Atoms, ions & isotopes
Atoms, ions & isotopes
How the atomic model changed
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Scientists do not keep a model just because it is familiar. When new evidence does not fit, the model needs to change. For each model below, connect what scientists observed with what that tells us about the atom.
For example, most alpha particles passing through the foil suggests that most of the atom is empty space. The few large deflections suggest that the positive charge is concentrated in a very small region. Those are two different observations supporting two different conclusions.
- Dalton (solid sphere model)
- Atoms pictured as tiny, solid, indivisible spheres.
- J. J. Thomson (plum pudding model)
- The discovery of electrons showed atoms were divisible. Electrons were placed within diffuse positive charge.
- Rutherford (nuclear model)
- Alpha scattering supported a tiny, dense, positive nucleus, with most of the atom empty space.
- Bohr (planetary model)
- Electrons placed in fixed energy levels.
- Chadwick
- The neutron completed the nuclear model and helped explain isotope masses.
Alpha scattering: observation → inference
Most alpha particles passed through; some were deflected through large angles; very few rebounded.
Explain what each observation tells you about the structure of the atom.
Worked answer
- Most pass through → most of the atom is empty space.
- Rare large deflections or rebounds → positive charge and most mass are concentrated in a tiny, dense nucleus.
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Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.
Positively charged particles are directed at a very thin metal foil. Most pass straight through. A very small proportion are deflected through large angles. Use these observations to explain two conclusions about atomic structure and why the results do not support a model in which positive charge is spread uniformly through the atom.
Total: 4 marks
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- Most particles pass through, so most of the atom is empty space. [1]
- Large deflections require positive charge concentrated in a very small central nucleus. [1]
- Most mass is also concentrated in the nucleus. [1]
- Diffuse positive charge would cause only small deflections, so cannot explain the rare large deflections. [1]
Fundamental particles and the atom
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An atom has a tiny central nucleus containing protons and neutrons. Electrons occupy regions outside the nucleus. The table gives the relative masses and charges you need to know.
Protons and neutrons are much heavier than electrons, so almost all the atom's mass is in its nucleus. An electron's mass is very small, but it is not zero.
In a neutral atom, each positive proton is balanced by one negative electron. That is why a neutral atom has equal numbers of protons and electrons.
| Particle | Relative charge | Relative mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | −1 | ≈ 1/1840 |
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Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.
State the relative charge and relative mass of a proton, a neutron and an electron.
Total: 6 marks
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- Proton: +1; relative mass 1. [2]
- Neutron: 0; relative mass 1. [2]
- Electron: −1; relative mass approximately 1/1840. [2]
The electron has a very small mass, not zero mass.
State where protons, neutrons and electrons are found in an atom. State where almost all of the mass of an atom is concentrated.
Total: 4 marks
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- Protons: nucleus. [1]
- Neutrons: nucleus. [1]
- Electrons: regions around the nucleus. [1]
- Almost all mass is in the nucleus. [1]
Mass number, atomic number and ions
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The atomic number tells you which element you have: it is the number of protons. The mass number counts protons and neutrons, so subtract the atomic number to find the neutrons. Both numbers are whole numbers with no units.
- Find the neutral atom first. Its proton number and electron number both equal the atomic number.
- For a positive ion, remove electrons. A 2+ charge means two electrons have been lost.
- For a negative ion, add electrons. A 1- charge means one electron has been gained.
- Leave the proton and neutron numbers alone. Making an ion changes the electrons, not the nucleus.
- Atomic number, Z
- The number of protons in the nucleus. It identifies the element.
- Mass number, A
- The total number of protons and neutrons in one atom of a particular isotope. It is a whole number.
A = Z + number of neutronsA mass number: total number of protons and neutrons; no unit; Z atomic number: number of protons; no unit.
Finding particle numbers in an ion
State the number of protons, neutrons and electrons in 56Fe2+. The atomic number of iron is 26.
Worked answer
- Iron has Z = 26, so it contains 26 protons.
- Neutrons = 56 − 26 = 30.
- Start with 26 electrons, then subtract 2 because the ion is 2+.
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Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.
An ion contains 17 protons, 18 neutrons and 18 electrons. Give its element, mass number and charge.
Total: 3 marks
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- Chlorine, because Z = 17. [1]
- A = number of protons + number of neutrons
A = 17 + 18 = 35. [1] - One extra electron gives a 1− charge: ³⁵Cl⁻. [1]
For the ion 3416Q2−, state the number of protons, neutrons and electrons.
Total: 3 marks
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- Protons = 16. [1]
- Neutrons = 34 − 16 = 18. [1]
- Electrons = 16 + 2 = 18. [1]
Isotopes
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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.
Comparing isotopes
Compare neutral atoms of 35Cl and 37Cl. State their numbers of protons, neutrons and electrons, then explain why they have the same chemical properties but different masses. The atomic number of chlorine is 17.
Worked answer
- 35Cl and 37Cl both contain 17 protons.
- 35Cl contains 18 neutrons; 37Cl contains 20 neutrons.
- Both neutral atoms contain 17 electrons and have the same electron configuration, so they react in the same way.
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.
- Define the term isotope. [2]
- Explain why isotopes of the same element have the same chemical properties. [1]
Total: 3 marks
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- Atoms with the same number of protons but different numbers of neutrons. [2]
- They have the same electron configuration. [1]