Proton number, nucleon number and isotopes
Section: 1 Atomic Structure | Syllabus: Cambridge AS Level Chemistry (9701)
Isotope Notation
Isotopes are represented using standard nuclear notation, where the mass number appears at the top left and the atomic number at the bottom left of the element symbol:
NotationAZX - where A = mass number, Z = atomic number, X = element symbol
Figure 1.6: Isotope Notation for Carbon Isotopes
(Three annotated symbols: ¹²₆C, ¹³₆C, ¹⁴₆C. For each, show arrows labelling A = mass number (top left) and Z = atomic number (bottom left). Below each symbol list: protons = 6, neutrons = 6/7/8 respectively.)
Examples: Isotopes of Carbon126C → 6 protons, 6 neutrons, 6 electrons
136C → 6 protons, 7 neutrons, 6 electrons
146C → 6 protons, 8 neutrons, 6 electrons
Definition of Isotopes
Isotopes Atoms of the same element that have the same number of protons (same atomic number) but different numbers of neutrons (different mass numbers).- Isotopes have the same atomic number (same number of protons) → same element.
- Isotopes have different mass numbers (different numbers of neutrons).
- In a neutral atom, the number of electrons equals the number of protons - so isotopes of the same element also have the same number of electrons.
Same Chemical Properties
Isotopes of the same element have identical chemical properties.
- Chemical properties are determined by the arrangement and number of electrons, particularly the outer (valence) electrons.
- Isotopes have the same number of protons → same number of electrons → same electron configuration → same chemical behaviour.
- For example, all three isotopes of hydrogen (¹H, ²H, ³H) react with oxygen to form water, and burn in air similarly.
The neutrons in the nucleus do not participate in chemical bonding. Adding or removing neutrons changes the mass of the atom but not its electron configuration, so chemical reactivity is unchanged.
Different Physical Properties
Isotopes of the same element have different physical properties, specifically mass and density.
- Different mass: Isotopes have different numbers of neutrons, giving them different mass numbers. Each atom of a heavier isotope has greater mass.
- Different density: Since density = mass ÷ volume, and atoms of different isotopes have approximately the same size (same electron cloud), the heavier isotope has a higher density.
Deuterium (²H, also written D) is an isotope of hydrogen with one neutron. Heavy water (D₂O) contains deuterium instead of ordinary hydrogen.
Density of H₂O = 1.00 g cm⁻³ → ice floats on liquid water
Density of D₂O ice ≈ 1.11 g cm⁻³ → D₂O ice sinks in ordinary water
This demonstrates that isotopes can have measurably different physical properties despite identical chemistry.
Figure 1.7: D₂O Ice Sinking in H₂O
(Two side-by-side beakers: Left - ordinary H₂O ice cube floating on liquid H₂O (density 1.00 g cm⁻³). Right - D₂O ice cube sinking to the bottom of liquid H₂O, labelled with density ≈ 1.11 g cm⁻³. Caption: same chemical behaviour, different density due to heavier deuterium.)
Exam TipQuestions often ask you to explain why isotopes have the same chemical properties but different physical properties. Always link chemical properties to electron configuration, and physical properties to mass/density differences from different neutron numbers.
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