MPSC 2021 Prelims Paper-I · Question 68 of 100

Correct — option (3), Mg2+. Ionisation energy is the minimum energy needed to remove the most loosely held electron from an isolated atom or ion in the gaseous state, and the question compares a neutral magnesium atom with the two ions successively derived from it. Two rules settle it. The first is that successive ionisation energies of the same element always increase, because each electron removed leaves the same nuclear charge holding on to one electron fewer: the remaining electrons feel a greater effective pull, the particle shrinks, and the next electron is harder to detach. That alone puts Mg2+ above Mg+ and Mg+ above Mg. The second rule is the one that makes the difference here dramatic rather than gradual. Magnesium has atomic number 12 and the arrangement 2, 8, 2. Its first electron comes out of the outermost shell, which holds two electrons rather loosely, and costs about 738 kilojoules per mole. Its second comes out of the same shell and costs about 1,451 kilojoules per mole, roughly twice as much. But once both are gone, Mg2+ has the arrangement 2, 8 — the closed, stable configuration of neon — and the next electron would have to be torn out of that complete inner shell, close to the nucleus and strongly held. That third ionisation energy is about 7,733 kilojoules per mole, more than five times the second. This jump is not a curiosity; it is the standard evidence for the shell model of the atom and the reason magnesium's valency is two and not one or three. It is also what makes Mg2+ the answer: of the three species offered, it is the one whose remaining electrons are held most tightly, so it demands the most energy to lose a further electron. Notice how the question is phrased. It does not ask which species is most easily ionised, nor which ionisation is easiest; it asks which has the maximum ionisation energy, so the answer is the species from which the next electron is hardest to remove. And notice that the option set is arranged as a ladder — the neutral atom, the singly charged ion, the doubly charged ion — with an escape option added, so a candidate who knows only that successive ionisation energies rise can pick out the top of the ladder without recalling a single number. The charges on the ions are printed in the booklet as superscripts inside small circles, and are transcribed here as plain Mg+ and Mg2+.

• (1)Mg — The neutral magnesium atom has the lowest ionisation energy of the three species offered, about 738 kilojoules per mole, which is the least of the values on this ladder rather than the greatest. Two of magnesium's electrons occupy the outermost shell and are relatively far from the nucleus and well screened from it by the inner shells, so one of them comes away comparatively easily — which is exactly why magnesium is a reactive metal that readily forms the Mg2+ ion in its compounds. A candidate lands on this option by reading the question as asking which species is ionised most easily, or by carrying a vague association between 'magnesium' and 'high first ionisation energy' — an association that is true only within its own period, where magnesium's filled outer subshell gives it a higher first ionisation energy than aluminium next door.
• (2)Mg+ — Mg+ sits in the middle of the ladder. Removing an electron from it — the second ionisation of magnesium — costs about 1,451 kilojoules per mole, roughly twice the first, because the electron is being pulled away from a particle that is already positively charged and has contracted around its nucleus. That is a substantial increase, and a candidate who stops after noticing that successive ionisation energies rise may settle here. But the same reasoning carried one step further gives a much larger increase, because Mg+ still has one electron left in its outermost shell and so is not a closed configuration; the electron removed from it comes out of the same loosely held outer shell as the first. Only when both outer electrons are gone does the next removal have to break into a complete shell, and that is where the value multiplies.
• (4)All of the above — This cannot be right as a matter of logic before any chemistry is considered. The question asks which of the listed species has the maximum ionisation energy, and three different species with three different values cannot all simultaneously be the maximum; an option asserting that they can is answering a question that was not asked. The chemistry is equally clear: the three values differ enormously, about 738, about 1,451 and about 7,733 kilojoules per mole respectively, so they are not even approximately equal. This is the second of only two places in this paper where the exact phrase 'All of the above' is printed, and it is a reminder that an escape option is not a safe default — it has to satisfy the stem like any other choice, and a stem asking for a single maximum rarely admits one.

Ionisation energy, or ionisation enthalpy, is the minimum energy required to remove the most loosely bound electron from one mole of isolated gaseous atoms or ions. It is always positive, because work must be done against the attraction of the nucleus. Successive ionisation energies of the same element rise steadily — the second is always greater than the first, the third than the second — and the rise is gentle while electrons are being taken from the same shell but becomes very steep as soon as the removal has to break into a completed inner shell. The size of that jump identifies the number of valence electrons and is the classic experimental support for the idea that electrons occupy shells at all: magnesium's first two ionisation energies are about 738 and about 1,451 kilojoules per mole, while the third is about 7,733, so magnesium plainly has two valence electrons and forms Mg2+. The main factors governing ionisation energy are the nuclear charge, the distance of the electron from the nucleus, the screening of the nuclear charge by inner electrons, and the stability of the resulting configuration. Across a period ionisation energy generally increases, because nuclear charge rises while the electrons enter the same shell; down a group it decreases, because the outer electrons are further away and better screened. The general trend across a period has well-known exceptions where a filled or half-filled subshell gives extra stability, which is why beryllium exceeds boron and nitrogen exceeds oxygen. The reverse quantity is electron affinity, the energy change when an atom gains an electron, and electronegativity is the related tendency of a bonded atom to attract shared electrons.

Periodic properties are examined in nearly every MPSC science section, and ionisation energy is the most heavily asked of them because it links the electron arrangement of an atom to its chemistry — to valency, to metallic character, to the ions an element forms. This question is a good example of the Commission's habit of testing a principle rather than a fact: no candidate is expected to recall that magnesium's third ionisation energy is about 7,733 kilojoules per mole, but every candidate is expected to know that removing an electron from a closed shell costs far more than removing one from an incomplete outer shell, and that alone answers the question. The other habit on display is the ladder option set, in which the choices are a graded series with an escape option appended. The right response to such a set is to establish the direction of the trend and then read off the end of it, and to remember that an escape option must satisfy the stem exactly like any other; here a stem asking for a single maximum makes 'All of the above' self-contradictory before any values are considered.

• Ionisation energy is the minimum energy needed to remove the most loosely held electron from an isolated gaseous atom or ion, and successive ionisation energies of an element always increase, because each removal leaves the same nuclear charge holding fewer electrons.
• Magnesium has atomic number 12 and the arrangement 2, 8, 2; its measured ionisation energies are about 737.7 kilojoules per mole for the first, about 1450.7 for the second and about 7732.7 for the third.
• The very large jump at the third ionisation is because Mg2+ already has the closed neon-like arrangement 2, 8, so a further electron must be pulled out of a complete inner shell — which is why magnesium's valency is two.
• Ionisation energy increases across a period as nuclear charge rises, and decreases down a group as the outermost electrons move further from the nucleus and are better screened; filled and half-filled subshells produce the familiar exceptions.
• The size of the jump between successive ionisation energies reveals the number of valence electrons an element has, and is a standard piece of experimental evidence for the arrangement of electrons in shells.

• Reading a question about maximum ionisation energy as though it asked which species is ionised most easily, which reverses the answer
• Stopping at the first increase in successive ionisation energies and missing the far larger jump that occurs when a closed shell has to be broken into
• Treating an escape option such as 'All of the above' as a safe default, when a stem asking for a single maximum cannot admit it
• Confusing ionisation energy, which concerns the loss of an electron, with electron affinity or electronegativity, which concern its gain or attraction

MPSC asks periodic properties as direct comparisons — which of four elements has the highest ionisation energy, which has the largest atomic radius — as trend questions across a period or down a group, and as reasoning questions of the kind set here, where the comparison is between an atom and its own ions. The Commission also asks the exceptions, since beryllium against boron and nitrogen against oxygen are favourites, and it asks the consequences, since the elements that lose electrons easily are the reactive metals and those that gain them are the reactive non-metals. Numerical values are rarely required; the trends and the reasons for them almost always are. A candidate who can write the electron arrangement of any element in the first three periods, and who knows that an ionisation energy jumps sharply as soon as a completed shell has to be disturbed, can reconstruct nearly every answer in this family from first principles.

No directly related past PYQ was found.

• practice — not a real PYQ

  The successive ionisation energies of an element are 578, 1817, 2745 and 11578 kilojoules per mole. How many electrons does an atom of this element have in its outermost shell ?

  • (a)One
  • (b)Two
  • (c)Three
  • (d)Four

  Answer(c) Three — the first three ionisation energies rise gradually, showing that those electrons are being taken from the same outermost shell, but the fourth is several times larger than the third, which means the fourth electron has to be removed from a completed inner shell. The position of the large jump therefore counts the valence electrons, and here it comes after the third, so the atom has three electrons in its outermost shell and will characteristically form an ion with a triple positive charge. The same reasoning applied to magnesium, whose jump comes after the second ionisation, shows that magnesium has two.

• practice — not a real PYQ

  Which one of the following statements about ionisation energy is correct ?

  • (a)It generally decreases across a period from left to right
  • (b)It generally decreases down a group from top to bottom
  • (c)The second ionisation energy of an element is always smaller than the first
  • (d)It is the energy released when an atom gains an electron

  Answer(b) It generally decreases down a group from top to bottom — as one descends a group a new shell is added at each step, so the outermost electron lies further from the nucleus and is better screened from it by the intervening shells, and less energy is needed to pull it away. Across a period the trend runs the other way, since nuclear charge increases while the electrons are added to the same shell. The second ionisation energy is always greater than the first, never smaller, and the energy change when an atom gains an electron is the electron affinity, a different quantity altogether.