Isotopes, ions and the scale of the nucleus
| English | Français |
|---|---|
| mass number/mæs ˈnʌmbə/ | nombre de masse |
| isotope/ˈaɪsətəʊp/ | isotope |
What would explain this observation?
- Two chlorine atoms can have the same proton number but different masses. Their nuclei contain different neutron numbers, while their element identity remains chlorine.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- A typical atom has radius about 0.1 nm, equal to 1×10⁻¹⁰ m. Its nucleus has a radius less than one ten-thousandth of the atom’s radius, of order 1×10⁻¹⁴ m. Almost all atomic mass is in the nucleus. Proton and neutron relative masses are each approximately one; an electron’s relative mass is very small. Mass number 质量数, A, counts protons plus neutrons. Isotopes 同位素 are atoms of the same element with different neutron numbers.
- isotope: Atoms of the same element with different numbers of neutrons; mass number: The total number of protons and neutrons in a nucleus.
What differs between chlorine-35 and chlorine-37 atoms?
In nuclide notation the upper-left number is A and the lower-left is Z. Protons=Z; neutrons=A−Z. A neutral atom has Z electrons; a positive ion has lost electrons and a negative ion has gained them. Chlorine-35 and chlorine-37 both have Z=17, with 18 and 20 neutrons respectively. Their mass numbers differ without a change in proton number.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- In nuclide notation the upper-left number is A and the lower-left is Z. Protons=Z; neutrons=A−Z. A neutral atom has Z electrons; a positive ion has lost electrons and a negative ion has gained them. Chlorine-35 and chlorine-37 both have Z=17, with 18 and 20 neutrons respectively. Their mass numbers differ without a change in proton number.
- Write a table with separate columns for A, Z, proton, neutron, electron and charge. Check A=protons+neutrons and charge=protons−electrons. For size comparisons convert radii into the same unit before dividing. A nucleus drawn clearly inside a circle is greatly enlarged relative to the atom; label the diagram not to scale and use numbers for a scale calculation.
Which two habits make the investigation or model in this case more defensible?
Write a table with separate columns for A, Z, proton, neutron, electron and charge. Check A=protons+neutrons and charge=protons−electrons. For size comparisons convert radii into the same unit before dividing. A nucleus drawn clearly inside a circle is greatly enlarged relative to the atom; label the diagram not to scale and use numbers for a scale calculation.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: chlorine-37 as Cl⁻ has 17 protons, 37−17=20 neutrons and 17+1=18 electrons. In a scale model where atom radius is 1 m and nuclear radius is 1/10,000 of it, the nuclear radius is 0.0001 m = 0.1 mm. This illustrates the order of size, not a precise measured radius for every isotope.
A magnesium-25 atom has atomic number 12. How many neutrons does it contain? Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A magnesium-25 atom has atomic number 12. How many neutrons does it contain?
The result is 13 neutrons. Known: chlorine-37 as Cl⁻ has 17 protons, 37−17=20 neutrons and 17+1=18 electrons. In a scale model where atom radius is 1 m and nuclear radius is 1/10,000 of it, the nuclear radius is 0.0001 m = 0.1 mm. This illustrates the order of size, not a precise measured radius for every isotope.
Check the conclusion and its limits
- Mass number is an integer particle count, whereas relative atomic mass is an abundance-weighted mean. Ions differ in electrons, isotopes in neutrons. Dividing two radii gives a length ratio; the volume ratio is different. Do not place neutrons outside the nucleus in a shell diagram.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Different isotopes of an element have different proton numbers. This claim is false: Mass number is an integer particle count, whereas relative atomic mass is an abundance-weighted mean. Ions differ in electrons, isotopes in neutrons. Dividing two radii gives a length ratio; the volume ratio is different. Do not place neutrons outside the nucleus in a shell diagram.
Isotopes, ions and the scale of the nucleus: In nuclide notation the upper-left number is A and the lower-left is Z. Protons=Z; neutrons=A−Z. A neutral atom has Z electrons; a positive ion has lost electrons and a negative ion has gained them. Chlorine-35 and chlorine-37 both have Z=17, with 18 and 20 neutrons respectively. Their mass numbers differ without a change in proton number.
Different isotopes of an element have different proton numbers.
Mass number is an integer particle count, whereas relative atomic mass is an abundance-weighted mean. Ions differ in electrons, isotopes in neutrons. Dividing two radii gives a length ratio; the volume ratio is different. Do not place neutrons outside the nucleus in a shell diagram.
Atoms of the same element with different numbers of neutrons: write the technical term.
isotope means Atoms of the same element with different numbers of neutrons.