Isotopes & relative masses
Isotopes & relative masses
Isotopes and their properties
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Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. The proton number keeps the element the same; the extra neutrons change its mass.
Neutral atoms of the isotopes have the same electron configuration. Since chemical reactions depend on electrons, the isotopes have the same chemical properties.
Their masses are different, so physical properties that depend on mass can differ. Keep mass number, which describes one isotope, separate from the element's weighted mean relative atomic mass.
- Isotopes
- Atoms of the same element with the same number of protons but different numbers of neutrons and different masses.
Compare ³⁵Cl and ³⁷Cl
For neutral atoms of ³⁵Cl and ³⁷Cl, state the numbers of protons, neutrons and electrons. Explain why they have the same chemical properties but different masses. Chlorine has atomic number 17.
Worked answer
| Isotope | Protons | Neutrons | Electrons |
|---|---|---|---|
| ³⁵Cl | 17 | 18 | 17 |
| ³⁷Cl | 17 | 20 | 17 |
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
- Define isotope. [2]
- Explain why isotopes of the same element have the same chemical properties. [1]
- Explain why they can have different physical properties. [2]
Total: 5 marks
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- Atoms of the same element with the same number of protons but different numbers of neutrons and different masses. [2]
- Their neutral atoms have the same electron configuration. [1]
- Different neutron numbers give different masses [1], so mass-dependent physical properties can differ [1].
| Species | Protons | Neutrons | Electrons |
|---|---|---|---|
| A | 17 | 18 | 18 |
| B | 17 | 20 | 17 |
| C | 18 | 20 | 18 |
| D | 18 | 22 | 20 |
- Identify both pairs that are species of different isotopes of the same element. [1]
- Justify the pairs using the particle data. [1]
Total: 2 marks
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- A and B; C and D. Both pairs required. [1]
- Each pair has the same number of protons but different numbers of neutrons. Ionic charge does not change isotope identity. [1]
The carbon-12 relative-mass scale
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A relative mass compares a particle's mass with a common reference: one-twelfth of the mass of one carbon-12 atom. It is a ratio of two masses, so it has no unit.
Read the particle named in the definition. Relative isotopic mass refers to one isotope. Relative atomic mass is a weighted mean that allows for an element's isotope abundances.
Relative molecular mass refers to molecules. For a substance without separate molecules, such as an ionic compound, use relative formula mass. Molar mass is a different quantity: the mass of one mole, usually in g mol⁻¹.
- Relative isotopic mass
- The mass of an atom of one isotope compared with one-twelfth of the mass of a carbon-12 atom.
- Relative atomic mass, Aᵣ
- The weighted mean mass of an atom of an element compared with one-twelfth of the mass of a carbon-12 atom.
| Quantity | Meaning |
|---|---|
| Mass number | Whole-number count of protons plus neutrons in one atom. |
| Relative isotopic mass | Measured relative mass of one isotope; close to, but not necessarily exactly equal to, mass number. |
| Relative atomic mass | Weighted mean across the isotopes present; often not a whole number. |
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
- Define atomic number. [1]
- Define mass number. [1]
- Explain why mass number is a whole number whereas relative atomic mass often is not. [2]
Total: 4 marks
Show answer & marking points
- The number of protons in the nucleus of an atom. [1]
- The total number of protons and neutrons in the nucleus of one atom. [1]
- Mass number counts whole particles in one isotope [1]. Relative atomic mass is a weighted mean including isotope abundances [1].
- Define relative isotopic mass. [2]
- Define relative atomic mass, Aᵣ. [2]
Total: 4 marks
Show answer & marking points
- The mass of an atom of one isotope compared with one-twelfth of the mass of a carbon-12 atom. [2]
- The weighted mean mass of an atom of an element compared with one-twelfth of the mass of a carbon-12 atom. [2]
Relative molecular and formula masses
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The formula tells you how many atoms to include. Add their Aᵣ contributions to find the relative molecular mass or relative formula mass.
Use relative molecular mass for substances made of separate molecules. Use relative formula mass when the formula describes a ratio in a giant structure, such as NaCl. Both relative masses have no units.
Use these terms and calculate the values; formal definitions of these two terms are not required here.
| Structure | Term | Examples |
|---|---|---|
| Discrete molecules | Relative molecular mass, Mᵣ | CO₂, H₂O, NH₃ |
| Giant ionic or covalent structure | Relative formula mass | NaCl, MgCl₂, SiO₂ |
Ar relative atomic mass; no unit; Σ sum of all the indicated contributions.
Choose the term and calculate
Use C = 12.0, O = 16.0, Mg = 24.3 and Cl = 35.5 to calculate relative masses for CO₂ and MgCl₂.
Worked answer
- CO₂: 12.0 + 2(16.0) = 44.0. It contains molecules.
- MgCl₂: 24.3 + 2(35.5) = 95.3. It has a giant ionic lattice.
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
Explain the difference between relative molecular mass and relative formula mass. State when each term is used and whether either quantity has a unit. [4]
Total: 4 marks
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- Relative molecular mass sums the Aᵣ values in one molecule and is used for discrete molecules. [1]
- Relative formula mass sums the Aᵣ values in one formula unit. [1]
- Use relative formula mass for ionic compounds or giant structures without discrete molecules. [1]
- Neither has units. [1]
Use Aᵣ: H 1.0, C 12.0, O 16.0, S 32.1, Ca 40.1.
- Calculate Mᵣ of H₂SO₄. [2]
- Calculate the relative formula mass of CaCO₃. [2]
Total: 4 marks
Show answer & marking points
- 2(1.0) + 32.1 + 4(16.0) [1] = 98.1 [1].
- 40.1 + 12.0 + 3(16.0) [1] = 100.1 [1].