Which of the following statements about the electron is incorrect? (i) It is a negatively charged particle. (i

NCERT Exemplar · Q5

Q. Which of the following statements about the electron is incorrect?

(i) It is a negatively charged particle.

(ii) The mass of electron is equal to the mass of neutron.

(iii) It is a basic constituent of all atoms.

(iv) It is a constituent of cathode rays.

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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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The electron is a fundamental subatomic particle with negative charge and negligible mass compared to nucleons. The incorrect statement is (B) — the electron's mass is roughly 1/1836 that of a neutron.

Understanding the Electron

The electron is one of the three fundamental particles that make up atoms, alongside protons and neutrons. Discovered by J.J. Thomson in 1897 through cathode ray experiments, it carries the elementary negative charge and has properties that define atomic structure and chemical behavior.

Let's examine each statement against what we know about electrons:

Statement (A): "It is a negatively charged particle."

This is correct. The electron carries a charge of $-1.6 \times 10^{-19}$ coulombs, which we conventionally denote as $-1e$ or simply $-1$ in atomic units. This negative charge is fundamental to all electromagnetic interactions involving electrons.
Statement (B): "The mass of electron is equal to the mass of neutron."

This is incorrect. The masses differ dramatically:
- Mass of electron: $m_e \approx 9.109 \times 10^{-31}$ kg
- Mass of neutron: $m_n \approx 1.675 \times 10^{-27}$ kg
The neutron is approximately 1839 times heavier than the electron. In atomic mass units, the electron has a mass of about $0.0005$ u while the neutron has a mass of about $1.008$ u. The electron's mass is so small that it contributes negligibly to an atom's total mass.
Statement (C): "It is a basic constituent of all atoms."

This is correct. Every neutral atom contains electrons equal in number to its protons. Even ions (charged atoms) are simply atoms that have gained or lost electrons. The arrangement of electrons in shells determines all of chemistry. …

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Q.Which of the following statements about the electron is incorrect?

Understanding the Electron

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