The Periodic Table

This lesson covers the history and structure of the periodic table as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 1: Key Concepts in Chemistry. You need to understand how Mendeleev developed the periodic table, how the modern table is arranged, and the key features of groups and periods.

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Early Attempts to Classify Elements

In the 19th century, scientists tried to organise the known elements. At this time, only about 60 elements had been discovered, and scientists knew little about atomic structure.

Newlands' Law of Octaves (1864)

John Newlands arranged the known elements in order of atomic weight (relative atomic mass). He noticed that every eighth element had similar properties — like the octaves in music.

Problems with Newlands' approach:

• It only worked for the first 20 or so elements.
• He forced elements into groups where they did not fit.
• He did not leave any gaps for undiscovered elements.
• His work was not taken seriously by other scientists at the time.

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Mendeleev's Periodic Table (1869)

Dmitri Mendeleev also arranged elements in order of atomic weight, but he made two crucial decisions that made his table far more successful:

What Mendeleev Did Differently

1. He left gaps — where the pattern of properties did not fit, Mendeleev left blank spaces and predicted that undiscovered elements would fill them.
2. He swapped the order of some elements — he placed elements in groups based on their chemical properties rather than strictly following atomic weight order.

Mendeleev's Predictions

Mendeleev predicted the properties of several undiscovered elements. For example, he predicted "eka-silicon" (later discovered as germanium in 1886):

Property	Mendeleev's Prediction (eka-silicon)	Actual Value (Germanium, discovered 1886)
Relative atomic mass	72	72.6
Density (g/cm³)	5.5	5.35
Appearance	Dark grey solid	Grey-white solid
Oxide formula	XO₂	GeO₂

The accuracy of these predictions convinced other scientists that Mendeleev's table was correct.

Exam Tip: A common exam question asks why Mendeleev's periodic table was accepted by other scientists. The answer is that he left gaps for undiscovered elements and predicted their properties, and when those elements were later discovered, the predictions were remarkably accurate. This is strong evidence that his table was based on a genuine pattern.

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The Modern Periodic Table

The modern periodic table arranges elements in order of atomic number (number of protons), not atomic weight. This resolved the problems Mendeleev had with swapping element orders.

Key Features

• Elements are arranged in order of increasing atomic number from left to right.
• Rows are called periods (there are 7 periods).
• Columns are called groups (there are 8 main groups, plus transition metals).
• Elements in the same group have the same number of outer shell electrons and therefore similar chemical properties.
• Elements in the same period have the same number of electron shells.

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Groups

The group number tells you the number of electrons in the outer shell:

Group	Outer Electrons	Examples	Key Features
1	1	Li, Na, K, Rb	Alkali metals — very reactive metals
2	2	Be, Mg, Ca, Sr	Alkaline earth metals — reactive metals
3	3	Al, Ga	Metals
4	4	C, Si, Ge	Mixed metals/non-metals
5	5	N, P, As	Mostly non-metals
6	6	O, S, Se	Non-metals
7	7	F, Cl, Br, I	Halogens — reactive non-metals
0	8 (or 2 for He)	He, Ne, Ar, Kr	Noble gases — very unreactive

Exam Tip: Group 0 elements have a full outer shell (8 electrons, or 2 for helium). This is why they are unreactive. In some periodic tables, Group 0 is labelled Group 8 or Group 18 — they all mean the same thing at GCSE level.

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Periods

The period number tells you how many electron shells an atom has:

Period	Electron Shells	First Element	Last Element
1	1	Hydrogen (H)	Helium (He)
2	2	Lithium (Li)	Neon (Ne)
3	3	Sodium (Na)	Argon (Ar)
4	4	Potassium (K)	Krypton (Kr)

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Metals and Non-Metals

The periodic table can be broadly divided into metals and non-metals:

Position on the Periodic Table

• Metals are found on the left-hand side and in the centre (including transition metals).
• Non-metals are found on the right-hand side.
• A stepped line (staircase) divides metals from non-metals.

Comparing Properties

Property	Metals	Non-Metals
State at room temperature	Mostly solids (except mercury)	Solids, liquids or gases
Melting/boiling points	Generally high	Generally low
Electrical conductivity	Good conductors	Poor conductors (insulators), except graphite
Thermal conductivity	Good conductors	Poor conductors
Malleability/ductility	Malleable and ductile	Brittle (if solid)
Appearance	Shiny (lustrous) when freshly cut	Dull (often)
Ion formation	Lose electrons → form positive ions (cations)	Gain electrons → form negative ions (anions)

Metalloids

Some elements on the boundary between metals and non-metals are called metalloids (or semi-metals). They have properties of both metals and non-metals. Examples include silicon (Si) and germanium (Ge). Silicon is used in semiconductors — it conducts electricity better than a non-metal but not as well as a metal.

Exam Tip: You should be able to classify an element as a metal or non-metal based on its position in the periodic table. Remember: metals are on the left and centre; non-metals are on the right.

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Why Mendeleev Swapped Element Order

Mendeleev occasionally placed elements out of strict atomic weight order to ensure they were in a group with elements of similar properties. For example: