Define mass number.

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Q. Define mass number.

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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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Mass number A = protons + neutrons (nucleons in the nucleus).

The mass number (denoted A) of an atom is the sum of the number of protons and the number of neutrons in its nucleus. Protons and neutrons together are called nucleons, since almost all the mass of an atom is concentrated in the nucleus.

A = number of protons (Z) + number of neutrons (N). …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 23 on this concept.

CBSE 2026Set ANNUAL1 markMCQ
$Q. If the number of protons and neutrons of an element is 13 and 14 respectively, then what are the atomic number (Z) and mass number (A)?$
(a) 13, 13
(b) 13, 27
(c) 14, 13
(d) 27, 14
›Reveal solutionSolution

Atomic number (Z) equals the number of protons; mass number (A) equals the sum of protons and neutrons.

Given: number of protons = 13, number of neutrons = 14.

Atomic number, Z = number of protons = 13.

Mass number, A = number of protons + number of neutrons = 13 + 14 = 27.

…

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CBSE 2026Set ANNUAL1 mark
$Q. In ⁸⁰₃₅Br, the number of neutrons is ______.$
›Reveal solutionSolution

The bromine nuclide with mass number 80 and atomic number 35 has 45 neutrons.

A nuclide is written as (mass number)/(atomic number) Symbol, i.e., mass number A on top and atomic number Z below, attached to the element symbol. Here A = 80 (mass number = total number of protons + neutrons) and Z = 35 (atomic number = numb …

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CBSE 2026Set ANNUAL1 markMCQ
$Q. If atomic mass of an element is 23 and its atomic number is 11, then the number of neutrons in an atom of the element would be$
(a) 23
(b) 12
(c) 11
(d) 34
›Reveal solutionSolution

Neutrons = mass number − atomic number = 23 − 11 = 12.

Atomic number (Z) = number of protons = 11. Mass number (A) = protons + neutrons = 23. …

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CBSE 2026Set ANNUAL1 markMCQ
$Q. Atoms having same atomic number but different mass numbers are called$
(a) Polymers
(b) Isomers
(c) Isotopes
(d) Isobars
›Reveal solutionSolution

Same atomic number, different mass number = isotopes.

Isotopes are atoms of the same element (same number of protons, same Z) that differ in the number of neutrons, hence different mass numbers. Example: 1H, 2H, 3 …

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CBSE 2026Set ANNUAL1 markMCQ
$Q. The chemical properties of atoms are controlled by number of$
(a) a) nucleus
(b) b) protons
(c) c) electrons
(d) d) neutrons
›Reveal solutionSolution
[!TLDR]

c) electrons

Why

Chemical behaviour is determined by the number and arrangement of electrons (especially valence el …
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CBSE 2026Set ANNUAL1 markMCQ
$Q. An atom of an element contains 19 electrons. Its nucleus has 20 neutrons. Its mass number is$
(a) a) 29
(b) b) 28
(c) c) 01
(d) d) 39
›Reveal solutionSolution
[!TLDR]

d) 39

Why

Neutral atom: protons = electrons = 19. Mass number = protons + neu …
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CBSE 2025Set ANNUAL1 markMCQ
$Q. Calculate the number of electrons, protons and neutrons in 35Br80.$
(a) 35, 45, 35
(b) 35, 35, 45
(c) 35, 36, 45
(d) 36, 35, 45
›Reveal solutionSolution

For a neutral atom, protons = electrons = atomic number (Z); neutrons = mass number (A) - Z. For 35Br80: 35 protons, 35 electrons, 45 neutrons.

The notation ZX^A (or here, 35Br80) gives the atomic number Z = 35 as a subscript and the mass number A = 80 as a superscript, for the element bromine (Br).

Step 1 — Protons: The atomic number Z directly equals the number of protons in the nucleus.

Protons = Z = 35

Step 2 — Electrons: In a neutral (uncharged) atom, the number of electrons equals the number of protons.

Electrons = 35

…

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CBSE 2025Set ANNUAL1 markMCQ
$Q. An ion B^2- has 18 electrons and 16 neutrons. Its mass number will be -$
(a) 34
(b) 32
(c) 18
(d) 16
›Reveal solutionSolution

For B2- with 18 electrons and 16 neutrons, the number of protons is 16 (since the 2- charge means 2 extra electrons over protons), giving mass number = 16 + 16 = 32.

A B2- ion carries a charge of -2, meaning it has 2 more electrons than protons (since each extra electron adds one unit of negative charge).

Number of electrons = 18 …

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CBSE 2025Set ANNUAL1 mark
$Q. Give one example of isotones.$
›Reveal solutionSolution

Isotones = same neutron count, different element; e.g. 14C (8n) and 15N (8n).

Isotones are atoms of different elements that have the same number of neutrons (but different atomic numbers and different mass numbers). Number of neutrons = mass number − atomic number.

…

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CBSE 2024Set ANNUAL1 markMCQ
$Q. By which of the following is positron represented?$
(a) 0/-1 e
(b) 0/+1 e
(c) 1/1 H
(d) None of these
›Reveal solutionSolution

Positron is represented as ⁰₊₁e — zero mass number (in nucleon terms), +1 charge.

A positron is a positively charged particle with the same (negligible, ~1/1837 amu) mass as an electron, so its mass number is written as 0, and …

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CBSE 2024Set ANNUAL1 markMCQ
$Q. Match the correct pair -- the isotope written as mass number 80, atomic number 35 (Bromine-80, ^80_35Br) should be matched with the correct item from Column B:$
(a) 45 Neutron
(b) Octahedral
(c) mol L^-1
(d) ΔS = q(rev)/T
(e) (triangle symbol, drawn as a downward-pointing triangle -- representing a 3-membered ring shape)
›Reveal solutionSolution

Number of neutrons = mass number (A) - atomic number (Z). For A = 80, Z = 35 (Bromine-80): neutrons = 80 - 35 = 45, matching option (a).

…

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CBSE 2024Set ANNUAL1 markMCQ
$Q. Isobars in the following pair is$
(a) A) 35/17Cl and 37/17Cl
(b) B) 12/6C and 13/6C
(c) C) 14/6C and 14/7N
(d) D) 12/6C and 14/6C
›Reveal solutionSolution
[!TLDR]

C) 14/6C and 14/7N

Why

Isobars have the same mass number but different atomic number; both have mass number 14 with Z = 6 and Z = 7 respectively. (A), (B), ( …
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Q.Define mass number.

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