The Periodic Table — History and Organisation

This lesson covers the history of the periodic table and how it is organised, as required by AQA GCSE Combined Science Trilogy (8464, Chemistry 4.1.2). You need to understand how early attempts to classify elements led to the modern periodic table and why elements are arranged the way they are.

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Early Classification Attempts

Before the periodic table was developed, scientists tried to organise the known elements in various ways:

John Newlands — The Law of Octaves (1866)

• Newlands arranged the known elements in order of atomic mass.
• He noticed that every eighth element had similar properties (like the octave in music).
• His law worked for the first few elements but broke down with heavier elements.
• He did not leave gaps for undiscovered elements.
• Other scientists criticised his work, and it was not widely accepted at the time.

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Mendeleev's Periodic Table (1869)

Dmitri Mendeleev created the first widely accepted periodic table:

Key Features of Mendeleev's Table

1. He arranged elements in order of atomic mass.
2. He placed elements with similar properties in the same vertical column (group).
3. He left gaps for elements that had not yet been discovered and predicted their properties.
4. He swapped the order of some elements so that elements with similar properties were in the same group (e.g. he placed tellurium before iodine despite tellurium having a higher atomic mass).

Why Mendeleev's Table Was Accepted

• His predictions for undiscovered elements were remarkably accurate. For example:
  • He predicted the existence and properties of "eka-silicon" — later discovered as germanium in 1886.
  • He predicted "eka-aluminium" — later discovered as gallium in 1875.
• When these elements were discovered and their properties closely matched Mendeleev's predictions, his table was accepted.

Exam Tip (AQA 8464): AQA loves to ask why Mendeleev left gaps and swapped element positions. The answer is always: to ensure elements with similar chemical properties were in the same group. He prioritised chemical properties over strict atomic mass order.

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The Modern Periodic Table

The modern periodic table arranges elements in order of atomic number (number of protons), not atomic mass. This was made possible by the discovery of the proton and the understanding that atomic number is the fundamental property of an element.

Key Terminology

Term	Definition
Group	A vertical column — elements have the same number of outer shell electrons
Period	A horizontal row — elements have the same number of electron shells
Metals	Found on the left-hand side and centre of the periodic table
Non-metals	Found on the right-hand side of the periodic table

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Organisation of the Modern Periodic Table

graph TD
    A["Modern Periodic Table"] --> B["Arranged by Atomic Number<br/>(number of protons)"]
    A --> C["Groups (Vertical Columns)"]
    A --> D["Periods (Horizontal Rows)"]
    C --> E["Same number of outer electrons<br/>→ Similar chemical properties"]
    D --> F["Same number of electron shells"]
    A --> G["Left/Centre: Metals"]
    A --> H["Right: Non-metals"]

    style A fill:#2c3e50,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#2980b9,color:#fff
    style E fill:#27ae60,color:#fff
    style F fill:#2980b9,color:#fff

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Why Elements in the Same Group Have Similar Properties

Elements in the same group have the same number of electrons in their outer shell. Since chemical reactions involve the outer shell electrons (gaining, losing or sharing them), elements with the same outer electron configuration react in similar ways.

Group	Outer Electrons	Example Elements	Typical Behaviour
1	1	Li, Na, K	Lose 1 electron → form 1+ ions
2	2	Be, Mg, Ca	Lose 2 electrons → form 2+ ions
6	6	O, S	Gain 2 electrons → form 2− ions
7	7	F, Cl, Br	Gain 1 electron → form 1− ions
0	Full shell (2 or 8)	He, Ne, Ar	Very unreactive — stable configuration

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Metals and Non-Metals

Property	Metals	Non-Metals
Position in periodic table	Left and centre	Right side
Electron behaviour	Lose electrons to form positive ions	Gain or share electrons to form negative ions or molecules
Bonding	Form ionic bonds with non-metals; metallic bonds with each other	Form covalent bonds with each other; ionic bonds with metals
Conductivity	Good conductors of heat and electricity	Generally poor conductors (except graphite)
State at room temperature	Usually solid (except mercury)	Can be solids, liquids or gases

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Worked Example

Question: Mendeleev placed tellurium (atomic mass 127.6) before iodine (atomic mass 126.9) in his periodic table, even though this reversed the order of atomic mass. Why?

Answer: Mendeleev swapped their positions so that tellurium was in the same group as sulfur and selenium (elements with similar chemical properties), and iodine was in the same group as fluorine, chlorine and bromine (the halogens). He prioritised chemical properties over strict atomic mass order. The modern periodic table confirms this arrangement because iodine has a higher atomic number (53) than tellurium (52).

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Common Mistakes

Mistake	Correction
Saying the modern periodic table is arranged by atomic mass	It is arranged by atomic number (protons)
Forgetting why Mendeleev left gaps	He predicted undiscovered elements would fill those positions
Saying groups are horizontal	Groups are vertical columns; periods are horizontal rows
Not knowing that Mendeleev swapped some elements	He swapped elements to keep elements with similar properties in the same group

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Summary

• Newlands arranged elements by atomic mass and noticed the Law of Octaves, but his pattern broke down with heavier elements.
• Mendeleev arranged elements by atomic mass, left gaps for undiscovered elements, and swapped some elements to keep similar properties in the same group.
• His predictions were confirmed when gallium and germanium were discovered with properties matching his predictions.
• The modern periodic table arranges elements by atomic number (protons).
• Groups (vertical columns) contain elements with the same number of outer shell electrons and similar chemical properties.
• Periods (horizontal rows) contain elements with the same number of electron shells.
• Metals are on the left/centre; non-metals are on the right.