify $N a_{2} O$ , $A l_{2} O_{3}$ and $S O_{2}$ as basic, amphoteric or acidic.

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Step 1: Recall the trend: left (metals) = basic, middle = amphoteric, right (non-metals) = acidic.
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(a) Sodium is a group 1 metal on the far left of period 3: $N a_{2} O$ is basic. It reacts with water to form sodium hydroxide: $N a_{2} O + H_{2} O \to 2 N a O H$
3
(b) Aluminium is in the middle of period 3: $A l_{2} O_{3}$ is amphoteric, it reacts with both acids and strong bases.
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(c) Sulfur is a non-metal on the right of period 3: $S O_{2}$ is acidic. It reacts with water to form sulfurous acid: $S O_{2} + H_{2} O \to H_{2} S O_{3}$

5. Common Pitfalls

Wrong move:

Assuming all reactivity trends down groups are the same

Why:

Reactivity increases down s-block metal groups but decreases down p-block non-metal groups like halogens

Correct move:

Remember: metals lose electrons to react (higher reactivity down), non-metals gain electrons (lower reactivity down for halogens)

Wrong move:

Confusing amphoteric and amphiprotic

Why:

Amphoteric means reacts with both acids and bases, while amphiprotic means can donate and accept protons. These terms are not interchangeable

Correct move:

For period 3 oxides, use amphoteric (not amphiprotic) to describe aluminium oxide

Wrong move:

Classifying d-block elements as p-block

Why:

Block is determined by the highest energy valence subshell, not the highest numbered filled subshell

Correct move:

For d-block elements, the highest energy valence subshell is d, so they are not classified as p-block

Wrong move:

Claiming all noble gases are completely unreactive

Why:

Heavier noble gases like xenon form stable covalent compounds, though IB SL focuses on their general inertness

Correct move:

For exams, state noble gases are generally unreactive due to their full valence shell, which meets marking criteria

6. Quick Reference Cheatsheet

Property	s-block (Groups 1-2)	p-block (Groups 13-18)
Location	Left of periodic table	Right of periodic table
Valence electrons	Occupy s-orbitals	Occupy p-orbitals
General classification	Reactive metals	Metals, metalloids, non-metals
Group 1 reactivity	Increases down group
Group 17 reactivity		Decreases down group
Period 3 oxide property	Basic (left)	Amphoteric/acidic (right)

7. Frequently Asked

Why is reactivity opposite for group 1 and group 17?

Group 1 elements lose electrons to react, so lower ionization energy (down the group) increases reactivity. Group 17 elements gain electrons to react, so lower electron affinity (down the group) decreases reactivity.

What does amphoteric mean for oxides?

An amphoteric oxide reacts with both strong acids and strong bases, showing both basic and acidic properties. Aluminium oxide ( $A l_{2} O_{3}$ ) is the key example for IB SL.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · 1
Group 1 reactivity trend explanation
2024 · 2
Period 3 oxide acidity classification
2023 · 1
Halogen displacement reaction prediction

Going deeper

syllabusOfficial IB Chemistry SL Topic 4 SyllabusFull syllabus learning outcomes

What's Next

Understanding s- and p-block element chemistry is the foundation for all inorganic chemistry topics in IB SL. This subtopic connects fundamental periodic trends to real observed chemical behavior, and the patterns you learn here help you predict reaction outcomes and properties of unfamiliar elements in exam questions. Most exam questions on periodicity require you to explain trends, so mastering the reasoning behind each trend (rather than just memorizing the direction of change) is critical to earn full marks. This topic also underpins all subsequent work on bonding, acid-base chemistry and inorganic reactions in the IB syllabus.