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I am learning Inorganic Chemistry recently, and doing some pass year paper, and found this question. i am not sure how to identify A3+. Pls Help for question 1 ,2 ,& 3

  • 2
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    – Poutnik
    Dec 25, 2021 at 7:39
  • 1
    $\begingroup$ For question ii, phosphorus-32 has the same electronic configuration as the usual phosphorus atom which is phosphorus-31. $\endgroup$
    – Maurice
    Dec 25, 2021 at 12:36
  • $\begingroup$ Noted, will improve for the next question $\endgroup$
    – James WOng
    Dec 26, 2021 at 4:09

1 Answer 1


Isoelectronic means it has the same electronic structure and number of electrons, for example a negatively charged hydrogen ion (hydride) is isoelectronic with a helium atom.

Now phosphorus forms a simple 3- ion which appears in solids such as aluminium phosphide, the phosphide anion is isoelectronic with an argon atom. If we count three atomic numbers forwards from argon we go

Argon, Potassium, Calcium, Scandium. So A is scandium. If we remove three electrons from a scandium atom then we end up with a trication which is isoelectronic with argon.

  • $\begingroup$ Thx for that explanation, i really appreciate ! $\endgroup$
    – James WOng
    Dec 26, 2021 at 4:08

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