The Periodic Table

This lesson covers the structure and history of the periodic table, as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand how the periodic table is organised, why elements in the same group share similar properties, and the contribution of Mendeleev to its development.

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Structure of the Modern Periodic Table

The modern periodic table arranges all known elements in order of increasing atomic (proton) number. It is organised into:

Groups (Columns)

• There are 8 main groups (plus transition metals in the middle).
• Elements in the same group have the same number of electrons in their outer shell.
• This gives them similar chemical properties.

Periods (Rows)

• There are 7 periods.
• Elements in the same period have the same number of electron shells.
• Moving across a period from left to right, the atomic number increases by one each time.

Metals and Non-Metals

Position	Type	General Properties
Left and centre	Metals	Good conductors of heat and electricity, malleable, ductile, shiny
Right	Non-metals	Poor conductors, brittle (if solid), dull
Staircase line	Boundary	Separates metals from non-metals

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Key Groups

Group	Name	Outer Electrons	Examples
Group 1	Alkali metals	1	Li, Na, K, Rb, Cs
Group 2	Alkaline earth metals	2	Be, Mg, Ca, Sr, Ba
Group 7	Halogens	7	F, Cl, Br, I, At
Group 0	Noble gases	Full outer shell (2 or 8)	He, Ne, Ar, Kr, Xe

Exam Tip: You may see Group 7 referred to as "Group 17" and Group 0 as "Group 18" in some resources. For the Edexcel GCSE specification, stick with the traditional numbering (Group 1, 7, 0) unless told otherwise.

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Mendeleev's Contribution

In the 1860s, Dmitri Mendeleev arranged the known elements in order of atomic mass and organised them into a table based on their properties. His key achievements were:

What Mendeleev Did

1. Arranged elements by increasing atomic mass.
2. Grouped elements with similar properties into the same column (group).
3. Left gaps in the table where he predicted that undiscovered elements would fit.
4. Swapped the positions of certain elements so that they were in groups with elements with similar properties (even though this meant they were slightly out of order by mass).

Why Mendeleev's Table Was So Significant

• He was able to predict the properties of undiscovered elements based on the properties of surrounding elements.
• When these elements were later discovered (e.g. germanium, gallium, scandium), their properties closely matched Mendeleev's predictions.
• This confirmed the validity of his periodic table.

Before Mendeleev

Before Mendeleev, other scientists such as John Newlands had attempted to organise the elements. Newlands proposed his "Law of Octaves" — the idea that every eighth element had similar properties. However, this pattern only worked for the lightest elements and broke down for heavier ones, so it was not widely accepted.

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The Modern Periodic Table vs Mendeleev's Table

Feature	Mendeleev's Table	Modern Periodic Table
Arrangement	By atomic mass	By atomic (proton) number
Gaps	Left for undiscovered elements	No gaps (all elements discovered up to 118)
Order issues	Some elements swapped to fit properties	No swapping needed — atomic number gives the correct order
Noble gases	Not included (undiscovered at the time)	Included as Group 0

Exam Tip: A common question asks why Mendeleev left gaps in his table. The answer is that he predicted elements had not yet been discovered and left spaces for them, even predicting their properties.

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Linking the Periodic Table to Electron Configuration

The periodic table is a direct reflection of electron configuration:

Periodic Table Feature	Electron Configuration Link
Group number	= number of outer-shell electrons
Period number	= number of occupied electron shells
Similar chemical properties	= same number of outer electrons

Example: Group 1 Elements

All Group 1 elements have 1 electron in their outer shell:

• Lithium: 2, 1
• Sodium: 2, 8, 1
• Potassium: 2, 8, 8, 1

Because they all have 1 outer electron, they all react in similar ways — they all lose that one electron easily to form a +1 ion.

Example: Group 7 Elements

All Group 7 elements have 7 electrons in their outer shell:

• Fluorine: 2, 7
• Chlorine: 2, 8, 7
• Bromine: 2, 8, 18, 7

Because they all have 7 outer electrons, they all gain one electron to form a −1 ion.

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Why the Periodic Table Is Useful

graph TD
    A["Periodic Table"] --> B["Predicts chemical<br/>properties"]
    A --> C["Shows patterns<br/>and trends"]
    A --> D["Groups elements<br/>with similar behaviour"]
    B --> E["Same group =<br/>same outer electrons =<br/>similar reactions"]
    C --> F["Reactivity trends<br/>down groups"]
    D --> G["Metals on left,<br/>non-metals on right"]

    style A fill:#2c3e50,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#1a1a2e,color:#fff
    style F fill:#1a1a2e,color:#fff
    style G fill:#1a1a2e,color:#fff

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Transition Metals

The transition metals are found in the central block of the periodic table (between Groups 2 and 3). At GCSE level, key points include:

• They are hard, strong metals with high melting points (except mercury, which is liquid).
• They are good conductors of heat and electricity.
• They are much less reactive than Group 1 metals.
• Many have coloured compounds (e.g. copper sulfate is blue, iron(III) oxide is orange-brown).
• Many are useful as catalysts (e.g. iron in the Haber process, platinum in catalytic converters).

Exam Tip: Transition metals are very different from Group 1 metals. If asked to compare them, mention: higher melting points, higher density, lower reactivity, coloured compounds, and catalytic activity.

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Summary