Ions, Isotopes and Relative Atomic Mass

This lesson covers the formation of ions, the definition of isotopes, and the calculation of relative atomic mass, as required by the Edexcel GCSE Combined Science specification (1SC0). These are fundamental concepts that underpin much of chemistry at GCSE level.

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What Is an Ion?

An ion is an atom (or group of atoms) that has gained or lost electrons and therefore has an overall electrical charge.

• Atoms are electrically neutral because they have equal numbers of protons and electrons.
• When an atom loses electrons, it becomes a positive ion (called a cation) because there are now more protons than electrons.
• When an atom gains electrons, it becomes a negative ion (called an anion) because there are now more electrons than protons.

Why Do Atoms Form Ions?

Atoms form ions to achieve a full outer electron shell — the same stable electron configuration as a noble gas. This is the most energetically stable arrangement.

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Formation of Positive Ions (Cations)

Metal atoms form positive ions by losing electrons from their outer shell.

Metal	Group	Electrons Lost	Ion Formed	Electron Configuration Change
Sodium (Na)	1	1	Na⁺	2, 8, 1 → 2, 8 (same as neon)
Magnesium (Mg)	2	2	Mg²⁺	2, 8, 2 → 2, 8 (same as neon)
Aluminium (Al)	3	3	Al³⁺	2, 8, 3 → 2, 8 (same as neon)
Potassium (K)	1	1	K⁺	2, 8, 8, 1 → 2, 8, 8 (same as argon)
Calcium (Ca)	2	2	Ca²⁺	2, 8, 8, 2 → 2, 8, 8 (same as argon)

The Pattern

• Group 1 metals lose 1 electron → form +1 ions (e.g. Na⁺, K⁺, Li⁺)
• Group 2 metals lose 2 electrons → form +2 ions (e.g. Mg²⁺, Ca²⁺)
• Group 3 metals lose 3 electrons → form +3 ions (e.g. Al³⁺)

Exam Tip: The charge on a metal ion equals the group number. Group 1 metals form +1 ions, Group 2 metals form +2 ions, and so on. The ion has the same electron configuration as the nearest noble gas.

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Formation of Negative Ions (Anions)

Non-metal atoms form negative ions by gaining electrons to fill their outer shell.

Non-Metal	Group	Electrons Gained	Ion Formed	Electron Configuration Change
Fluorine (F)	7	1	F⁻	2, 7 → 2, 8 (same as neon)
Chlorine (Cl)	7	1	Cl⁻	2, 8, 7 → 2, 8, 8 (same as argon)
Oxygen (O)	6	2	O²⁻	2, 6 → 2, 8 (same as neon)
Sulfur (S)	6	2	S²⁻	2, 8, 6 → 2, 8, 8 (same as argon)

The Pattern

• Group 7 non-metals gain 1 electron → form −1 ions (e.g. Cl⁻, F⁻, Br⁻)
• Group 6 non-metals gain 2 electrons → form −2 ions (e.g. O²⁻, S²⁻)

Naming Ions

• Positive ions keep the name of the element: sodium → sodium ion (Na⁺).
• Negative ions change their ending to -ide: chlorine → chloride ion (Cl⁻), oxygen → oxide ion (O²⁻).

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What Are Isotopes?

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

Key Points About Isotopes

Feature	Same or Different?
Number of protons	Same
Number of electrons (in neutral atoms)	Same
Number of neutrons	Different
Atomic number	Same
Mass number	Different
Chemical properties	Same (because they have the same electron configuration)
Physical properties	Slightly different (e.g. different density, different rate of diffusion)

Examples of Isotopes

Carbon isotopes:

Isotope	Protons	Neutrons	Mass Number
Carbon-12 (¹²C)	6	6	12
Carbon-13 (¹³C)	6	7	13
Carbon-14 (¹⁴C)	6	8	14

Chlorine isotopes:

Isotope	Protons	Neutrons	Mass Number
Chlorine-35 (³⁵Cl)	17	18	35
Chlorine-37 (³⁷Cl)	17	20	37

Exam Tip: Isotopes have the same chemical properties because they have the same number of electrons (and therefore the same electron configuration). Chemical reactions involve electrons, not neutrons, so the number of neutrons makes no difference to how an atom reacts.

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Relative Atomic Mass (Aᵣ)

The relative atomic mass (Aᵣ) of an element is the weighted mean mass of an atom of that element, compared to 1/12 of the mass of a carbon-12 atom.

Because most elements exist as a mixture of isotopes, the relative atomic mass is not usually a whole number — it is an average that takes into account the mass and abundance (percentage) of each isotope.

The Formula

Aᵣ = Σ (mass of isotope × percentage abundance) ÷ 100

Or in words: multiply each isotope's mass by its percentage abundance, add them all up, then divide by 100.

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Worked Examples — Calculating Relative Atomic Mass

Worked Example 1: Chlorine

Chlorine has two isotopes:

• Cl-35 with 75% abundance
• Cl-37 with 25% abundance

Aᵣ = (35 × 75 + 37 × 25) ÷ 100 Aᵣ = (2625 + 925) ÷ 100 Aᵣ = 3550 ÷ 100 Aᵣ = 35.5

Worked Example 2: Copper

Copper has two isotopes:

• Cu-63 with 69% abundance
• Cu-65 with 31% abundance

Aᵣ = (63 × 69 + 65 × 31) ÷ 100 Aᵣ = (4347 + 2015) ÷ 100 Aᵣ = 6362 ÷ 100 Aᵣ = 63.62 (which rounds to 63.5 on the periodic table)

Worked Example 3: Boron

Boron has two isotopes:

• B-10 with 20% abundance
• B-11 with 80% abundance

Aᵣ = (10 × 20 + 11 × 80) ÷ 100 Aᵣ = (200 + 880) ÷ 100 Aᵣ = 1080 ÷ 100 Aᵣ = 10.8

Exam Tip: The relative atomic mass shown on the periodic table is already the weighted average. If a question gives you isotope data and asks you to calculate Aᵣ, use the formula above. Always show your working clearly — method marks are available even if your final answer is slightly wrong.

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Why Relative Atomic Mass Is Not a Whole Number

Reason	Explanation
Isotopes exist	Most elements have two or more isotopes with different masses
Weighted average	Aᵣ takes into account the abundance of each isotope
Not a simple average	The more abundant isotope has a greater influence on the Aᵣ

For example, chlorine's Aᵣ is 35.5 (not 36, which would be a simple average of 35 and 37) because the lighter isotope (Cl-35) is three times more abundant than the heavier one (Cl-37).

graph TD
    A["Element has<br/>multiple isotopes"] --> B["Each isotope has<br/>different mass number"]
    B --> C["Each isotope has<br/>different abundance"]
    C --> D["Aᵣ = weighted mean<br/>of all isotope masses"]
    D --> E["Aᵣ is usually<br/>NOT a whole number"]

    style A fill:#2c3e50,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#c0392b,color:#fff

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Putting It All Together

Practice Question

An element has the following isotopes:

• Isotope X-24 with 79% abundance
• Isotope X-25 with 10% abundance
• Isotope X-26 with 11% abundance

Calculate the relative atomic mass:

Aᵣ = (24 × 79 + 25 × 10 + 26 × 11) ÷ 100 Aᵣ = (1896 + 250 + 286) ÷ 100 Aᵣ = 2432 ÷ 100 Aᵣ = 24.32

(This is magnesium — the periodic table shows Aᵣ ≈ 24.3.)