1.1.2Physical Chemistry
Isotopes — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 2 of 42
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5What Are Isotopes?
Isotopes show that the number of neutrons in an element's atoms can vary.
Definition
- Definition Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Same atomic number
- Same atomic number Because the proton number is fixed, isotopes belong to the same element and have the same number of electrons when neutral.
Different mass number
- Different mass number Extra neutrons change the mass number, so isotopes are named by their mass number, such as carbon-12 and carbon-14.
Natural mixtures
- Natural mixtures Most elements exist in nature as a mixture of isotopes in fixed proportions,
- which is why relative atomic masses are not whole numbers.
Part 2 of 5Common Examples
Common Examples
Hydrogen
- Hydrogen Protium ¹H has no neutrons, deuterium ²H has one neutron and tritium ³H has two neutrons, all with one proton.
Carbon
- Carbon Carbon-12 has six neutrons and carbon-13 has seven, and carbon-12 is the standard for the relative atomic mass scale.
Chlorine
- Chlorine Chlorine-35 and chlorine-37 occur naturally in a ratio of roughly 3 to 1, giving an average near 35.5.
Oxygen
- Oxygen Oxygen-16, oxygen-17 and oxygen-18 all have eight protons but eight, nine and ten neutrons respectively.
| Isotope | Protons | Neutrons | Electrons (neutral) |
|---|---|---|---|
| ¹H | 1 | 0 | 1 |
| ²H | 1 | 1 | 1 |
| ³⁵Cl | 17 | 18 | 17 |
| ³⁷Cl | 17 | 20 | 17 |
Part 3 of 5Chemical versus Physical Properties
1
Chemical reactions are controlled by the arrangement of electrons, particularly in the outer shell.
2
Isotopes of an element have identical electron arrangements, so they react in the same way and form the same compounds.
3
Neutrons add mass but no charge, so they do not alter the attraction between nucleus and electrons.
4
Physical properties that depend on mass, such as density, boiling point and the rate of diffusion, differ slightly between isotopes.
5
A heavier isotope diffuses more slowly than a lighter one at the same temperature, and mass spectrometry exploits mass differences to separate them.
Part 4 of 5Worked Example
Question
- Chlorine-35 and chlorine-37 are isotopes of chlorine (atomic number 17).
- Give the number of neutrons in each and the number of electrons in a ³⁷Cl⁻ ion.
- Neutrons in ³⁵Cl = 35 - 17 = 18.
- Neutrons in ³⁷Cl = 37 - 17 = 20.
- A neutral chlorine atom has 17 electrons.
- The Cl⁻ ion has gained one electron, so it has 17 + 1 = 18 electrons.
Answer ³⁵Cl has 18 neutrons, ³⁷Cl has 20 neutrons, and ³⁷Cl⁻ has 18 electrons.
Part 5 of 5Exam Checklist & Tips
1.1.2 – Key Points for the Exam
- Isotopes Atoms of the same element with different numbers of neutrons.
- Neutron number The number of neutrons, found from mass number minus atomic number.
- Relative isotopic mass The mass of an atom of an isotope compared with one twelfth of the mass of a carbon-12 atom.
- Abundance The percentage of atoms of an element that are a particular isotope.
Common Mistakes
- Saying isotopes have different numbers of protons; the proton number never changes within an element.
- Claiming isotopes react differently because they have different masses; their chemistry is the same.
- Forgetting that the number of electrons in a neutral isotope equals its proton number.
- Writing the mass number as the number of neutrons when naming an isotope.
Last updated:
\n