Introduction

The modern periodic table is one of the most important tools in chemistry. It organises all known chemical elements into a logical arrangement that reveals patterns in their physical and chemical properties. Chemists use the periodic table to identify elements, predict their behaviour, and understand how they form compounds.

The periodic table used today is based on the work of Dmitri Mendeleev, who recognised repeating patterns among the elements, and Henry Moseley, who showed that elements should be arranged according to their atomic number. Because of this arrangement, elements with similar electron arrangements and chemical properties appear in the same groups.

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What Is the Modern Periodic Table?

The modern periodic table is a chart that organises all known chemical elements in order of increasing atomic number.

Each element occupies a unique position determined by:

• its atomic number,
• its electron arrangement,
• its chemical properties.

Definition:
The modern periodic table is the organised arrangement of all known chemical elements in order of increasing atomic number.

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Arrangement by Atomic Number

The atomic number is the number of protons in an atom's nucleus.

The periodic table begins with:

Every element has an atomic number one greater than the element before it.

This arrangement ensures that:

• every element has a unique position,
• similar elements appear together,
• chemical properties repeat in a predictable pattern.

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The Structure of the Periodic Table

The periodic table is organised into:

• Periods (horizontal rows)
• Groups (vertical columns)

There are:

• 7 periods
• 18 groups

Each position provides information about an element's structure and properties.

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Periods

A period is a horizontal row in the periodic table.

There are 7 periods.

As you move across a period:

• atomic number increases,
• one proton is added to each successive element,
• one electron is added to each successive neutral atom.

Elements in the same period have:

• the same number of occupied electron shells.

For example:

• Period 2 elements have 2 occupied electron shells.
• Period 3 elements have 3 occupied electron shells.

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Groups

A group is a vertical column in the periodic table.

Elements in the same group have:

• similar chemical properties,
• similar electron arrangements,
• the same number of outer-shell (valence) electrons (for the main-group elements).

Examples:

Because they have similar outer electrons, elements in the same group often react in similar ways.

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Major Regions of the Periodic Table

The periodic table can be divided into several important regions.

Metals

Metals are found mainly on the left-hand side and in the centre.

Most metals:

• conduct heat and electricity,
• are shiny,
• are malleable,
• form positive ions.

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Non-metals

Non-metals are found mainly on the right-hand side.

Most non-metals:

• are poor conductors,
• are dull,
• often form negative ions or share electrons.

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Metalloids

Metalloids lie along the "staircase" between metals and non-metals.

They have properties of both.

Examples include:

• silicon,
• germanium,
• arsenic.

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Transition Metals

These occupy the centre of the table.

They often:

• have variable oxidation states,
• form coloured compounds,
• are good conductors,
• are used in engineering and industry.

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Lanthanides and Actinides

These two rows are shown below the main table.

• Lanthanides are known as the rare earth elements.
• Actinides include radioactive elements such as uranium.

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Electron Arrangement and Chemical Properties

One of the greatest strengths of the periodic table is that it reflects the arrangement of electrons in atoms.

Elements in the same group have:

• the same number of outer-shell electrons,
• similar chemical properties,
• similar types of chemical reactions.

For example:

Group 1 (Alkali Metals)

• 1 outer electron
• Very reactive
• Form +1 ions

Group 17 (Halogens)

• 7 outer electrons
• Reactive non-metals
• Form –1 ions

Group 18 (Noble Gases)

• Full outer electron shell
• Very unreactive

These repeating electron arrangements explain why the properties of elements repeat across the periodic table.

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Locating Elements in the Periodic Table

Every element can be identified by its position.

For example:

Sodium (Na)

• Atomic number: 11
• Period: 3
• Group: 1
• Metal

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Oxygen (O)

• Atomic number: 8
• Period: 2
• Group: 16
• Non-metal

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Chlorine (Cl)

• Atomic number: 17
• Period: 3
• Group: 17
• Halogen

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Iron (Fe)

• Atomic number: 26
• Period: 4
• Transition metal

Knowing an element's position allows us to predict many of its properties without memorising every detail.

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Worked Example 1

An element is located in Period 3, Group 17.

Question

Identify the element.

Solution

Period 3, Group 17 corresponds to chlorine (Cl).

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Worked Example 2

A student says:

"Elements in the same period have similar chemical properties."

Question

Is this correct?

Solution

No.

Elements in the same group have similar chemical properties because they have similar outer electron arrangements.

Elements in the same period have the same number of occupied electron shells.

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Worked Example 3

Complete the table.

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Worked Example 4

An unknown element is located in Group 1, Period 4.

Predict two of its properties.

Solution

The element is potassium (K).

Predicted properties:

• It is an alkali metal.
• It has one outer electron.
• It forms +1 ions.
• It reacts readily with water.

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Real-World Connection

The periodic table is used every day by scientists, engineers, doctors, and manufacturers. It helps chemists design new medicines, develop stronger alloys, improve batteries, produce fertilisers, and create advanced electronic devices. Because the table reveals relationships between elements, it allows scientists to predict the behaviour of substances even before experiments are carried out.

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Did You Know?

There are currently 118 confirmed chemical elements in the modern periodic table. While many occur naturally on Earth, others have been created artificially in laboratories using particle accelerators. These synthetic elements are often highly unstable and may exist for only fractions of a second before they decay into other elements.

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Key Terms

• Modern periodic table — the arrangement of all known elements in order of increasing atomic number.
• Atomic number — the number of protons in the nucleus of an atom.
• Period — a horizontal row in the periodic table.
• Group — a vertical column of elements with similar chemical properties.
• Metal — an element that is typically shiny, conducts heat and electricity, and forms positive ions.
• Non-metal — an element that is generally a poor conductor and often gains or shares electrons.
• Metalloid — an element with properties intermediate between metals and non-metals.
• Transition metal — an element in the central block of the periodic table with characteristic metallic properties and variable oxidation states.
• Valence electrons — the electrons in the outermost occupied shell of an atom that are involved in chemical bonding.

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Key Takeaways

• The modern periodic table arranges elements by increasing atomic number.
• The table is organised into 7 periods (rows) and 18 groups (columns).
• Elements in the same group have similar outer electron arrangements and therefore similar chemical properties.
• Elements in the same period have the same number of occupied electron shells.
• The periodic table contains major regions, including metals, non-metals, metalloids, transition metals, lanthanides, and actinides.
• An element's position in the periodic table provides valuable information about its structure, electron arrangement, and chemical behaviour, making the periodic table one of the most powerful tools in chemistry.