Classification of an Element (AQA A-Level Chemistry): Revision Notes

2.1.1 Classification of an Element

Periodic Table Organization

The Periodic Table organises elements based on their proton number (atomic number), which defines the element and its properties. The arrangement of elements follows specific patterns related to their electron configurations and recurring trends in chemical and physical properties, a concept known as periodicity.

Groups and Periods

• Periods: Elements in the same period have the same number of electron shells. The period number corresponds to the highest energy level occupied by electrons in that row.
• Groups: Elements in the same group have the same number of valence electrons (electrons in their outer shell). This number is indicated by the group number, affecting the element's reactivity and bonding behaviour.

Blocks of the Periodic Table

Elements are classified into different blocks depending on their electron configuration, specifically the type of atomic orbital in which the outermost electrons are found:

• s-block: Groups 1 and 2. Elements in these groups have their outermost electrons in an s orbital. Example: Sodium ($Na$) and Magnesium ($Mg$).
• p-block: Groups 3 to 0 (including the noble gases). These elements have their outermost electrons in a p orbital. Example: Oxygen ($O$) and Chlorine ($Cl$).
• d-block: Transition metals, located between groups 2 and 3. These elements have electrons filling a d orbital. Example: Iron ($Fe$) and Copper ($Cu$).
• f-block: These elements are the lanthanides and actinides, often shown below the main body of the periodic table. They have electrons in an f orbital and include radioactive elements like Uranium ($U$).

Significance of Electron Configuration

The electron configuration of an element determines many of its properties, including its chemical reactivity, ionisation energy, and atomic radius. The periodic table's structure highlights trends that occur across periods and down groups, such as atomic size, electronegativity, and ionisation energy, which are all influenced by electron configuration.

Understanding how elements are classified in the periodic table allows chemists to predict their behaviour and reactions.