The Periodic Table

The Periodic Table organises elements to showcase trends in their physical and chemical properties. It arranges elements by atomic number, with rows called periods and columns known as groups. Each element has a unique number of protons, influencing its position in the table, and leading to predictable behaviours in similar groups of elements.

Structure of the Periodic Table

Groups (Vertical Columns)

Elements in the same group have the same number of valence electrons, which gives them similar chemical properties.

Group 1: Alkali Metals

• Highly reactive, especially with water.

  • Reaction with Water: Demonstrates the increasing reactivity of alkali metals as you move down the group (lithium, sodium, potassium).

• Their reactivity increases down the group as the atoms get larger, making it easier to lose the outer electron. Group 2: Alkaline Earth Metals

• Reactive but less so than alkali metals.

• They have two outer electrons. Group 7: Halogens

• Highly reactive, needing just one electron to complete their outer shell. Group 8: Noble Gases

• Stable and unreactive due to their full outer electron shells.

Periods (Horizontal Rows)

Elements in the same period have the same number of electron shells, but increasing atomic number across the period results in different chemical properties.

History of the Periodic Table

1. The Greeks had early atomic theories.
2. Robert Boyle (1661) defined an element as a substance that cannot be broken down into simpler substances by chemical means.
3. Dobereiner's Triads: Johann Dobereiner grouped elements in threes, where the atomic mass of the middle element was approximately the average of the other two. These elements shared similar properties.
4. Newlands' Octaves: John Newlands arranged elements by increasing atomic mass and noticed that every eighth element had similar properties, laying the groundwork for the concept of periodicity.
5. Mendeleev's Periodic Table (1869): Dmitri Mendeleev arranged elements by increasing atomic weight and predicted the properties of undiscovered elements by leaving gaps in his table. This arrangement was the first successful version of the periodic table but had inconsistencies due to atomic weight anomalies.
6. Moseley's Modern Periodic Table (1913): Henry Moseley reorganised the table based on atomic number (number of protons), solving inconsistencies in Mendeleev's table. This is the foundation of the modern table used today.

Key Trends in the Periodic Table

Atomic Radius

• Increases down a group because additional electron shells are added, increasing the distance between the nucleus and the outer electrons.
• Decreases across a period as increasing nuclear charge pulls electrons closer to the nucleus.

Ionisation Energy

• The first ionisation energy is the energy required to remove the most loosely bound electron.
• It decreases down a group as the atomic size increases, making it easier to remove an electron.
• It increases across a period due to the stronger attraction between the nucleus and electrons.

Electronegativity

• Electronegativity is the ability of an atom to attract electrons in a chemical bond.
• It decreases down a group due to the increased atomic radius.
• It increases across a period because of stronger nuclear attraction.

Differences between the Modern Table and Mendeleev's Table

Mendeleev's Table	Modern Table
Elements are ordered in terms of atomic weight	Elements are ordered in terms of atomic number
Less elements	More elements
Gaps left for undiscovered elements	No gaps