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Elements in Group 1

4.4 Elements in Group 1
CHEMISTRY • Form 4 • Chapter 4: The Periodic Table of Elements

4.4Elements in Group 1

Group 1 of the Periodic Table consists of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). These elements are known as alkali metals. They have one valence electron, low melting and boiling points compared to other metals, and low densities. Chemically, they are highly reactive, readily donating their single valence electron to form ions with a +1 charge. Their reactivity increases down the group as the atomic size expands and the valence electron is lost more easily.

Learning Objectives

  • Generalise the changes in physical properties of elements when going down Group 1.
  • Investigate through experiment the chemical properties of Group 1 elements with water, oxygen gas, and chlorine gas.
  • Generalise the changes in the reactivity of elements when going down Group 1.
  • Reason out the physical and chemical properties of the other elements in Group 1.

Trend of Atomic Size and Reactivity Down Group 1

More electron shells mean a larger atomic radius, weaker nuclear attraction on the valence electron, and higher reactivity.

Li
Lithium atom model Two electron shells with one valence electron in the outer shell.
2.1
Na
Sodium atom model Three electron shells with one valence electron in the outer shell.
2.8.1
K
Potassium atom model Four electron shells with one valence electron in the outer shell.
2.8.8.1
Li → Na → K → Increasing reactivity

Brief Explanation

Understand the key ideas about Group 1 alkali metals.

Alkali metals are soft, low-density metals with one valence electron. When moving down the group from lithium to potassium, additional filled electron shells cause the atomic radius to enlarge.

1. Physical Trend

The metallic bonding between the atoms weakens as the atomic size expands. Less thermal energy is required to separate the atoms, leading to lower melting and boiling points down the group.

2. Chemical Trend

The outermost valence electron experiences a weaker electrostatic pull from the nucleus due to the increased distance. Consequently, the atom donates its valence electron much more readily, resulting in increased reactivity when reacting with water, oxygen, and chlorine gas down Group 1.

Property Li Na K
Atomic radius (nm) 0.133 0.186 0.203
Melting point (°C) 186 98 64
Boiling point (°C) 1 342 880 760
In chemical reactions, each Group 1 atom donates one valence electron to form a univalent cation with a +1 charge:
\(\text{M} \rightarrow \text{M}^+ + \text{e}^-\)

Reaction with Water

Alkali metals react with water to produce an alkaline metal hydroxide solution and hydrogen gas. The resulting solution turns red litmus paper blue.

\(2\text{Li}(\text{s}) + 2\text{H}_2\text{O}(\text{l}) \rightarrow 2\text{LiOH}(\text{aq}) + \text{H}_2(\text{g})\)

Reaction with Oxygen Gas

Alkali metals burn in oxygen gas to produce a white solid metal oxide. The oxide dissolves in water to form an alkaline metal hydroxide solution.

\(4\text{Li}(\text{s}) + \text{O}_2(\text{g}) \rightarrow 2\text{Li}_2\text{O}(\text{s})\)

\(\text{Li}_2\text{O}(\text{s}) + \text{H}_2\text{O}(\text{l}) \rightarrow 2\text{LiOH}(\text{aq})\)

Reaction with Chlorine Gas: Alkali metals burn in chlorine gas to form a white solid metal chloride.
\(2\text{Na}(\text{s}) + \text{Cl}_2(\text{g}) \rightarrow 2\text{NaCl}(\text{s})\)
Trend in Reactivity Down Group 1: Reactivity increases when going down the group: \(\text{Li} < \text{Na} < \text{K} < \text{Rb} < \text{Cs} < \text{Fr}\).
  • The number of shells filled with electrons increases going down the group, causing the atomic size to increase.
  • The single valence electron is placed further away from the nucleus.
  • The nuclear attraction force towards the valence electron weakens.
  • Hence, the single valence electron is more easily donated to achieve a stable octet or duplet electron arrangement.
Properties of Other Group 1 Elements: Rubidium (Rb) and caesium (Cs) are soft metals with shiny surfaces, low melting and boiling points, and are extremely reactive. Francium (Fr) is an unstable radioactive isotope with a short half-life.

Try to Answer First

Answer mentally, then check the explanation.

1

Why does the melting point of alkali metals decrease down Group 1?

Answer: The atomic size increases down the group, which weakens the attraction force (metallic bond) between the metal atoms. Therefore, less heat energy is required to overcome these attraction forces.
2

What are the products formed when sodium metal reacts with water?

Answer: Sodium hydroxide (\(\text{NaOH}\)) solution and hydrogen gas (\(\text{H}_2\)).
3

Why is potassium more reactive than sodium?

Answer: The atomic size of potassium is larger than sodium because it has more shells filled with electrons. The valence electron in potassium is further from the nucleus, weakening the nuclear attraction force, so the valence electron is donated more easily.

Common Mistakes

Avoid these errors when studying Group 1 elements.

!
Writing the chemical equation for the reaction of alkali metals with water incorrectly by producing metal oxide instead of metal hydroxide, for example writing \(\text{Na} + \text{H}_2\text{O} \rightarrow \text{Na}_2\text{O} + \text{H}_2\) instead of forming \(\text{NaOH}\).
!
Forgetting that the solution formed after dissolving alkali metal oxides in water turns red litmus paper blue because it forms an alkaline metal hydroxide solution (\(\text{OH}^-\) ions).
!
Confusing physical and chemical trends: stating that melting point increases down Group 1, whereas melting point actually decreases down the group.

Concept Misunderstandings

Compare each misconception with the correct fact.

Misconception

Alkali metals have high melting points because they are metals.

Correct Fact

Unlike dense transition metals (e.g., iron with a melting point of \(1\,540\ ^\circ\text{C}\)), alkali metals have relatively weak metallic bonds and large atomic radii, giving them characteristically low melting points (\(186\ ^\circ\text{C}\) for Li, down to \(64\ ^\circ\text{C}\) for K).

Misconception

Reactivity decreases down Group 1 because the nucleus has more protons to attract electrons.

Correct Fact

Group 1 metals react by donating electrons, not attracting them. Even though proton number increases, the increased number of inner shells shields the valence electron and moves it further away, making it easier to lose the single valence electron.

Summary

Key facts to remember.

  • Group 1 alkali metals (\(\text{Li}\), \(\text{Na}\), \(\text{K}\), \(\text{Rb}\), \(\text{Cs}\), \(\text{Fr}\)) are soft, low-density metals with one valence electron.
  • Down the group, atomic radius increases, causing the melting and boiling points to decrease due to weaker attraction forces between atoms.
  • Reactivity increases down the group because the valence electron is further from the nucleus, experiencing weaker nuclear attraction and thus being donated more readily.
  • They react with water to produce metal hydroxides and hydrogen gas, with oxygen gas to yield metal oxides (which dissolve in water to form alkaline solutions), and with chlorine gas to form metal chlorides.

Short Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which of the following statements correctly explains why potassium is more reactive than sodium?

2

What product is formed when lithium oxide dissolves in water?

B. Fill in the Blanks

3

Down Group 1, the melting point of elements decreases because the increase in atomic size will weaken the attraction force between the metal ________.

4

When potassium burns in chlorine gas, a white solid called potassium ________ is formed.

C. Match the Element with Its Description

Drag each element on desktop. On mobile, tap an element and then tap the matching box.

Element / Term

Lithium Sodium Potassium Francium

Description / Application

1Used in the production of rechargeable batteries for devices like smartphones.
2Used in yellow vapour lamps for street and outdoor lighting.
3An essential Group 1 element widely used in agricultural fertilisers.
4An unstable radioactive isotope in Group 1 with a very short half-life.
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Alkali metals Valence electron Atomic radius Metal hydroxide Radioactive isotope
The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.

Elements in Group 1

4.4 Elements in Group 1
CHEMISTRY • Form 4 • Chapter 4: The Periodic Table of Elements

4.4Elements in Group 1

Group 1 of the Periodic Table consists of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). These elements are known as alkali metals. They have one valence electron, low melting and boiling points compared to other metals, and low densities. Chemically, they are highly reactive, readily donating their single valence electron to form ions with a +1 charge. Their reactivity increases down the group as the atomic size expands and the valence electron is lost more easily.

Learning Objectives

  • Generalise the changes in physical properties of elements when going down Group 1.
  • Investigate through experiment the chemical properties of Group 1 elements with water, oxygen gas, and chlorine gas.
  • Generalise the changes in the reactivity of elements when going down Group 1.
  • Reason out the physical and chemical properties of the other elements in Group 1.

Trend of Atomic Size and Reactivity Down Group 1

More electron shells mean a larger atomic radius, weaker nuclear attraction on the valence electron, and higher reactivity.

Li
Lithium atom model Two electron shells with one valence electron in the outer shell.
2.1
Na
Sodium atom model Three electron shells with one valence electron in the outer shell.
2.8.1
K
Potassium atom model Four electron shells with one valence electron in the outer shell.
2.8.8.1
Li → Na → K → Increasing reactivity

Brief Explanation

Understand the key ideas about Group 1 alkali metals.

Alkali metals are soft, low-density metals with one valence electron. When moving down the group from lithium to potassium, additional filled electron shells cause the atomic radius to enlarge.

1. Physical Trend

The metallic bonding between the atoms weakens as the atomic size expands. Less thermal energy is required to separate the atoms, leading to lower melting and boiling points down the group.

2. Chemical Trend

The outermost valence electron experiences a weaker electrostatic pull from the nucleus due to the increased distance. Consequently, the atom donates its valence electron much more readily, resulting in increased reactivity when reacting with water, oxygen, and chlorine gas down Group 1.

Property Li Na K
Atomic radius (nm) 0.133 0.186 0.203
Melting point (°C) 186 98 64
Boiling point (°C) 1 342 880 760
In chemical reactions, each Group 1 atom donates one valence electron to form a univalent cation with a +1 charge:
\(\text{M} \rightarrow \text{M}^+ + \text{e}^-\)

Reaction with Water

Alkali metals react with water to produce an alkaline metal hydroxide solution and hydrogen gas. The resulting solution turns red litmus paper blue.

\(2\text{Li}(\text{s}) + 2\text{H}_2\text{O}(\text{l}) \rightarrow 2\text{LiOH}(\text{aq}) + \text{H}_2(\text{g})\)

Reaction with Oxygen Gas

Alkali metals burn in oxygen gas to produce a white solid metal oxide. The oxide dissolves in water to form an alkaline metal hydroxide solution.

\(4\text{Li}(\text{s}) + \text{O}_2(\text{g}) \rightarrow 2\text{Li}_2\text{O}(\text{s})\)

\(\text{Li}_2\text{O}(\text{s}) + \text{H}_2\text{O}(\text{l}) \rightarrow 2\text{LiOH}(\text{aq})\)

Reaction with Chlorine Gas: Alkali metals burn in chlorine gas to form a white solid metal chloride.
\(2\text{Na}(\text{s}) + \text{Cl}_2(\text{g}) \rightarrow 2\text{NaCl}(\text{s})\)
Trend in Reactivity Down Group 1: Reactivity increases when going down the group: \(\text{Li} < \text{Na} < \text{K} < \text{Rb} < \text{Cs} < \text{Fr}\).
  • The number of shells filled with electrons increases going down the group, causing the atomic size to increase.
  • The single valence electron is placed further away from the nucleus.
  • The nuclear attraction force towards the valence electron weakens.
  • Hence, the single valence electron is more easily donated to achieve a stable octet or duplet electron arrangement.
Properties of Other Group 1 Elements: Rubidium (Rb) and caesium (Cs) are soft metals with shiny surfaces, low melting and boiling points, and are extremely reactive. Francium (Fr) is an unstable radioactive isotope with a short half-life.

Try to Answer First

Answer mentally, then check the explanation.

1

Why does the melting point of alkali metals decrease down Group 1?

Answer: The atomic size increases down the group, which weakens the attraction force (metallic bond) between the metal atoms. Therefore, less heat energy is required to overcome these attraction forces.
2

What are the products formed when sodium metal reacts with water?

Answer: Sodium hydroxide (\(\text{NaOH}\)) solution and hydrogen gas (\(\text{H}_2\)).
3

Why is potassium more reactive than sodium?

Answer: The atomic size of potassium is larger than sodium because it has more shells filled with electrons. The valence electron in potassium is further from the nucleus, weakening the nuclear attraction force, so the valence electron is donated more easily.

Common Mistakes

Avoid these errors when studying Group 1 elements.

!
Writing the chemical equation for the reaction of alkali metals with water incorrectly by producing metal oxide instead of metal hydroxide, for example writing \(\text{Na} + \text{H}_2\text{O} \rightarrow \text{Na}_2\text{O} + \text{H}_2\) instead of forming \(\text{NaOH}\).
!
Forgetting that the solution formed after dissolving alkali metal oxides in water turns red litmus paper blue because it forms an alkaline metal hydroxide solution (\(\text{OH}^-\) ions).
!
Confusing physical and chemical trends: stating that melting point increases down Group 1, whereas melting point actually decreases down the group.

Concept Misunderstandings

Compare each misconception with the correct fact.

Misconception

Alkali metals have high melting points because they are metals.

Correct Fact

Unlike dense transition metals (e.g., iron with a melting point of \(1\,540\ ^\circ\text{C}\)), alkali metals have relatively weak metallic bonds and large atomic radii, giving them characteristically low melting points (\(186\ ^\circ\text{C}\) for Li, down to \(64\ ^\circ\text{C}\) for K).

Misconception

Reactivity decreases down Group 1 because the nucleus has more protons to attract electrons.

Correct Fact

Group 1 metals react by donating electrons, not attracting them. Even though proton number increases, the increased number of inner shells shields the valence electron and moves it further away, making it easier to lose the single valence electron.

Summary

Key facts to remember.

  • Group 1 alkali metals (\(\text{Li}\), \(\text{Na}\), \(\text{K}\), \(\text{Rb}\), \(\text{Cs}\), \(\text{Fr}\)) are soft, low-density metals with one valence electron.
  • Down the group, atomic radius increases, causing the melting and boiling points to decrease due to weaker attraction forces between atoms.
  • Reactivity increases down the group because the valence electron is further from the nucleus, experiencing weaker nuclear attraction and thus being donated more readily.
  • They react with water to produce metal hydroxides and hydrogen gas, with oxygen gas to yield metal oxides (which dissolve in water to form alkaline solutions), and with chlorine gas to form metal chlorides.

Short Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which of the following statements correctly explains why potassium is more reactive than sodium?

2

What product is formed when lithium oxide dissolves in water?

B. Fill in the Blanks

3

Down Group 1, the melting point of elements decreases because the increase in atomic size will weaken the attraction force between the metal ________.

4

When potassium burns in chlorine gas, a white solid called potassium ________ is formed.

C. Match the Element with Its Description

Drag each element on desktop. On mobile, tap an element and then tap the matching box.

Element / Term

Lithium Sodium Potassium Francium

Description / Application

1Used in the production of rechargeable batteries for devices like smartphones.
2Used in yellow vapour lamps for street and outdoor lighting.
3An essential Group 1 element widely used in agricultural fertilisers.
4An unstable radioactive isotope in Group 1 with a very short half-life.
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Alkali metals Valence electron Atomic radius Metal hydroxide Radioactive isotope
The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.
Chapter : Periodic Table of Elements
Topic : Reasoning out the physical and chemical properties of the other elements in Group 1
Form 4 Chemistry
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