The Periodic Table - Group 1 Alkali Metals

Group 1: The Alkali Metals

Properties, Reactions and Trends

What Are the Alkali Metals?

Group 1 metals are some of the most reactive elements in the periodic table. Understanding how they behave helps us predict the properties of other metals and explains many everyday chemical reactions, from the fizzing of antacid tablets to the bright colours in fireworks.

The Position of Group 1

Group 1 is the first vertical column on the left-hand side of the modern periodic table. All the atoms in this group have a single electron in their outermost energy level, written as ns1ns1. Because they form alkaline solutions when they react with water, they are called the alkali metals.

Think of it like this: all Group 1 atoms have just one electron in their "outer coat" - this makes them behave in very similar ways.

Electronic Structure and Why These Metals Are So Reactive

The outer electron is weakly held because:

• It is in a shell relatively far from the nucleus

• There is only a small effective nuclear attraction after inner-shell electrons shield the positive charge

As a result, Group 1 elements:

• have low first ionisation energies (they lose the outer electron easily)

• are soft metals that can be cut with a knife

• have low densities (Li, Na and K float on water)

• are excellent reducing agents, readily forming 1+1+ ions

Electron configurations

1. Lithium: 2,12,1

2. Sodium: 2,8,12,8,1

3. Potassium: 2,8,8,12,8,8,1

Notice how they all end in 1 - this similarity in their outer electron explains why their chemistry is so similar.

Physical Properties Down the Group

As we go down Group 1 from lithium to potassium:

• Melting point and boiling point decrease

• Density increases slightly (though all remain less dense than water up to potassium)

• The metals become softer and more shiny when freshly cut

Why do melting points decrease?

As atomic radius increases down the group, the metallic bonds become weaker. Think of it like trying to hold hands in a circle - the bigger the circle gets, the weaker the grip becomes. This means less energy is needed to break the bonds, so melting and boiling points get lower.

Chemical Reactions of Lithium, Sodium and Potassium

Reaction with Oxygen

When freshly cut surfaces are exposed to air, they quickly tarnish (go dull) as they form oxides.

Comparison table

When burned in air, lithium produces a red flame, sodium gives a bright yellow flame, and potassium burns with a lilac (pale purple) flame.

Reaction with Chlorine

All three metals react vigorously with chlorine gas, forming white crystalline chlorides that dissolve to give colourless solutions.

2M+Cl2→2MCl(M=Li,Na,K)2M+Cl2​→2MCl(M=Li,Na,K)

The reaction gets faster as we go from Li to K. Sodium often needs gentle heating to start the reaction, but potassium may burst into flames spontaneously in chlorine gas.

Reaction with Water

This is the most dramatic reaction to observe! The metals float and move around on the water surface, fizzing as hydrogen gas is released.

2M+2H2O→2M++2OH−+H22M+2H2​O→2M++2OH−+H2​

Observations:

• Lithium: steady fizzing; solution becomes alkaline

• Sodium: melts into a silvery ball, darts around the surface; may show an orange flame

• Potassium: ignites the hydrogen gas produced, burning with a lilac flame; sometimes makes small popping sounds

The Trend in Reactivity

Reactivity increases down the group: Li < Na < K

This happens because:

1. Atomic radius increases → the outer electron is further from the nucleus

2. Inner electron shells provide greater shielding → they "block" some of the nuclear attraction

3. Although nuclear charge increases, effects (1) and (2) are stronger

Therefore, the outer electron is lost more easily as we go down the group. It's like trying to hold onto a ball while wearing thicker and thicker gloves - the grip gets weaker!

Worked example

Question: A student observes that when a small piece of sodium is added to water, it moves around on the surface and the solution becomes alkaline.

(a) Write the balanced symbol equation for this reaction. (b) Explain why the solution becomes alkaline. (c) Predict what would happen if potassium was used instead of sodium.

Solution:

(a) 2Na+2H2O→2NaOH+H22Na+2H2​O→2NaOH+H2​

(b) The reaction produces sodium hydroxide (NaOH), which releases hydroxide ions (OH−OH−) in solution. These hydroxide ions make the solution alkaline.

(c) Potassium would react more vigorously than sodium. It would move faster on the surface, the hydrogen gas produced would ignite with a lilac flame, and there might be small popping sounds due to rapid gas production.

Predicting the Properties of Rubidium and Caesium

Using the trends we've observed, we can predict that rubidium and caesium will:

• React even more violently with water, probably exploding on contact

• Have melting points lower than potassium's 63°C63°C (caesium is actually liquid just above room temperature at 28°C28°C!)

• Form white oxides and chlorides that dissolve to give colourless alkaline solutions

• Be even softer than potassium

Aim: To compare the reactivity of Group 1 metals with water

Apparatus:

• Three troughs half-filled with distilled water

• Universal indicator solution

• Small pieces of Li, Na, K (stored under oil)

• Tweezers, safety screen, safety goggles

Method:

1. Add 5 drops of universal indicator to each trough of water

2. Using tweezers, place a rice-grain-sized piece of lithium onto the surface of the first trough

3. Observe and record what happens

4. Repeat with sodium in the second trough, then potassium in the third

Safety:

• Wear safety goggles and stand behind safety screen

• Use only very small pieces of metal

• Keep flames away unless observing flame colour

• Teacher demonstration only

Expected Observations:

• Lithium: fizzes steadily, moves slowly, indicator turns purple

• Sodium: melts into a ball, moves rapidly around surface, may show faint orange flame

• Potassium: bursts into lilac flame immediately, moves very rapidly, may hear slight "pop"

Conclusion: The increasing vigour of reaction confirms that reactivity increases down Group 1. All solutions become alkaline due to formation of metal hydroxides.

Real-world Application: Sodium Street Lights

High-pressure sodium lamps contain sodium metal and an inert gas. When switched on, the sodium vaporises and an electric current passes through the vapour, producing the characteristic bright yellow glow used in street lighting. This application uses sodium's relatively low melting point (98°C98°C) and the ease with which its outer electron can be excited to produce light. The yellow colour is exactly the same as the flame test colour for sodium!

Key terms

Alkali metal - A metal in Group 1 of the periodic table that forms alkaline solutions when it reacts with water

Group - A vertical column in the periodic table containing elements with similar properties

Ionisation energy - The energy needed to remove an electron from an atom

Shielding effect - When inner electrons reduce the attraction between the nucleus and outer electrons

Reactivity trend - The pattern of how reactive elements become as you move through a group or period

Electron configuration - The arrangement of electrons in shells around an atom's nucl