4.2 Group Number and Charge on an Ion

This topic explains the relationship between an element's group number in the periodic table and the charge on the ion it forms.

Definition

The group number of an element in the periodic table can provide information about the charge of an ion formed by an element.

Simple explanation: If you know an element's group number, you can predict what charge its ion will have.

Main Idea

Valence electrons are involved in the formation of ions. The relationship between group number and ions formed by elements is based on the number of valence electrons in the element.

• s-block elements: group number = number of valence electrons.
• p-block elements: number of valence electrons = group number − 10.

Elements tend to achieve a stable electron configuration such as the noble gases. Remember that the 2 or 8 electron configuration is the most stable configuration.

Simple explanation: Atoms gain or lose electrons to become stable like noble gases. The number of electrons they gain or lose depends on their group.

Core Concept

Rule for Ion Formation

Group Number and Ion Charge

Detailed Explanation

Why Elements Form Ions

Elements tend to achieve a stable electron configuration such as the noble gases. The 2 or 8 electron configuration is the most stable configuration.

• Elements with 1–3 electrons in their valence shell tend to lose those electrons and form +1, +2, +3 ions respectively.
• Elements with 5–7 electrons in their valence shell tend to gain 3, 2, 1 electrons respectively and form negatively charged ions with −3, −2, −1 charges respectively.
• Elements with 4 valence electrons can lose 4 electrons to form +4 ions. They can also gain 4 electrons and form −4 ions.

Simple explanation: Atoms want to be like noble gases — with a full outer shell. Metals lose electrons; non-metals gain electrons.

Group 1 (Alkali Metals)

Group 1 elements such as lithium (Li), sodium (Na), and potassium (K) have one valence electron and belong to the s-block.

• s-block elements lose electrons equal to their group number.
• They tend to lose this electron to form a +1 ion, also known as a mono-valent cation.

• Lithium (Li) loses one valence electron to form Li⁺
• Sodium (Na) loses one valence electron to form Na⁺
• Potassium (K) loses one valence electron to form K⁺

Group 2 (Alkaline Earth Metals)

Group 2 elements such as magnesium (Mg) and calcium (Ca) have two valence electrons and belong to the s-block.

- They tend to lose both electrons to form a +2 ion, also known as a divalent cation.

• Magnesium (Mg) loses two valence electrons to form Mg²⁺
• Calcium (Ca) loses two valence electrons to form Ca²⁺

Group 13 (IIIA)

Group 13 elements such as aluminium (Al) have three valence electrons.

- They tend to lose three electrons to form a +3 ion.

Example: - Aluminium (Al) loses three valence electrons to form Al³⁺

Group 14 (IVA)

Group 14 elements such as carbon (C) have four valence electrons.

• They can lose 4 electrons to form +4 ions.
• They can also gain 4 electrons to form −4 ions.
• In practice, carbon rarely forms simple ions — it usually forms covalent bonds.

Groups 15, 16, 17

Group 15 (VA): 5 valence electrons → gain 3 → −3 ion - Example: Nitrogen (N) → N³⁻

Group 16 (VIA): 6 valence electrons → gain 2 → −2 ion - Example: Oxygen (O) → O²⁻

Group 17 (VIIA): 7 valence electrons → gain 1 → −1 ion - Example: Chlorine (Cl) → Cl⁻

Group 18 (VIIIA) — Noble Gases

Group 18 elements have 8 valence electrons (except He, which has 2). They are already stable and do not form ions under normal conditions.

Simple explanation: Noble gases are already stable — they don't need to lose or gain electrons.

Summary Table

Intermediate Level

Why Group Number Predicts Ion Charge

Atoms form ions to achieve a noble gas configuration. The number of electrons they need to lose or gain depends on their group number:

• s-block elements (Groups 1, 2): Lose electrons equal to their group number.
• p-block elements (Groups 13–18): Gain or lose electrons to reach 8 valence electrons.

• Group 13 → lose 3 → +3
• Group 14 → lose 4 or gain 4 → +4 or −4
• Group 15 → gain 3 → −3
• Group 16 → gain 2 → −2
• Group 17 → gain 1 → −1

Ionic Charge and Chemical Formula

The charge on an ion determines the formula of ionic compounds:

Example: Sodium (Na⁺) and chlorine (Cl⁻) combine in a 1:1 ratio to form NaCl.

Example: Magnesium (Mg²⁺) and chlorine (Cl⁻) combine in a 1:2 ratio to form MgCl₂.

Example: Aluminium (Al³⁺) and oxygen (O²⁻) combine in a 2:3 ratio to form Al₂O₃.

Transition Elements — A Special Case

Transition elements (d-block) can form ions with different charges because they can lose different numbers of electrons from their s and d sub-shells.

• Iron (Fe) can form Fe²⁺ and Fe³⁺
• Copper (Cu) can form Cu⁺ and Cu²⁺

This is why the group number rule applies mainly to s-block and p-block elements, not transition elements.

Important Points

• - Group number helps predict the charge on an ion.
• - s-block elements: group number = number of valence electrons.
• - p-block elements: valence electrons = group number − 10.
• - Elements with 1–3 valence electrons tend to lose electrons and form +1, +2, +3 ions.
• - Elements with 5–7 valence electrons tend to gain electrons and form −3, −2, −1 ions.
• - Elements with 4 valence electrons can lose 4 or gain 4.
• - Group 1: +1 ion (Na⁺)
• - Group 2: +2 ion (Mg²⁺)
• - Group 13: +3 ion (Al³⁺)
• - Group 15: −3 ion (N³⁻)
• - Group 16: −2 ion (O²⁻)
• - Group 17: −1 ion (Cl⁻)
• - Group 18: no ion (stable)
• - Transition elements can form ions with different charges.

Remember

• - Group 1 → +1 (Na⁺)
• - Group 2 → +2 (Mg²⁺)
• - Group 13 → +3 (Al³⁺)
• - Group 15 → −3 (N³⁻)
• - Group 16 → −2 (O²⁻)
• - Group 17 → −1 (Cl⁻)
• - Group 18 → no ion
• - Metals lose electrons → positive ions.
• - Non-metals gain electrons → negative ions.
• - Transition elements can have variable charges.

Common Mistakes

• 1. Thinking all elements form ions. Noble gases (Group 18) do not form ions under normal conditions.
• 2. Confusing the charge on cations and anions. Cations are positive; anions are negative.
• 3. Forgetting the group number–charge relationship. Group 1 → +1; Group 2 → +2; Group 17 → −1; etc.
• 4. Thinking transition elements follow the same rule. They can form ions with different charges.
• 5. Thinking Group 14 elements always form ions. Carbon rarely forms simple ions; it usually forms covalent bonds.
• 6. Forgetting that elements form ions to achieve noble gas configuration. This is the driving force behind ion formation.

Quick Revision

• Group number helps predict the charge on an ion.
• s-block elements: group number = number of valence electrons.
• p-block elements: valence electrons = group number − 10.
• Elements with 1–3 valence electrons lose electrons → +1, +2, +3 ions.
• Elements with 5–7 valence electrons gain electrons → −3, −2, −1 ions.
• Elements with 4 valence electrons can lose 4 or gain 4.
• Group 1: +1 (Na⁺); Group 2: +2 (Mg²⁺); Group 13: +3 (Al³⁺).
• Group 15: −3 (N³⁻); Group 16: −2 (O²⁻); Group 17: −1 (Cl⁻).
• Group 18: no ion (stable).
• Transition elements can form ions with different charges.

60-Second Revision

• Group 1 → +1; Group 2 → +2; Group 13 → +3.
• Group 15 → −3; Group 16 → −2; Group 17 → −1.
• Group 18 → no ion.
• Metals lose; non-metals gain.
• Warning: transition elements can have variable charges.