Periodic table and element classification

IB Chemistry HL· Topic 3.1· 15 min read

1. Development of the Modern Periodic Table★☆☆☆☆⏱ 3 min

Early attempts to classify elements included Dobereiner's triads and Newlands' Law of Octaves. Dmitri Mendeleev created the first widely accepted periodic table, ordering elements by atomic mass and leaving gaps for undiscovered elements, successfully predicting their properties.

📘 Definition

Modern Periodic Law

The physical and chemical properties of elements are a periodic function of their atomic number (proton number).

Example:

All group 1 elements have 1 valence electron and share similar reactivity, regardless of their mass.

📐 Worked Example

Explain why tellurium (atomic mass 127.6, $Z = 52$ ) is placed before iodine (atomic mass 126.9, $Z = 53$ ) in the modern periodic table.

1
Recall the modern periodic table is ordered by increasing atomic number, not atomic mass.
2
Compare the atomic numbers: tellurium has $Z = 52$ , iodine has $Z = 53$ . Since $52 < 53$ , tellurium comes first.
3
This order also matches chemical properties: tellurium fits group 16, iodine fits group 17, confirming the arrangement.

Exam tip:

Multiple choice questions often trick students into choosing atomic mass as the modern ordering principle — always select atomic number.

2. Groups and Periods★☆☆☆☆⏱ 4 min

The periodic table is arranged into vertical groups and horizontal periods, each with a clear meaning linked to electron structure:

Groups (vertical): Same number of valence electrons, so similar chemical properties. Common named groups: Group 1 = Alkali metals, Group 2 = Alkaline earth metals, Group 17 = Halogens, Group 18 = Noble gases, Groups 3-12 = Transition metals.
Periods (horizontal): Same number of occupied electron shells. Properties change predictably across a period from metallic to non-metallic.

📐 Worked Example

State the number of occupied electron shells and valence electrons for an element in period 4, group 16.

1
Period number equals the number of occupied electron shells, so period 4 = 4 occupied shells.
2
For main group elements, group number equals the number of valence electrons, so group 16 = 6 valence electrons.
3
Confirm with selenium ( $Z = 34$ ): electron configuration $[A r] 4 s^{2} 3 d^{10} 4 p^{4}$ , which has 4 shells and 6 valence electrons.

3. s, p, d and f Block Classification★★☆☆☆⏱ 5 min

Elements are divided into blocks based on which sub-level holds their highest energy valence electron. This classification directly connects electron configuration to the arrangement of the periodic table.

📘 Definition

Block Classification

Grouping of elements by the sub-orbital containing the highest energy valence electron.

Example:

The highest energy electron in calcium is in 4s, so calcium is an s-block element.

📐 Worked Example

Classify each element by block: (a) Chlorine ( $Z = 17$ ), (b) Vanadium ( $Z = 23$ ), (c) Uranium ( $Z = 92$ ).

1
Write the electron configuration for each element, identify the sub-level with the highest energy electron.
2
(a) Chlorine: $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{5}$ . Highest energy electron is in 3p → p-block.
3
(b) Vanadium: $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{2} 3 d^{3}$ . Highest energy electron is in 3d → d-block.
4
(c) Uranium: $[R n] 7 s^{2} 5 f^{3} 6 d^{1}$ . Highest energy electron is in 5f → f-block.

4. Metals, Non-Metals and Metalloids★☆☆☆☆⏱ 3 min

Elements are broadly classified into three categories based on physical and chemical properties, which helps predict bonding and reactivity.

Class	Key Properties	Location
Metals	Good conductors, malleable, ductile, form cations	Left and lower region of the periodic table
Non-metals	Poor conductors, brittle, often gaseous, form anions	Right and upper region of the periodic table
Metalloids	Intermediate properties, semiconductors	Along the diagonal between groups 13 and 16

📐 Worked Example

Classify arsenic (As) and state one practical use based on its classification.

1
Locate arsenic on the periodic table: it falls on the diagonal line between metals and non-metals.
2
Elements on this diagonal are classified as metalloids.
3
Arsenic is a semiconductor, so it is used in semiconductor electronic devices.

5. Common Pitfalls

Wrong move:

Ordering the modern periodic table by increasing atomic mass.

Why:

This matches Mendeleev's original table but is incorrect for the modern definition.

Correct move:

Always use atomic number (proton number) as the ordering principle for the modern periodic table.

Wrong move:

Confusing the meaning of group and period numbers: stating period number equals valence electron count.

Why:

This mixes up the definition of groups (vertical) and periods (horizontal).

Correct move:

Period number = number of occupied electron shells; main group number = number of valence electrons.

Wrong move:

Classifying potassium ( $[A r] 4 s^{1}$ ) as a d-block element because it has a 4th shell.

Why:

Blocks are classified by the highest energy valence electron, not the highest shell number.

Correct move:

Potassium's highest energy electron is in 4s, so it is an s-block element.

Wrong move:

Claiming all d-block elements are transition metals per IB definition.

Why:

IB defines transition metals as elements with a partially filled d sub-level, so scandium and zinc are excluded.

Correct move:

Only d-block elements with incomplete d sub-levels are classified as transition metals.

6. Quick Reference Cheatsheet

Feature	Key Fact
Ordering principle	Increasing atomic number
Groups (vertical)	Same number of valence electrons, similar properties
Periods (horizontal)	Same number of occupied electron shells
s-block	Groups 1-2, highest energy electron in s sub-level
p-block	Groups 13-18, highest energy electron in p sub-level
d-block	Groups 3-12, highest energy electron in d sub-level
f-block	Lanthanides/actinides, highest energy electron in f sub-level
Metalloids	B, Si, Ge, As, Sb, Te, At
Transition metal (IB def)	Partially filled d sub-level

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 · Paper 1
Block classification multiple choice
2024 · Paper 2
Periodic table structure short answer
2023 · Paper 1
Modern periodic table ordering

Going deeper

syllabusIB Chemistry HL Topic 3 Syllabus

What's Next

This subtopic is the foundation for all periodicity and inorganic chemistry in IB Chemistry HL. The classification system you learned here allows you to predict periodic trends in atomic radius, ionization energy, and electronegativity, which are frequently tested in both Paper 1 and Paper 2. It also underpins the study of transition metal chemistry and bonding, so mastering these basics will make all subsequent topics much easier to understand. Review the key definitions here before moving on to more advanced periodicity concepts.