Explain what is meant by an isotope and state that an element may have more than one isotope

5.1.2 The Nucleus

1. Composition of the Nucleus

• Protons (p⁺) – charge + 1 e, mass ≈ 1.0073 u.
• Neutrons (n⁰) – charge 0, mass ≈ 1.0087 u.
• Electrons (e⁻) – charge – 1 e, mass ≈ 0.00055 u (shown for completeness; they lie outside the nucleus).

2. Proton Number (Z), Mass Number (A) and Neutron Number (N)

• Z (atomic number) – number of protons in the nucleus.
• A (mass number) – total number of nucleons (protons + neutrons).
• N (neutron number) – number of neutrons; calculated by N = A – Z.

Worked example: For the nuclide ⁴⁰₁₉K, Z = 19 and A = 40.  N = A – Z = 40 – 19 = 21 neutrons.

3. Nuclide Notation

The standard way of writing a specific nucleus is:

⁽ᴬ⁾₍ᴢ₎X or ⁽ᴬ⁾X

• Superscript A – mass number.
• Subscript Z – proton number (often omitted in IGCSE questions).
• ‘X’ – chemical symbol of the element.

Examples:

• ⁽¹⁴⁾₆C (or simply ⁽¹⁴⁾C) – carbon nucleus with A = 14, Z = 6.
• ⁽²³⁵⁾₉₂U – uranium‑235 nucleus with A = 235, Z = 92.

4. What Is an Isotope?

An isotope is a variant of a chemical element that has the same proton number (Z) but a different neutron number (N). Consequently isotopes of the same element have the same atomic number but different mass numbers.

5. Key Points About Isotopes

• All atoms of a given element have identical Z (same number of protons).
• Isotopes differ only in N, giving different mass numbers A.
• Chemical properties are virtually identical because they depend on electron configuration, which is set by Z.
• Physical properties that depend on mass (density, atomic mass, nuclear stability) can vary between isotopes.
• Stability is governed mainly by the proton‑to‑neutron ratio; unfavourable ratios lead to radioactivity.

6. Radioactive Decay Types (Core Requirement)

• Alpha (α) decay: emission of an α‑particle (⁴₂He).
  Example: ²³⁸₉₂U → ^{234₉₀Th + 4₂He}
• Beta (β⁻) decay: a neutron converts to a proton, emitting an electron and an antineutrino.
  Example: ¹⁴₆C → ¹⁴₇N + β⁻ + ν̅
• Gamma (γ) radiation: emission of high‑energy photons from an excited nucleus; no change in A or Z.
  Example: ⁶⁰₂₇Co* → ⁶⁰₂₇Co + γ

7. Nuclear Fission and Fusion (Supplementary)

• Fission: a heavy nucleus splits into two (or more) lighter nuclei, releasing neutrons and energy.
  Typical reaction: ²³⁵₉₂U + n → ⁹⁴₃₆Kr + ^{141₅₆Ba + 3 n + energy}
• Fusion: two light nuclei combine to form a heavier nucleus, releasing large amounts of energy.
  Typical reaction (deuterium‑tritium fusion): ²₁H + ³₁H → ⁴₂He + n + energy

8. Example: Carbon Isotopes

Carbon (Z = 6) has several naturally occurring isotopes.

9. Why an Element Can Have More Than One Isotope

Natural processes (e.g., cosmic‑ray interactions) and nuclear reactions can add or remove neutrons from a nucleus without changing the number of protons. Each distinct neutron count produces a separate isotope of the same element, differing in mass and, if the neutron‑to‑proton ratio is unsuitable, in stability.

10. Exam‑Ready Statement

“An isotope is a form of an element that has the same number of protons but a different number of neutrons, giving it a different mass number. Consequently, an element may have more than one isotope.”

11. Suggested Diagram (for classroom use)