Atoms & ions
Atoms & ions
How evidence changed the atomic model
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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.
| Model | Evidence and interpretation |
|---|---|
| Dalton (solid sphere model) | Atoms pictured as tiny, solid, indivisible spheres. |
| J. J. Thomson (plum pudding model) | Discovery of the electron showed atoms were divisible; electrons were placed within diffuse positive charge. |
| Rutherford (nuclear model) | Alpha scattering showed a tiny, dense, positively charged nucleus and an atom that is mostly empty space. |
| Bohr (planetary model) | Electrons occupy fixed energy levels, helping explain periodic properties. |
| Chadwick | Discovery of the neutron completed the nuclear model and helped explain isotope masses. |
Observation → inference
Most alpha particles pass through thin foil; some undergo large deflections and a few rebound.
Explain what these observations tell you about the structure of the atom.
Worked answer
- Most pass through: most of the atom is empty space.
- Rare large deflections: positive charge and most mass are concentrated in a tiny, dense nucleus.
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
- Explain how the discovery of electrons challenged Dalton’s indivisible-atom model. [2]
- State one feature of Bohr’s model and explain how it helps account for repeating chemical properties. [2]
Total: 4 marks
Show answer & marking points
- Atoms contain smaller negatively charged particles [1], so they are not indivisible [1].
- Electrons occupy fixed energy levels [1]; repeating outer-shell arrangements help explain repeating chemical properties [1].
Fundamental particles
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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 | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | Approximately 1/1840 | Orbitals around the nucleus |
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
Complete the table for the three fundamental particles. [9]
| Particle | Relative charge | Relative mass | Location |
|---|---|---|---|
| Proton | … | … | … |
| Neutron | … | … | … |
| Electron | … | … | … |
Total: 9 marks
Show answer & marking points
| Particle | Relative charge | Relative mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | Approximately 1/1840 | Region around the nucleus |
One mark for each correct entry: 9 marks. The electron has a very small mass, not zero mass.
Particle numbers in atoms 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 an isotope.
A mass number: total number of protons and neutrons; no unit; Z atomic number: number of protons; no unit.
Count an ion’s particles
State the numbers of protons, neutrons and electrons in a ⁵⁹Co²⁺ ion. Cobalt has atomic number 27.
Worked answer
- Protons = 27.
- Neutrons = 59 − 27 = 32.
- Electrons = 27 − 2 = 25.
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
An ion is represented as ⁵⁶Fe²⁺. The atomic number of iron is 26. State the numbers of protons, neutrons and electrons. [3]
Total: 3 marks
Show answer & marking points
- Protons = 26. [1]
- Neutrons = 56 − 26 = 30. [1]
- Electrons = 26 − 2 = 24. [1]
An ion contains 17 protons, 18 neutrons and 18 electrons. State its element, mass number and charge. Use atomic numbers S = 16, Cl = 17 and Ar = 18. [3]
Total: 3 marks
Show answer & marking points
- Chlorine: Z = 17. [1]
- Mass number = 17 + 18 = 35. [1]
- Charge = −1: one more electron than proton. [1]
Complete the missing entries. [6]
| Species | Atomic number | Mass number | Protons | Neutrons | Electrons |
|---|---|---|---|---|---|
| ²⁷Al³⁺ | 13 | 27 | … | … | … |
| ³⁷Cl⁻ | 17 | 37 | … | … | … |
Total: 6 marks
Show answer & marking points
| Species | Protons | Neutrons | Electrons |
|---|---|---|---|
| ²⁷Al³⁺ | 13 | 14 | 10 |
| ³⁷Cl⁻ | 17 | 20 | 18 |
One mark for each correct particle count: 6 marks.