Trends Across Period 3

What are the period 3 elements?

• The elements from sodium () to argon () are in the third row of the periodic table.

• We refer to the elements of this row as period 3 elements.

Periodic trends:

• Periodic trends are predictable patterns in the properties of elements across a period or down a group.

• In A-Level chemistry, we discuss trends in atomic radius, first ionisation energy and melting point.

Basic principles to consider when discussing trends across period 3:

Atomic structure - explains the trends in size and ionisation energy

Structure and bonding – determine the melting points of elements:

The elements across period 3 have varying types of bonding

• to : Metallic bonding.

• : Giant Covalent structure.

• , , : Simple molecular substances.

• : Monatomic gas.

Atomic Radius

Trend:

The atomic radius decreases across Period 3 ()

Explanation:

• Across period 3, there is an increasing nuclear charge as there is an increase in protons

• Electrons are added to the same energy level and so no significant increase in shielding.

• There is a greater electrostatic attraction between the nucleus and the outer electrons.

• Therefore, the atomic radius decreases as you move across period 3 from left to right

First Ionisation Energy

First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms

Trend:

Overall, there is a general increase in first ionisation energy for the same reason as atomic radius above leading to more energy being required to remove the outermost electron.

There are two anomalies.

Anomalies:

Drop from to :

• ’s outer electron is in a 3p subshell, which is higher in energy than ’s 3s.

• This means less energy is required to remove Al’s electron than ’s

Drop from P to S:

• S has paired electrons in a 3p orbital, causing electron-electron repulsion, making it easier to remove.

• Melting points depend on the type of structure and bonding present

• The table below explains the melting point of the elements in terms of their structure and bonding

Atomic Radii Graph

The graph shows a decrease in the atomic radius of elements across period 3

Ionisation Energy Graph

The graph shows a general increase with drops at and

Melting Point Graph

Melting point rises from to , then drops sharply and gradually decreases from to