Predict particle deflection and atomic-radius trends
Use particle charge and mass to compare beam deflection, then explain qualitative atomic- and ionic-radius patterns.
Section 1 · Atomic structure
AS Level · Cambridge International Chemistry 9701, 2025–2027 syllabus. Electronic configurations are limited to ground-state atoms and ions of elements H to Kr. Written practice is self-checked; it does not replace cumulative exam practice or human subject-teacher review.
Before you begin
- Use atomic symbols and the Periodic Table, and distinguish an atom from an ion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which particle is not deflected by an electric field?
What you will learn
- Predict how proton, neutron and electron beams move in an electric field
- Explain qualitative atomic- and ionic-radius trends across a period and down a group
Electric fields reveal charge and mass
A proton is attracted towards the negative plate and an electron towards the positive plate. A neutron has no charge, so an electric field does not deflect a neutron beam. If protons and electrons enter with the same speed, they experience forces of equal magnitude in opposite directions because their charges have equal magnitude.
Acceleration depends on force divided by mass. The electron has a much smaller mass than a proton, so it is deflected much more strongly in the opposite direction. Describe direction from charge and relative deflection from charge-to-mass ratio; do not confuse either with the particles' initial direction of travel.
Explain radius with nuclear attraction and occupied shells
Across a period, proton number increases while electrons are added to the same main shell. The increased nuclear attraction generally pulls the electron cloud closer, so atomic radius decreases. Down a group, a new occupied shell is added; greater distance and shielding generally outweigh the increased nuclear charge, so atomic radius increases.
Cations are smaller than their parent atoms because electron loss can remove an outer shell or reduce electron–electron repulsion. Anions are larger than their parent atoms because adding electrons increases repulsion within the occupied shells. For ions with the same number of electrons, greater nuclear charge gives the smaller radius.
Pause and explain
Why does atomic radius generally decrease across a period?
Worked example
Explain why an electron beam bends more than a proton beam in opposite directions in the same uniform electric field when both enter at the same speed.
Show the worked solution
- Their charges have equal magnitude and opposite signs, so the forces are equal in magnitude and opposite in direction.
- Acceleration is force divided by mass.
- The electron has far less mass than the proton.
Answer The beams bend in opposite directions because their charges have opposite signs. The electron's much smaller mass gives it a much greater acceleration, so its deflection is larger.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare the direction and relative size of deflection of proton, neutron and electron beams in a uniform electric field.
Give me a hint
Use charge for direction and mass for relative acceleration.
Compare my reasoning
- Protons and electrons have opposite charges.
- Neutrons have no charge.
- For equal speeds, electron and proton force magnitudes match, but their masses differ greatly.
Protons bend towards the negative plate; electrons bend towards the positive plate and are deflected much more because their mass is far smaller. Neutrons are not deflected.
Look for these in your work
- Directions follow charge
- Neutron is undeflected
- Electron deflection is larger because of lower mass
Explain the change in atomic radius from lithium to fluorine and from fluorine to chlorine.
Give me a hint
Compare shell number and nuclear attraction for each direction.
Compare my reasoning
- Lithium to fluorine stays within Period 2.
- Nuclear charge rises while added electrons enter the same shell.
- Fluorine to chlorine adds an occupied shell.
Li to F: radius generally decreases because increasing nuclear charge pulls the same outer shell closer. F to Cl: radius increases because chlorine has an additional occupied shell and greater shielding.
Look for these in your work
- Across-period cause stated
- Down-group cause stated
Which is larger: Mg²⁺ or Na⁺? Explain using electron number and nuclear charge.
Give me a hint
Both ions have ten electrons; compare the number of protons attracting them.
Compare my reasoning
- Mg²⁺ and Na⁺ are isoelectronic.
- Mg²⁺ has 12 protons; Na⁺ has 11.
- The stronger nuclear attraction draws the same number of electrons closer.
Na⁺ is larger. Both ions have ten electrons, but Mg²⁺ has the greater nuclear charge and pulls its electron cloud closer, making Mg²⁺ smaller.
Look for these in your work
- Electron numbers compared
- Nuclear-charge reasoning leads to correct order
Common mistakes
- Deflecting a neutron because it has mass
- Explaining atomic-radius trends using proton number alone
- Saying a cation is larger because it has lost electrons
What can you explain now?
Which particle is not deflected by an electric field?
Compare with the explanation
Neutron
A neutron has no electric charge.
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Make it stick
How do charge-to-mass ratio and occupied shells help predict beam deflection and atomic radius?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.