What Is the Periodic Table?

The periodic table is a chart that organises all known chemical elements according to their atomic number and properties.

Elements in the periodic table are arranged so that elements with similar chemical properties appear in the same vertical columns, called groups.

The periodic table helps chemists understand:

• Relationships between elements
• Chemical behaviour of elements
• Patterns and trends in properties

It is one of the most important tools used in chemistry.

Structure of the Periodic Table

The periodic table is arranged in rows and columns.

Periods (Rows)

Horizontal rows in the periodic table are called periods.

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

Example:

Elements in Period 3 include:

• Sodium
• Magnesium
• Aluminium
• Silicon
• Phosphorus
• Sulfur
• Chlorine
• Argon

All these elements have three electron shells.

Groups (Columns)

Vertical columns in the periodic table are called groups.

Elements in the same group have:

• The same number of outer shell electrons
• Similar chemical properties

For example:

Group 1 elements all have one electron in their outer shell.

Group 1 – Alkali Metals

Group 1 elements are called alkali metals.

Examples include:

• Lithium
• Sodium
• Potassium

Properties of Group 1 metals:

• Very reactive
• React strongly with water
• Form alkaline solutions

Their reactivity increases down the group.

Group 7 – Halogens

Group 7 elements are known as halogens.

Examples include:

• Fluorine
• Chlorine
• Bromine
• Iodine

Properties of halogens:

• Very reactive non-metals
• Form salts with metals
• Exist as diatomic molecules (X₂)

Their reactivity decreases down the group.

Group 0 – Noble Gases

Group 0 elements are called noble gases.

Examples include:

• Helium
• Neon
• Argon

Properties of noble gases:

• Very unreactive
• Exist as single atoms (monatomic gases)
• Have full outer electron shells

Because of their stable electron configurations, they rarely form compounds.

Periodic Trends

The periodic table also shows important trends in properties.

Some important trends include:

Atomic Size

Atomic size generally increases down a group because atoms have more electron shells.

Reactivity

• Metal reactivity increases down Group 1
• Non-metal reactivity decreases down Group 7

Number of Electrons

The group number usually corresponds to the number of outer shell electrons.

Example:

Group 2 elements have two outer electrons.

Importance of the Periodic Table

The periodic table helps scientists:

• Predict chemical reactions
• Understand atomic structure
• Identify patterns in element properties
• Discover new elements

It is essential for studying all areas of chemistry.

Exam Tip (5070)

Students are often asked to:

• Identify groups and periods
• Describe properties of Group 1, Group 7 and Group 0 elements
• Explain periodic trends

Example exam question:

Why are noble gases unreactive?

Answer:

Noble gases are unreactive because they have full outer electron shells, which makes them very stable.

Practice Question

What does the group number indicate about an element?

Answer

The group number usually indicates the number of electrons in the outer shell.

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