Enforcement Officer / Accounts Officer (EO/AO) 2017 recruitment test. Transcribed from the Commission's own booklet (series A, GRS-A-TNS); answer as per the official UPSC final answer key. This question is in Part B. Answer last reviewed 24 August 2026.

Which one of the following materials is not diamagnetic at Standard Temperature and Pressure (STP)?

(a)Nitrogen
(b)Sodium chloride
(c)Water
(d)Iron

Answer

Why

Correct — D, (d) Iron. Read to the end of the stem: the word not is printed in bold italic, and the question asks which of the four is not diamagnetic. Three of them are; iron is not. Iron is the standard example of a ferromagnetic material, and ferromagnetism sits at the opposite end of the scale from diamagnetism.

The underlying physics is a question about electrons. A substance is diamagnetic when the resultant magnetic moment of each of its atoms is zero — every electron is paired, and the orbital and spin moments cancel out. Apply a magnetic field to such a substance and, by Lenz’s law, the induced changes in the electron motion produce a net magnetic moment directed against the applied field. The substance is therefore weakly repelled: field lines are expelled, the field inside is slightly reduced, and a bar of the material placed in a non-uniform field moves from the stronger part of the field towards the weaker. Its magnetic susceptibility is small and negative. Iron behaves in the opposite way. Its atoms carry permanent magnetic dipole moments which interact so strongly that they align spontaneously over regions called domains; an applied field grows and orients the domains, the material is strongly magnetised and strongly attracted, and its susceptibility is large and positive.

The three wrong options are not a random selection. Nitrogen at standard temperature and pressure, water and sodium chloride are three of the seven substances the standard school textbook lists as diamagnetic — the full list runs bismuth, copper, lead, silicon, nitrogen at STP, water and sodium chloride — and iron heads its list of ferromagnetic elements, which runs iron, cobalt, nickel and gadolinium. Recognising that the item has been built out of two textbook lists is the quickest way through it.

The phrase ’at Standard Temperature and Pressure’ in the stem is not decoration either. Nitrogen and oxygen are gases, and their magnetic behaviour is quoted at a stated temperature and pressure because it depends on the state they are in; the textbook itself writes ’nitrogen (at STP)’ in the diamagnetic list and ’oxygen (at STP)’ in the paramagnetic one. That pairing is worth memorising, because the two gases sit on opposite sides of the line: molecular oxygen has two unpaired electrons and is paramagnetic, while molecular nitrogen has none and is diamagnetic.

Why the others are wrong

(a)Nitrogen — Nitrogen at standard temperature and pressure is diamagnetic, so it cannot be the answer to a question asking which is not. Molecular nitrogen has a triple bond and every one of its electrons is paired, leaving no net magnetic moment on the molecule — which is exactly the condition for diamagnetism. It is worth learning against its neighbour in the periodic table: molecular oxygen has two unpaired electrons in antibonding orbitals, carries a permanent moment, and is paramagnetic. A candidate who remembers that liquid oxygen is attracted to a magnet and generalises from it to all atmospheric gases will mark this option and lose the mark.
(b)Sodium chloride — Sodium chloride is diamagnetic and appears by name in the textbook list. Its ions explain why: the sodium ion has given away its outer electron and the chloride ion has taken one up, so both have the closed-shell configuration of a noble gas, every electron is paired, and neither ion carries a resultant magnetic moment. The lesson generalises to a large family of ionic salts of the main-group elements. Where an ionic solid does show paramagnetism, it is almost always because a transition metal ion with unpaired d electrons is present — copper chloride, which the same textbook lists as paramagnetic, is the standard contrast.
(c)Water — Water is diamagnetic and is also in the textbook list. In the water molecule the ten electrons are fully paired in bonding and lone pairs, leaving no net moment. This is the most useful item on the list to remember, because it makes the point that diamagnetism is ordinary rather than exotic: most everyday matter, including living tissue, which is largely water, is weakly repelled by a magnetic field. In fact diamagnetism is present in every substance; it is simply masked wherever paramagnetism or ferromagnetism is also present, both of which are far stronger effects.

Concept

Materials are classified magnetically by their susceptibility, the ratio of the magnetisation induced in them to the applied magnetising field. DIAMAGNETIC materials have a small negative susceptibility, between minus one and zero, and a relative permeability just below one; their atoms have no resultant magnetic moment, and an applied field induces one opposing itself, so they are weakly repelled and tend to move from the stronger part of a field to the weaker. Bismuth, copper, lead, silicon, nitrogen at STP, water and sodium chloride are the standard examples. Superconductors are the extreme case: they expel the field completely, with susceptibility exactly minus one, a phenomenon called the Meissner effect. PARAMAGNETIC materials have a small positive susceptibility; their atoms carry permanent dipole moments which thermal motion keeps randomly oriented, so an applied field produces only weak alignment and weak attraction, and the effect strengthens as the field rises or the temperature falls. Aluminium, sodium, calcium, oxygen at STP and copper chloride are the textbook examples. FERROMAGNETIC materials have a very large positive susceptibility and a relative permeability above a thousand; their atomic dipoles interact so strongly that they align spontaneously within domains, and an applied field grows the favourably oriented domains until the sample is strongly magnetised. Iron, cobalt, nickel and gadolinium are the elements. Ferromagnetism depends on temperature: heat a ferromagnet enough and its domain structure breaks up and it becomes a paramagnet. Materials that keep their magnetisation when the field is removed are hard ferromagnets and make permanent magnets; those that lose it, such as soft iron, are soft ferromagnets and make electromagnet cores.

Physics in EPFO general ability papers is drawn from the school syllabus and is often lifted almost directly from its example lists, which makes those lists worth learning as lists rather than as principles alone. The magnetic classification is a good instance: three short lists of examples, one line of physics for each class, and the ability to say which way a sample moves in a non-uniform field. The examiner’s method on this item is to put three members of one list against one member of another, so a candidate who knows the lists answers in seconds while a candidate who knows only the definitions has to reason about the electronic structure of four substances under time pressure. Both routes work; only one is quick.

Key facts

Iron is ferromagnetic, not diamagnetic; the ferromagnetic elements are iron, cobalt, nickel and gadolinium.
The standard list of diamagnetic materials is bismuth, copper, lead, silicon, nitrogen at STP, water and sodium chloride.
The standard list of paramagnetic materials is aluminium, sodium, calcium, oxygen at STP and copper chloride.
A diamagnetic substance has no resultant atomic magnetic moment; an applied field induces one opposing itself, so it is weakly repelled.
Susceptibility is small and negative for diamagnetic materials, small and positive for paramagnetic materials, and very large and positive for ferromagnetic materials.
A diamagnetic sample in a non-uniform field moves from the stronger part of the field towards the weaker; a paramagnetic or ferromagnetic sample moves the other way.
Diamagnetism is present in all substances but is masked wherever the much stronger paramagnetic or ferromagnetic effects are present.
Superconductors are perfect diamagnets, expelling the field entirely — the Meissner effect — with susceptibility of minus one.
Molecular nitrogen has all its electrons paired and is diamagnetic; molecular oxygen has two unpaired electrons and is paramagnetic, which is why both are quoted at STP.

Three classes, three susceptibilities, and three textbook lists

(d) Iron — ferromagnetic→Its atoms carry permanent magnetic dipole moments which interact so strongly that they align spontaneously over regions called domains. An applied field grows and orients the domains, the material is strongly magnetised and strongly attracted, and its susceptibility is large and positive. The ferromagnetic elements are iron, cobalt, nickel and gadolinium.

Diamagnetic — the other end of the scale→The resultant magnetic moment of each atom is zero: every electron is paired, and the orbital and spin moments cancel. An applied field induces, by Lenz’s law, a moment directed against itself, so the substance is weakly repelled and moves from the stronger part of a non-uniform field towards the weaker. Susceptibility small and negative.

(a) Nitrogen, (b) Sodium chloride, (c) Water→Three of the seven substances the standard list names as diamagnetic — bismuth, copper, lead, silicon, nitrogen at STP, water and sodium chloride. Recognising that the item has been built out of two textbook lists is the quickest way through it.

Why each of those three has no moment→Molecular nitrogen has a triple bond and every one of its electrons paired. In sodium chloride the sodium ion has given away its outer electron and the chloride ion has taken one up, so both have the closed-shell configuration of a noble gas. In water the ten electrons are fully paired in bonding pairs and lone pairs.

Why the stem says ‘at STP’→Nitrogen and oxygen are gases, and their magnetic behaviour is quoted at a stated temperature and pressure. The two sit on opposite sides of the line: molecular oxygen has two unpaired electrons and is paramagnetic, molecular nitrogen has none and is diamagnetic. A candidate who remembers liquid oxygen being drawn to a magnet and generalises will mark nitrogen.

Paramagnetic, the class in between→Atoms carry permanent dipole moments, but thermal motion keeps them randomly oriented, so an applied field produces only weak alignment and weak attraction; susceptibility small and positive. Aluminium, sodium, calcium, oxygen at STP and copper chloride. Diamagnetism is in fact present in every substance — it is simply masked wherever these much stronger effects are.

Study next

Magnetic susceptibility, relative permeability and the classification of magnetic materialsDomains, hysteresis, retentivity and coercivity, and the difference between hard and soft magnetic materialsThe Curie temperature and the loss of ferromagnetism on heatingSuperconductivity and the Meissner effectWhy oxygen is paramagnetic and nitrogen diamagnetic — molecular orbitals and unpaired electrons

Common traps

Missing the negative ask; the word not is in bold italic and three of the four options are diamagnetic.
Generalising from liquid oxygen’s attraction to a magnet and assuming all gases behave alike. Oxygen is paramagnetic, nitrogen is diamagnetic.
Assuming that anything non-magnetic in ordinary speech must be diamagnetic. Aluminium feels non-magnetic and is paramagnetic.
Ignoring the phrase ’at STP’, which is in the stem because the magnetic behaviour of a gas is quoted for a stated state.

Magnetic classification items are almost always built by mixing the textbook example lists, so the fastest preparation is to learn the three lists and one distinguishing behaviour for each class. Expect the negative form — which is not diamagnetic, which is not ferromagnetic — because it lets the examiner use three members of one list, and expect gases to be qualified by a state. A related warning worth carrying: a badly built question of this shape can leave more than one defensible answer, so read all four options before marking even when the first one you meet looks right.

Related PYQs

EPFO_EOAO_2017_Q94Open & attempt →

Which one of the following gases has the highest solubility in water?

(a) Chlorine
(b) Ammonia
(c) Carbon dioxide
(d) Nitrogen

Answer(b) Ammonia

Asks which gas has the highest solubility in water — the next item in this paper’s science run, and another question answered from a short list of standard substances rather than from first principles.

Practice

practice — not a real PYQ
Which one of the following sets contains only ferromagnetic substances?
- (a)Iron, cobalt, nickel, gadolinium
- (b)Bismuth, copper, lead, silicon
- (c)Aluminium, sodium, calcium, platinum
- (d)Water, sodium chloride, nitrogen, silicon
Answer(a) Iron, cobalt, nickel, gadolinium
practice — not a real PYQ
A small bar of a certain material, when placed in a non-uniform magnetic field, moves from the stronger part of the field towards the weaker part. The material is
- (a)paramagnetic
- (b)ferromagnetic
- (c)diamagnetic
- (d)ferrimagnetic
Answer(c) diamagnetic

Why