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General ScienceMediumAsked in: NDA · NDA-I 2019, GAT

In which of the following pairs are the ions isoelectronic?

  1. A.Mg²⁺, Ar
  2. B.Na⁺, O²⁻
  3. C.Al³⁺, Cl⁻
  4. D.K⁺, Ne

Correct answer

B. Na⁺, O²⁻

Explanation

The correct answer is B, Na⁺ and O²⁻. Both have 10 electrons, the same number as a neon atom, so they are isoelectronic.

Isoelectronic species have the same number of electrons. Sodium has 11 electrons and loses one to form Na⁺, leaving 10. Oxygen has 8 electrons and gains two to form O²⁻, giving 10. Other members of this neon family are F⁻, Mg²⁺ and Al³⁺. Among isoelectronic ions, the one with more protons pulls the electrons in more tightly, so O²⁻ is larger than Na⁺.

A pairs Mg²⁺, with 12 − 2 = 10 electrons, with argon, which has 18.

C pairs Al³⁺, with 13 − 3 = 10 electrons, with Cl⁻, which has 17 + 1 = 18.

D pairs K⁺, with 19 − 1 = 18 electrons, with neon, which has 10.

Exam tip: count electrons as atomic number minus the positive charge, or plus the negative charge; 10-electron set N³⁻, O²⁻, F⁻, Ne, Na⁺, Mg²⁺, Al³⁺.

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Practice Questions

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Q2.General ScienceAsked in: NDA · NDA-I 2018, GATEasy

Which of the following properties is true for a tooth paste?

  1. A.It is acidic
  2. B.It is neutral
  3. C.It is basic
  4. D.It is made up of Calcium phosphate, the material of tooth enamel
Show answer
Correct answer: C. It is basic

Toothpaste is mildly basic so that it can neutralise the acid that causes tooth decay. Bacteria in the mouth break down sugar and food particles left on the teeth and produce acids.

Q3.General ScienceAsked in: NDA · NDA-II 2018, GATMedium

A stainless steel chamber contains Ar gas at a temperature T and pressure P. The total number of Ar atoms in the chamber is n. Now Ar gas in the chamber is replaced by CO₂ gas and the total number of CO₂ molecules in the chamber is n/2 at the same temperature T. The pressure in the chamber now is P'. Which one of the following relations holds true? (Both the gases behave as ideal gases)

  1. A.P' = P
  2. B.P' = 2P
  3. C.P' = P/2
  4. D.P' = P/4
Show answer
Correct answer: C. P' = P/2

For an ideal gas in a fixed volume at a fixed temperature, pressure is proportional to the number of molecules, so half the molecules give half the pressure.

Q4.General ScienceAsked in: NDA · NDA-II 2019, GATMedium

A solid disc and a solid sphere have the same mass and same radius. Which one has the higher moment of inertia about its centre of mass?

  1. A.The disc
  2. B.The sphere
  3. C.Both have the same moment of inertia
  4. D.The information provided is not sufficient to answer the question
Show answer
Correct answer: A. The disc

About an axis through the centre, a solid disc has I = ½MR² while a solid sphere has I = ⅖MR², and ½ is larger than ⅖.