Group I of the Periodic Table, known as the Alkali Metals, contains some of the most reactive elements in chemistry: Lithium ($Li$), Sodium ($Na$), Potassium ($Kb$), Rubidium ($Rb$), Caesium ($Cs$), and Francium ($Fr$). Unlike the familiar transition metals (like iron or copper), alkali metals possess unique physical properties and exhibit highly predictable trends as you move down the group. The IGCSE 0654 Core syllabus requires a description of these elements' softness and density, and an analysis of their reaction with water.

Alkali metals are atypical metals. While they are shiny when freshly cut, they differ from "traditional" metals in several key ways:

• Softness: They are remarkably soft and can be easily cut with a laboratory knife.
• Density: They have very low densities compared to other metals. Lithium, sodium, and potassium are all less dense than water, meaning they will float on its surface.
• Melting Points: They have relatively low melting points for metals.

As you move down Group I (from Lithium to Francium), three significant physical and chemical trends emerge:

(a) Decreasing Melting Point

The melting point decreases as the atoms become larger. The metallic bonds holding the atoms together become weaker because the outer electrons are further from the nucleus.

(b) Increasing Density

Generally, the density of the elements increases as you move down the group (though there are small irregularities).

(c) Increasing Reactivity

Reactivity increases significantly down the group. This is because all Group I atoms have one electron in their outer shell. As the atoms get larger down the group, the outer electron is further from the positive nucleus and is more shielded by inner shells. This makes the outer electron easier to lose, making the element more reactive.

All alkali metals react vigorously with water to produce a metal hydroxide and hydrogen gas.

Observations

• Lithium ($Li$): Fizzes steadily and gradually disappears.
• Sodium ($Na$): Reacts more vigorously. It melts into a shiny silver ball and skims rapidly across the surface of the water.
• Potassium ($K$): Reacts violently. The heat produced is sufficient to ignite the hydrogen gas, which burns with a characteristic lilac flame.

The resulting solution is highly alkaline (pH 12–14) because of the hydroxide ions produced.

The alkali metals provide a perfect demonstration of periodic trends. Their extreme reactivity and unique physical states are direct consequences of having a single valence electron. By understanding how their properties shift with increasing atomic size, chemists can safely store these elements (under oil) and utilize their high reactivity in a variety of industrial and scientific applications.