An atom having atomic mass number 13 has 7 neutrons. What is the atomic number of the atom?

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Q. An atom having atomic mass number 13 has 7 neutrons. What is the atomic number of the atom?

Andhra Pradesh BieapShort· 2mImportance★★★★★est

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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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Atomic number equals the number of protons, which we find by subtracting neutrons from the mass number: $13 - 7 = 6$ .

Understanding Atomic Notation

Every atom is built from three particles: protons, neutrons, and electrons. The atomic number $Z$ counts the protons in the nucleus and defines the element's identity. The mass number $A$ is the total count of nucleons—protons plus neutrons—in the nucleus. Electrons contribute negligibly to mass, so they don't appear in $A$ .

The relationship connecting these quantities is straightforward:

A = Z + N

where $N$ is the number of neutrons. This equation captures the fact that the nucleus's mass comes entirely from its protons and neutrons.

A = Z + N

Finding the Atomic Number

We're given two pieces of information about this atom:

Mass number $A = 13$
Number of neutrons $N = 7$

We need to find the atomic number $Z$ .

Write down the fundamental relation.

Start with $A = Z + N$ .
Substitute the known values.

13 = Z + 7

Solve for the atomic number. …

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

AP EAPCET 2026Set eng-2026-05-18-AN1 markMCQ
$Q. The total number of electrons, protons and neutrons present in the three isotopes of hydrogen is (A) 3 (B) 5 (C) 6 (D) 9$
›Reveal solutionSolution
Add up protons + electrons + neutrons for protium, deuterium, and tritium separately, then sum all three totals to get 9.

Concept and Intuition

All three isotopes of hydrogen have the same atomic number (1 proton, 1 electron in the neutral atom) but differ in neutron number: protium has none, deuterium has one, and tritium has two. Summing every subatomic particle across all three isotopes is a simple counting exercise once the neutron counts are known.

Step-by-Step Solution

Protium ( $_1^1H$ ): 1 proton + 1 electron + 0 neutrons = 2.

Deuterium ( $_1^2H$ ): 1 proton + 1 electron + 1 neutron = 3.

Tritium ( $_1^3H$ ): 1 proton + 1 electron + 2 neutrons = 4. …
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AP EAPCET 2023Set ap-2023-05-22-AN1 markMCQ
$^{23}\mathrm{Na^+}$
›Reveal solutionSolution
11 protons fixes the element as sodium; 12 neutrons gives mass number 23; 10 electrons (one fewer than protons) means it's the +1 cation, Na⁺.

Concept and Intuition

The number of protons defines the element (atomic number $Z$ ). The mass number $A=Z+$ neutrons. If the number of electrons differs from the number of protons, the species is an ion, with charge $=(\text{protons})-(\text{electrons})$ .

Step-by-Step Solution

Protons $=11 \Rightarrow$ element is sodium (Na, $Z=11$ ).

Mass number $A=$ protons + neutrons $=11+12=23$ .

Electrons $=10$ , protons $=11$ : charge $=+1$ (11 positive charges, 10 negative charges).

So the species is $^{23}\mathrm{Na^+}$ .

Common Mistakes …
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AP EAPCET 2022Set ap-2022-07-11-AN1 markMCQ
$Q. Identify the group number of the element X which has 1.44% more number of neutrons than the protons. (Mass number of X is 127) (A) group 15 (B) group 16 (C) group 17 (D) group 14$
›Reveal solutionSolution
Solving $Z+1.44Z=127$ for the proton count gives $Z\approx52$ (with $N=75$ , matching the stated neutron-proton relationship almost exactly), placing element X in group 16.

Concept and Intuition

For a nuclide of known mass number $A=N+Z$ , if the neutron count is a known multiple/percentage-relation of the proton count, the two can be combined into a single equation in one unknown ( $Z$ ), pinning down the element uniquely from the periodic table, and hence its period and group.

Step-by-Step Solution

Let $Z$ = number of protons, $N$ = number of neutrons, with $N \approx 1.44Z$ (neutrons in a 1.44 : 1 relation to protons) and $N+Z=127$ .

Substitute: $Z + 1.44Z = 127 \Rightarrow 2.44Z = 127 \Rightarrow Z \approx 52.05$ , so $Z=52$ .

Check: $N = 127-52 = 75$ , and $75/52 = 1.442$ , consistent with the given relation.

Atomic number 52 corresponds to tellurium (Te), a p-block element. …
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AP EAPCET 2022Set ap-2022-07-12-AN1 markMCQ
$Q. Which of the following statement is correct about the isotopes of hydrogen? (A) Internuclear distance in hydrogen and deuterium is same (B) Enthalpy of bond dissociation of deuterium is less than hydrogen. (C) Order of relative abundance of isotopes is H > T > D (D) Ratio of neutrons to protons is greater for D than T.$
›Reveal solutionSolution
Bond length is set by electron distribution, not nuclear mass, so H₂ and D₂ have essentially the same internuclear distance — making (A) the only correct statement.

Concept and Intuition

Isotopes have identical electron configurations and hence nearly identical chemical/bonding behavior (bond lengths depend on electronic structure). However, the heavier isotope's nucleus has lower zero-point vibrational energy (due to larger reduced mass), making its bond dissociation enthalpy slightly higher, not lower — a classic isotope effect that trips up statement (B).

Step-by-Step Solution

(A): Bond length in a diatomic molecule depends on the electron cloud/orbital overlap, which is essentially unchanged by isotopic substitution (protons, deuterons and tritons have the same charge and nearly the same electronic environment). So $r(H_2)\approx r(D_2)$ . TRUE.

(B): Because $D$ is heavier, its zero-point energy is lower, meaning MORE energy is needed to dissociate it — so BDE(D₂) > BDE(H₂), making the statement ("enthalpy of D is less than H") FALSE.

(C): Natural abundance is H (99.98%) ≫ D (0.02%) ≫ T (trace, radioactive), so the correct order is H > D > T, not H > T > D. FALSE. …
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AP EAPCET 2022Set eng-2022-07-07-FN1 markMCQ
$^{12}\mathrm{C}$
›Reveal solutionSolution
Natural carbon's two stable isotopes are $^{12}\mathrm{C}$ and $^{13}\mathrm{C}$ ; $^{14}\mathrm{C}$ , though naturally occurring in trace amounts, is radioactive, not stable.

Concept and Intuition

Isotopes of an element share the same atomic number but differ in mass number (neutron count). Naturally occurring carbon is a mixture of isotopes, but only two of them are stable indefinitely — the rest, like $^{14}\mathrm{C}$ , undergo radioactive decay (in $^{14}\mathrm{C}$ 's case, this decay is the basis of radiocarbon dating).

Step-by-Step Solution

Carbon's naturally occurring isotopes include $^{12}\mathrm{C}$ , $^{13}\mathrm{C}$ , and trace $^{14}\mathrm{C}$ .

$^{12}\mathrm{C}$ (about 98.9% abundance) and $^{13}\mathrm{C}$ (about 1.1% abundance) are both stable (non-radioactive).

$^{14}\mathrm{C}$ is present only in trace amounts and is radioactive, decaying via beta emission with a half-life of about 5730 years — it is not classified as "stable." …
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AP EAPCET 2021Set ap-2021-09-03-AN1 markMCQ
$+3$
›Reveal solutionSolution
$X^{3+} = [Ar]3d^6$ has 24 electrons, so the atomic number of $X$ is $27$ .

Concept and Intuition

The electronic configuration of an ion tells us its electron count directly. A $+3$ charge means three electrons were lost relative to the neutral atom, so the atomic number equals the ion's electron count plus 3.

Step-by-Step Solution

Argon core $[Ar]$ has $18$ electrons.

$[Ar]3d^6$ therefore has $18 + 6 = 24$ electrons — this is $X^{3+}$ .

Since $X^{3+}$ lost 3 electrons, neutral $X$ has $24 + 3 = 27$ electrons.

Atomic number $Z = 27$ , which is cobalt (Co).

Common Mistakes

Forgetting to add back the 3 lost electrons and reporting $24$ (option B is $24$ ). …
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AP EAPCET 2021Set eng-2021-08-20-FN1 markMCQ
$^{13}_{6}C$
›Reveal solutionSolution
This tests basic nuclear notation $^A_ZX$ : protons $=Z$ , neutrons $=A-Z$ , electrons $=Z$ for a neutral atom — giving 6, 7, 6 for $^{13}_6C$ — answer (A).

Concept and Intuition

In the notation $^A_Z X$ , the subscript $Z$ (atomic number) is the number of protons, which also equals the number of electrons in a neutral atom, and the superscript $A$ (mass number) is the total number of nucleons (protons + neutrons). So neutrons are simply obtained by subtracting protons from the mass number.

Step-by-Step Solution

From $^{13}_6C$ : $Z=6$ (atomic number), $A=13$ (mass number).

Protons $= Z = 6$ .

Electrons (neutral atom) $= Z = 6$ . …
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Q.An atom having atomic mass number 13 has 7 neutrons. What is the atomic number of the atom?

Understanding Atomic Notation

Finding the Atomic Number

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