Basic Structure of the Periodic Table

Understanding the Layout

The Periodic Table arranges all known elements based on their atomic number, electron configuration, and recurring trends in their physical and chemical properties.
Horizontally, the Periodic Table is divided into rows called periods. The number of each period corresponds to the number of energy levels or shells in the atoms of the elements in that period.
Vertically, it is divided into columns called groups - elements in the same group have similar chemical properties because they have the same number of electrons in their outermost energy level.

Main Blocks of the Periodic Table

The Periodic Table is broadly divided into two types of elements - metals on the left and non-metals on the right, separated by a ‘staircase’ line.
The table can also be divided into blocks. S-block elements are groups 1 and 2, which include alkali metals and alkaline earth metals.
P-block elements are from groups 13 to 18 and include the halogens and noble gases.
The centre of the table makes up the D-block (transition metals), and the separate, two-row block at the bottom is the F-block (lanthanides and actinides).
Central transition metals often have coloured compounds, multiple oxidation states and are often used as catalysts.

Key Groups

Group 1 elements (alkali metals) are very reactive, as they readily lose their single valence electron to achieve a stable electron configuration.
Group 2 elements (alkaline earth metals) are somewhat less reactive than the alkali metals, as they have two valence electrons.
Group 17 elements (halogens) are highly reactive nonmetals, as they aim to gain an extra electron.
Group 18 elements (noble gases) are virtually unreactive, as their electron configuration is already stable.

Periodic Trends

The Periodic Table displays some clear trends (or patterns) in elemental properties. These include atomic radius, electronegativity, first ionisation energy and electron affinity.
Atomic radius decreases across a period from left to right (due to increased nuclear charge without much increase in shielding effect) and increases down a group (due to addition of energy levels).
Electronegativity and ionisation energy generally increase across a period (as it gets increasingly difficult to remove electrons or add extra ones) and decrease down a group. These trends allow predicting behaviour of elements and their compounds.

Revising the Periodic Table

When learning the Periodic Table, it can be helpful to focus on understanding the reasons behind its structure and trends, rather than just memorizing it. Understanding its underlying principles will help you to predict how elements will behave.