The species that has the same number of electrons as ³⁵₁₇Cl is
- (a)³²₁₆S
- (b)³⁴₁₆S⁺
- (c)⁴⁰₁₈Ar⁺
- (d)³⁵₁₆S²⁻
Correct — C, ⁴⁰₁₈Ar⁺. In the stacked notation the printed booklet uses, the lower number is the atomic number, which equals the number of protons, and the upper number is the mass number, which equals protons plus neutrons. A neutral atom has as many electrons as protons; a positive charge means that many electrons have been removed, and a negative charge means that many have been added. Chlorine here is neutral with atomic number 17, so it carries 17 electrons. Argon has atomic number 18, and the single positive charge takes one electron away, leaving 18 minus 1, which is 17. That matches. Note that the mass numbers 35 and 40 play no part in the count at all — they are there to be ignored.
- (a)³²₁₆S — Sulphur has atomic number 16 and this species is neutral, so it has 16 electrons — one short of chlorine's 17.
- (b)³⁴₁₆S⁺ — Starting from sulphur's 16 electrons and removing one for the positive charge leaves 15, which is two short of 17.
- (d)³⁵₁₆S²⁻ — Sulphur's 16 electrons plus the two gained with the double negative charge give 18. This is the sulphide ion, and 18 is the electron count of neutral argon and of the chloride ion Cl⁻, not of neutral chlorine.
Species carrying the same number of electrons are called isoelectronic. The count is fixed entirely by the atomic number and the charge, and never by the mass number, which only records how many neutrons sit alongside the protons. So Cl⁻, Ar, K⁺ and Ca²⁺ all carry 18 electrons and are isoelectronic with one another, while neutral chlorine, with 17, belongs to a different set that includes S⁻ and Ar⁺.
This question is a trap laid with mass numbers. Three of the four species are sulphur, tempting the eye because sulphur sits next to chlorine, and the mass numbers 32, 34, 35 and 40 are strewn about to draw attention away from the subscript that actually matters. Work strictly to the rule that electrons equal the atomic number minus the charge — 17 for the chlorine given, and then 16, 15, 17 and 18 for the four options in order. Only argon with one electron removed matches.
- In the stacked notation the subscript is the atomic number and the superscript is the mass number.
- Number of electrons equals the atomic number minus the charge on the species.
- The mass number affects the number of neutrons, not the number of electrons.
- Cl⁻, Ar, K⁺ and Ca²⁺ each carry 18 electrons and form a classic isoelectronic set.
- Isotopes of one element share the atomic number but differ in mass number, so they have identical electron counts and identical chemistry.
The mass numbers 32, 34, 35 and 40 are decoration; only the atomic number and the charge decide — option (c).
- Using the mass number to count electrons — it counts nucleons, not electrons.
- Getting the sign of the charge backwards; a positive charge means fewer electrons, not more.
- Matching chlorine with the chloride ion out of habit; neutral chlorine has 17 electrons and the chloride ion has 18.
NDA sets isoelectronic and nuclide-composition items regularly, sometimes asking for the number of neutrons instead, which is where the mass number finally matters.
The alpha particle carries two positive charges. Its mass is very nearly equal to that of
- (a) two protons
- (b) an atom of helium
- (c) sum of masses of two positrons and two neutrons
- (d) two positrons as each positron carries a single positive charge
Answer(b) an atom of helium
Reads a charged species the same way — an alpha particle is a helium nucleus that has lost both its electrons, so its charge comes from the protons while its mass comes from protons plus neutrons.
There are six electrons, six protons and six neutrons in an atom of an element. What is the atomic number of the element ?
- (a) 6
- (b) 12
- (c) 18
- (d) 24
Answer(a) 6
Tests the same rule in its simplest form — the atomic number is fixed by the protons alone, and the neutrons, which set the mass number, are the distractor in both questions.
- practice — not a real PYQ
How many neutrons are present in ³⁵₁₇Cl?
- (a)17
- (b)18
- (c)35
- (d)52
Answer(b) 18 — neutrons equal the mass number minus the atomic number, that is 35 - 17.
- practice — not a real PYQ
Which one of the following is isoelectronic with the neon atom?
- (a)Na⁺
- (b)Cl⁻
- (c)K⁺
- (d)Ca²⁺
Answer(a) Na⁺ — sodium's 11 electrons minus one gives 10, the same as neutral neon; the other three all have 18.