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?

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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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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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