The short answer
What's inside an atom, why isotopes exist, and how the table's shape predicts an element's behaviour.
Written and checked by GAMSAT tutors — not AI-generated.
Try the reasoning style
We treat forgetting as a failure — a lapse to be patched with reminders and records. Yet a mind that kept everything could not think; it would drown in the undifferentiated noise of every moment it had ever lived. To forget is not so much to lose information as to decide, mostly without our noticing, what was never worth keeping.
The author's argument relies most directly on which unstated assumption?
Pick an option to see how the tutor reasons to the answer — not just whether you were right.
Not quite — the answer is B.
Work backwards from the conclusion: a mind that ‘kept everything’ supposedly ‘could not think.’ That only follows if thinking means leaving most of experience out — so B is the premise the argument quietly rests on. A raises reliability, which the passage never weighs; C contradicts ‘mostly without our noticing’; D smuggles in a claim about intellect the passage never makes. The question rewards finding the hidden premise, not recalling a fact.
Everything is built from atoms, and the periodic table is the cheat sheet — its shape isn't decorative, it predicts how each element behaves. Start with what's inside an atom, and the trends fall out naturally.
Three particles, three jobs
Protons (+) set the element's identity — the atomic number. Neutrons (0) add mass and give isotopes. Electrons (−) live in shells and do all the chemistry — bonding and reactivity are about the outer (valence) electrons.
Atomic number vs mass number
Atomic number (Z) = number of protons = the element's identity. Mass number (A) = protons + neutrons. Isotopes of an element have the same Z but different A (different neutron counts) — same chemistry, different mass.
Periodic trends
Across a period (left → right)
- Atomic radius decreases
- Electronegativity increases
- Ionisation energy increases
Down a group (top → bottom)
- Atomic radius increases
- Electronegativity decreases
- Ionisation energy decreases
Worked example
Carbon-12 and carbon-14 are both carbon. What is the same about them, and what differs — and do they behave differently chemically?
Check yourself
Two atoms have the same number of protons but different numbers of neutrons. They are best described as:
Key takeaways
- Protons set identity (atomic number); electrons drive chemistry; neutrons add mass.
- Mass number = protons + neutrons; isotopes share protons but differ in neutrons.
- Across a period: radius ↓, electronegativity ↑, ionisation energy ↑.
- Down a group: radius ↑, electronegativity ↓, ionisation energy ↓.
- Reactivity is about valence (outer-shell) electrons.
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