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will 88Sr have an atomic radius that is larger, smaller or same as 87 Rb?

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    i thought that they'd be the same but the answer is SMALLER but i don't understand why. 1) Smaller2)Bigger nuclear charge / more protons in Sr3) Similar/same shieldingthese are the 3 marks but aren't they supposed to have the same radius as they are both period 5 meanig they both have 5 principle levels. or Sr is bigger as it has 6 electrons in the 3d sub level compared to Rb which has 5. but the answer is that Sr is smaller which i don't understand at all.pls help

    The most important point is 2), they are in the same period so yes the number of shells is the same however Sr has 1 more proton&electron to interact with, so there's a greater charge difference between the nucleus and the surrounding electrons meaning there's a stronger attraction between them which pulls the electrons towards the nucleus (slightly).
    Trend down a group - Atomic radius increases as there are extra shells
    Trend across a period - Atomic radius decreases as nuclear charge is greater whilst electron shielding remains constant/similar
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