The Periodic Table

The Periodic Table is one of the most important tools in chemistry. It helps us understand how elements behave, how they react, and how they’re related to each other. It’s not just a list of elements — it’s a powerful system built from years of scientific discovery.

Early Periodic Tables

In the early 1800s, scientists began trying to organise elements. They arranged them by atomic mass, but the patterns didn’t always work — elements with similar properties sometimes ended up in different groups. Not all elements had been discovered yet, so the system was incomplete and often confusing.

Dmitri Mendeleev’s Periodic Table

In 1869, Dmitri Mendeleev revolutionised the periodic table. He:

• Arranged elements by atomic mass
• Left gaps for elements that hadn’t been discovered yet
• Swapped some elements to group them by similar chemical properties

Mendeleev’s predictions were later proven correct when new elements were discovered to fill the gaps. His work laid the foundation for the modern periodic table.

The Modern Periodic Table

Today’s periodic table is arranged by atomic number (the number of protons), not atomic mass. This fixes many of the problems in earlier versions. Elements are arranged in rows (periods) and columns (groups) based on their atomic structure and properties.

• Groups: Elements in the same column have the same number of outer electrons and similar chemical properties.
• Periods: Elements in the same row have the same number of electron shells.

Some isotopes are stable, while others are radioactive and used in medical and scientific applications.

Metals and Non-Metals

The periodic table is divided into:

• Metals: Found on the left and centre; they’re typically shiny, good conductors, and form positive ions.
• Non-metals: Found on the right; they’re usually dull, poor conductors, and can form negative ions.

A zig-zag line separates metals from non-metals.

Transition Metals

The transition metals are in the middle of the table (groups 3–12). They are:

• Good conductors
• Strong and dense
• Capable of forming multiple ions
• Often used as catalysts

Examples include iron, copper, and silver.

Group 1 – The Alkali Metals

Group 1 contains the alkali metals (e.g. lithium, sodium, potassium). They:

• Have 1 electron in their outer shell
• Are very reactive, especially with water
• Become more reactive down the group
• Form +1 ions and white compounds that dissolve in water

Group 2 and Group 3

• Group 2 elements are called the alkaline earth metals. They are less reactive than Group 1 but still form basic oxides and +2 ions.
• Group 3 includes elements like aluminium, which is a lightweight metal used in many industries. Group 3 metals form +3 ions and have moderate reactivity.

Group 7 – The Halogens

Group 7 elements are the halogens (e.g. fluorine, chlorine, bromine). They:

• Have 7 electrons in their outer shell
• Are non-metals and form -1 ions
• Become less reactive down the group
• Can displace less reactive halogens in reactions

Group 0 – The Noble Gases

Group 0 elements are the noble gases (e.g. helium, neon, argon). They:

• Have full outer shells, making them very unreactive
• Exist as monatomic gases
• Are used in lighting, balloons, and as protective atmospheres

Also see Atomic Structure, Structure & Bonding, Chemical Changes

Links for Learning

BBC Bitesize - Developing the periodic table
BBC Bitesize -Mendeleev's periodic table
BBC Bitesize - Groups and periods
Revise Chemistry - Groups in the periodic table
Study Mind - The modern periodic table