Identify the pairs which are not of isotopes? (Note: more than one of the given options may be correct.) (i) ¹

NCERT Exemplar · Q17

Q. Identify the pairs which are not of isotopes? (Note: more than one of the given options may be correct.)

(i) ^12_6 X , ^13_6 Y

(ii) ^35_17 X, ^37_17 Y

(iii) ^14_6 X, ^14_7 Y

(iv) ^8_4 X, ^8_5 Y

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Concept understanding — Atomic Notation

Atomic Notation: The ID Card of an Atom

Imagine you're at a huge stadium filled with people. To identify any one person, you'd need more than just a name — you'd need their jersey number, their team, maybe their position. An atom is the same way. Just saying "carbon" or "oxygen" tells you the kind of atom, but not the full story. Atomic notation is the shorthand that gives you the complete identity.

The Intuition: Three Numbers, One Symbol

Every atom is built from three key particles:

Protons (positive charge) — these define what element it is.
Neutrons (no charge) — these add mass and stability.
Electrons (negative charge) — these orbit the nucleus and determine chemical behaviour.

Atomic notation packs all this information around the element's symbol. Think of it as a label with two numbers attached:

^{A}_{Z}\text{X}

Where:

X is the element symbol (e.g., C for carbon, O for oxygen).
Z is the atomic number (number of protons).
A is the mass number (protons + neutrons).

Note

The atomic number $Z$ is what makes an atom that element. Change $Z$ , and you change the element entirely. Carbon always has $Z=6$ ; if you change it to $7$ , it's nitrogen.

The Precise Statement

For any atom represented as $^{A}_{Z}\text{X}$ :

Atomic number $Z$ = number of protons = number of electrons (in a neutral atom).
Mass number $A$ = number of protons + number of neutrons.
Number of neutrons $N = A - Z$ .

A neutral atom has equal protons and electrons. If the atom gains or loses electrons, it becomes an ion — but the notation for the nucleus stays the same.

Example: Carbon-12

The most common carbon atom is written as:

^{12}_{6}\text{C}

From this:

$Z=6$ → 6 protons, and in a neutral atom, 6 electrons.
$A=12$ → total nucleons (protons + neutrons) = 12.
Neutrons $N = 12 - 6 = 6$ .

Why Two Numbers? Why Not Just One?

If you only knew the mass number $A$ , you wouldn't know the element — because different elements can have the same mass number (e.g., $^{14}\text{C}$ and $^{14}\text{N}$ both have $A=14$ ). If you only knew the atomic number $Z$ , you wouldn't know how many neutrons are present — and that matters for isotopes.

Important

Isotopes are atoms of the same element (same $Z$ ) with different numbers of neutrons (different $A$ ). For example:

$^{12}_{6}\text{C}$ (6 neutrons)
$^{13}_{6}\text{C}$ (7 neutrons)
$^{14}_{6}\text{C}$ (8 neutrons) All are carbon, but they have different masses and some are radioactive.

A Quick Table for Clarity

Notation	Element	Protons ( $Z$ )	Neutrons ( $N$ )	Mass Number ( $A$ )
$^{1}_{1}\text{H}$	Hydrogen	1	0	1
$^{4}_{2}\text{He}$	Helium	2	2	4
$^{23}_{11}\text{Na}$	Sodium	11	12	23

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Isotopes share the same atomic number (same element, same protons) but differ in mass number (different neutrons). Pairs (C) and (D) have different atomic numbers, so they are not isotopes.

Understanding Isotopes Through Atomic Notation

The notation $^A_Z X$ tells you everything about an atom's nucleus:

$Z$ (subscript) = atomic number = number of protons
$A$ (superscript) = mass number = protons + neutrons

Isotopes are atoms of the same element with different numbers of neutrons. The defining feature of an element is its atomic number $Z$ . If two atoms have the same $Z$ , they're the same element; if they also have different $A$ , they're isotopes of one another.

The question asks which pairs are not isotopes, meaning we're looking for pairs where $Z$ differs.

Examining Each Pair

Option (A): $^{12}_6 X$ and $^{13}_6 Y$

Both have $Z = 6$ (both are carbon atoms). The mass numbers are $A = 12$ and $A = 13$ , so they differ by one neutron. These are carbon-12 and carbon-13, classic isotopes.
Option (B): $^{35}_{17} X$ and $^{37}_{17} Y$

Both have $Z = 17$ (both are chlorine atoms). The mass numbers are $A = 35$ and $A = 37$ . These are the two stable isotopes of chlorine found in nature.
Option (C): $^{14}_6 X$ and $^{14}_7 Y$ …

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Q.Identify the pairs which are not of isotopes? (Note: more than one of the given options may be correct.)

Understanding Isotopes Through Atomic Notation

Examining Each Pair

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